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TECHNICAL PAPER NO. 49

.THE EFFECTS OF RADIATA PINE PLANTATION

ESTABLISHMENT AND MANAGEMENT ON

WATER YIELDS AND WATER QUALITY

-A REVIEW

BY

P.M. CORNISH

FORESTRY COMMISSION OF NEW SOUTH WALES

THE EFFECTS OF RADIATA PINE PLANTATION ESTABLISHMENTAND MANAGEMENT ON WATER YIELDS AND WATER QUALITY

-A REVIEW

by

P.M. Cornish

HYDROLOGY SECTIONWOOD TECHNOLOGY AND FOREST RESEARCH DIVISION

FORESTRY COMMISSION OF NEW SOUTH WALESSYDNEY

1989

Technical Paper No. 49December, 1989

Published by:Forestry Commission of New South Wales,Wood Technology and Forest Research Division27 Oratava Avenue, West Pennant Hills, 2120P.O. Box 100, Beecroft N.S.W. 2119Australia

Copyright © 1989 Forestry Commission of New South Wales

ODCISSNISBN

232.4:174.4:116.280548-68070730575640

Technical Paper No. 49 Forestry Commission of New South Wales

• i • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality· A Review

TABLE OF CONTENTS PAGE

SUMMARy 1

INTRODUCTION 2

EFFECTS ON WATER YIELDS 4:',

1. Important Hydrological Processes in Relation To Vegetation Types 4

(a) Precipitation 5(b) Evapotranspiration 5(c) Transpiration 5(d) Interception 6(e) Evapotranspiration differences between vegetation types 8(f) Energy supply 9

2. Differences in Water Yields between Conifers, Eucalypts and Pasture 10

(a) Water yield differences between eucalypt and pasture 10(b) Water yield and P. radiata in Australia 11(c) Water yield andP. radiata in New Zealand 12(d) Water yield and pine afforestation in South Africa 13(e) Water yield changes and annual precipitation 14(f) Water yield changes and plantation age 16(g) Other hydrologic consequences of afforestation 18(h) Transpiration efficiency of P. radiata and eucalypts and its relationship 20

to productivity.(i) Effects of fertilisation on water yields in P. radiata 23

(j) Present understanding of P. radiata water use 24(k) Amendments required to Boughton's water yield conclusions 25

EFFECTS ON WATER QUALITY 26

1. Soil Erosion and Sediment Production 26

(a) Influence of rainfall and running water 26(b) Factors influencing surface runoff 27(c) Slope stability and landslips 28(d) Forest operations and stream sediment levels 29

(i) General 29(ii) Suspended sediment and P. radiata 30(iii) Erosion mitigation measures in P. radiata plantations 32

2. Chemical Water Quality 33

(a) Nutrients and major ions 33(b) Applied forestry chemicals 34(c) Bacterial water quality 35(d) Salinity 36

CONCLUSIONS 38

1. Water Use and Water yield 38

2. Water Quality 39

Forestry Commission of New South Wales Technical Paper No. 4~

------------------------ -------

The E;ffeQts of Radia~a Pine Plantation Establlshment and - ii •Man'agement on Water Yields and Water Ouallty- A Review

REFERENCES ; 42

LIST OF TABLES

Table 1.

Table 2.

Table 3.

Table 4.

Table 5.

Table 6.

Table 7.

Interception (I) as an overali.percentage of gross precipitation (P) for different forest 8types and ages as reported by various,workers.

Ranking of the processes of transpiration (T) and interception (1) for pasture, eucalyptus ...9forest and pine forest.

Summary of streamflow changes in three of the Jonkershoek catchments, South Africa....... 15(after Van Wyck, 1987).

Water yield differences between eucalypt and pasture for a range of mean annual 17precipitation vlaues, 'A' values derived from Holmes and Sinclair (1986). 'B'valuesfrom Fig. 4.

Comparison of timber yields and estimated biomass production for P. radiata and 23E. delegatensis at Bago S.F. N.S.W.

Estimated mean annual evapotranspiration of a fully stock P. radiata plantation for 25a range of mean annual precipitation values.

Effects of forest operations on stream turbidity at Fernow (after Kochenderfer and 30Aubertin, 1975).

LIST OF FIGURESFigure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

General rend lines relating annual streamflow increase to percentage reduction 2jIJ. cover for conifers, deciduous hardwood/mixed. hardwood and scrub. (After Boschand Hewlett, 1982).

Envelope curves depicting mean evapotranspiration for fully afforested catchments 11and completly pastured catchments in Victoria (after Holmes andSinclair, 1986).Evapotanspiration differences between pasture and eucalypt for particular values ofannual precipitation are given by the vertical difference 'A'. Evapotranspiration ofP. radiata plantations in South Australia is indicated by the closed circles.

Trends in total and dry periods streamflows following afforestation of native grassland ....... 14with P. patula in South Africa (after Bosch, 1979).

Suggested relationships between water yield decrease and mean annual precipitation ......... 15for the complete afforestation of eucalypt and pasture by pine. Evapotranspirationdifferences between pasture and eucalypt for particular values of annual precipitationare given by the vertical difference 'B'.

Water yield decrease versus plantation age for areas fully afforested with P. radiata 18(after Van Wyck, 1987) and P. panda (after Bosch, 1979}: squares - P. patula;closedcircles - Biesievlei catchment; open circles - Bosbouklook catchment; dotted circles Tierkloof catchment; all P. radiata.

Hypothetical storm hydrographs for grassed and afforested catchments (after Holmes ....... 20and Wronski, 1982).

Relationships between biomass and net primary productivity for E. obliqua (circles) 21andE. grandis (triangles) for the range of ages indicated (after Turner, 1986). Alsoinclude.d are P. radiata data from Australia ~tars1taken from Forrest and Ovington(1970) (including an estimated 1.0-2.5 t ha - yr- of litterfall), Baker and Attiwell(1985) and Lambert (1978) and from New Zealand (squares) taken from Madgwicket al. (1977) and Beets and Pollock (1987). Stand ages are indicated.


- 1 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouallty- A Review

SUMMARY

This paper examines the effects of P. radiata establishment and management on watervalues in Australia through a comprehensive review of relevant published information.The most important hydrologic processes that contribute to ultimate hydrologicdifferences between vegetation types are outlined. Those processes that determine therelative hydrological behaviour of grasslands, eucalypt forests and pine plantations areconsidered in more detail and related to the effects that forest management practiceshave on water yield and water quality.

Recent findings suggest that the complete afforestation of cleared pasture areas inAustralia by P. radiata is likely to result in maximum reductions in water yield in excess of400 mm per annum where precipitation exceeds 1300 mm annually, although actualreductions in any year will depend on the precipitation received and the age of the stand.Baseflow in streams in such afforested areas may decline significantly, and some perennialstreams may become ephemeral, particularly in drier years. A systematic examination ofrelevant factors should be undertaken in a lengthy study of growth rates and water use inpine to refine predictions of P. radiata water use with age.

Suspended sediment and turbidity are the most widespread pollutants of streamwater inP. radiata plantations, particularly in areas of higher rainfall such as the N.S.W.tablelands. The imposition of various controls to minimise streamwater sedimentincreases is now standard practice in many plantations but turbidity levels remain higherthan in managed hardwood forests. Various factors that contribute to sedimentproduction in plantations are discussed.


The Effects of Radiata Pine Plantation Establishment and • 2 •Management on Water Yields and Water Quality· A Review

INTRODUCTION

The influence of differing land uses on the hydrology of river basins has been a topic ofconsiderable scientific interest since the early years of the twentieth century. In particularthe role of forests in moderating streamflow and the impact of forests (and forestry) onthe hydrological cycle have received a great deal of research attention around the world.Early studies concentrated on quantifying water yield differences between catchments ofdiffering types, densities or ages of vegetation in the same climatic environment.Following the studies begun in the D.S.A. at Wagon Wheel Gap, Colorado in 1909 (Batesand Henry, 1928) many workers were able to show that a significant reduction of forestcover resulted in increases in water yield, and that these increases gradually diminished asforest cover re-established.

A review of 39 water yield catchment studies worldwide by Hibbert (1967) led hiID toconclude that, generally»

"1. Reductionafforest coverincreases wateryield.2. Establishment offorestcoveron sparsely vegetated land decreases wateryield.3. Responseto treatment is highly variable, and,for themost part, unpredictable."

Bosch and.Hewlett (1982) further reviewed these studies and another 55 that had beenreported after 1967. They considered that the first two of Hibbert's conclusions werereinforced by these additional studies»

"Decreases in wateryieldfollowing afforestation seem to beproportional to thegrowth rateofthe stand while gains, in wateryieldafterclearjelling diminish inproportion to the rate ofrecovery ofthe vegetation It. '

500

1007550

% REDUCTION IN COVER

25

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Fig.!. General trend lines relating annual sreamflow increase to percentage reduction in cover forconifers, deciduous hardwood/mixed hardwood and scrub. (After Bosch and Hewlett,1975).

Technical Paper No, 49 Forestry Comrrusslon of New South Wales

- 3 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality - A Review

Bosch and Hewlett (1982) were less inclined to support Hibbert's (1967) third conclusion,however, and put forward the opinion that water yield changes were, to some reasonabledegree, predictable. Fig. 1 reproduces the general trend lines relating annual streamflowincrease to percentage reduction in cover for conifers, deciduous hardwood/mixedhardwood and scrub as depicted in Fig. 1 of Bosch and Hewlett (1982). While the actualdata from the studies reviewed scatter considerably around these trend lines theconclusion is inescapable that a similar manipulation of these three vegetation types islikely to result in water yield changes that are greatest in conifers and least in scrub.

Boughton (1970) reviewed the general effects of land management on water yield andwater quality with particular emphasis on the Australian environment and on Australianresearch. From the fairly limited amount of published Australian information available atthe time this review concluded that, in relation to the effects of forests on water yields:-

"There is considerable evidence to showthat trees usemore water thangrass and that morerunoff orgroundwater recharge couldbe expected whena tree covered catchmentis convertedto a grass cover. It must be emphasised that thedifferences in wateryield willbe small andmay be insignificant in relation to the very large natural variability of theflow in manyAustralianrivers.

All studies whichhave beenmade to compare the water use ofhardwood and softwoodtreespecieshave not delineated any differences in water use otherthan could be attributed todifferences in rootdepth. It appears thatmatureforests ofdifferent species underthe sameconditionsgenerally use aboutthe same amountsofwater.

Studies ofthedifferences in interception, infiltration, and depression storage capacitiesbetween trees and grass covered areas do not show anysystematic effects on wateryieldof thesame magnitudeas that causedbydifference in rootdepth. "

Boughton (1970) also concluded that certain forestry practices could affect water qualityin streams.-

"Three majorforestry practices are significant in affecting water quality - the construction oJaccessroads, logging, and clearingfor establishment ofsoftwoodplantations. In the eastemstatesof Australia, thepractices are widely associated with problemsof raising turbidity levelsin watersupplies and to a lesser extent, promoting erosion. 11

The 1970's saw the emergence in Australia of widespread community interest in theenvironment, and forests became a natural focus for some of this interest. This periodcoincided with the introduction of more intensive forestry practices with the potential fora greater impact on water values. The recognition of this potential by forest managers,water supply authorities, researchers and interested sections of the public resulted inmuch relevant research being initiated at this time.

Research, then current, into the effects of forestry on water values was reviewed byLangford and O'Shaughnessy (1977), with particular emphasis on Australian work. Thisreview showed that by this time research was firmly established into the hydrologicprocesses responsible for measurable effects such as differing water yields. It alsohighlighted areas of possible future concern to water supplies such as extensive P. radiataplantations. It suggested that water yields from forested areas could be controlled bysuitable forest management strategies. The introduction of good management practicesthat resulted in minimal diminition of water quality was advocated.

Forestry Commission of New South Wales Technical Paper No. 49

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The Effects of Radiata Pine Plantation Establishment andManagl3ment on Water Yields and Water Quality - A Review

- 4 -

Considerable research, principally in North America, had established a firm link betweenforestry operations (including roading) and soil erosion and water quality degradation by1970. This led to the adoption of forestry practices designed to minimise these problems(e.g., Rothwell, 1971). Guidelines more in keeping with local Australian forestry practicesfollowed later (Cameron and Henderson, 1979; Cornish, 1983a), and many of theseguidelines form the basis of conditions of logging now insisted upon by most forestservices in Australia.

Exotic pine plantations underpin the supply of sawn timber in Australia in the short tomedium term (Anon., 1989a), and also provide much wood pulp from thinnings. Theseplantations tend to be concentrated in areas where climate and soils are suitable, land isavailable and processing industries are established. Management efficiencies and fireprotection dictate that they will become large homogenous forest areas differinginternally only in the relative ages and thinning histories of the componentcompartments. This expansion of plantations in areas of relatively high rainfall leads toinevitable questions concerning their impact on water values.

The purpose of this paper is to update and discuss the known and likely effects of exoticpine establishment and management on water values in Australia by reference toinformation generally published in the last 25 'years and.readily accessible. The hydrologicprocesses that contribute to observable hydrologic differences between vegetation typeswill be examined and processes important in pine plantations will be related tomanagement practices. Particular emphasis will be given to P. radiata in the N.S.W.environment.

Although no major review of the effects of forest management on water values inAustralia has been published since that of Langford and O'Shaughnessy (1977), thisauthor is aware of a cop-current review of the effects of Australian land use generally onthe water resource . This review, initiated by the Department of Water Resources,Victoria, is being conducted by the CSIRO Division of Water Resources as AWRCPartnership Project 86/12.

EFFECTSON WATER YIELD

1. Important Hydrologic Processes in Relation to Vegetation Types

It is obvious that all vegetation requires water for growth and survival. Vegetationtherefore' 'uses' Water and it is not surprising that different types of vegetation 'use'differing quantities of water when the vast range of morphological and growth .characteristics of these different types is considered. Much research has been carried outto determine water 'use' by vegetation.

To examine what is meant by water 'use' it is meaningful to consider the variouscomponents of the water balance of a catchment, these components being linked by ageneral equation: .

P = RO + U ± AS + ET Equation (I)whereP gross precipitation,RO runoff, or more precisely stream discharge,U deep seepage to groundwater,AS additions to or subtractions from the soil moisture store, andET evapotranspiration; all components being determined over the same time interval.



The evapotranspiration term, which includes contributions from transpiration byvegetation, interception and evaporation of precipitation on plant surfaces andevaporation from the soil surface, can be equated with total water 'use'.

In order to quantify hydrologic processes that differ between vegetation types it isnecessary to identify which of the terms in equation (1) are independent of vegetationtype, which are directly dependent on vegetation type and which are indirectly dependent

,~ on vegetation type. These important processes are discussed below in relation to thefundamental components of the water balance equation and the supply of energy.

(a) Precipitation. Precipitation (P) is the driving term in the hydrological cycle.Much scientific debate took place in the first half of this century concerningthe influence of vegetation on precipitation. Were the forests there because ofthe rain or vice versa? Penman (1963) reviewed the scientific evidence andconcluded:-

"Theimplication to be carried forward is that though vegetation may affect thedisposalofprecipitation, it cannotaffect theamount'ofprecipitation to bedisposed. "

Pereira (1973) in a later review agrees with two exceptions. Fog-drip has beenfound to be an important contributor of total precipitation in some coastalforested areas, and the presence or absence of forests appears to influencesnowfall depths, particularly in the U.S.S.R. Precipitation in Australia fallsmainly as rain over most areas and can be considered essentially independentof vegetation type.

(b) Evapotranspiration. Evapotranspiration (ET) is indisputably linked withvegetation type as its two principal components, transpiration (T) andinterception (1), have been shown to depend on vegetation type in numerousstudies. These two component terms will be discussed in detail in thefollowing sections. The other component, soil evaporation (ES), is generallyless important but can constitute up to 40% of daily ET in forests after rain(Denmead, 1982). O'Shaughnessy and Moran (1983) contend that when thereis a complete vegetative cover direct evaporative losses from the soil surfaceare small and that evapotranspiration consists principally of transpiration andinterception.

Two important processes that effect evapotranspiration in different types ofvegetation were recognised by Holmes and Wronski (1982). They concludedthan-

"Twohydrologicalprocesses cancontribute to the larger consumption of rain byforests.Theseare a larger soil water deficit created during dry weather by thedeeper and moreactive root- zone of the trees, which requires greater replenishment before nmoffcanoccurand afaster rate ofevaporation ofwaterfrom thetree canopy when it is wetthanfrom the wetted foliage ofthepasture species. 11


The Effects of Radiata Pine Plantation Establishment and - 6 -Management on Water Yields and Water Quality - A Review

(c) Transpiration. Rooting depth is an important factor in relation to differencesin transpiration that exist between forests and shallow-rooted vegetation suchas pasture. Penman (1963) reported numerous studies that demonstrated thisfact. In areas of winter rainfall dominance pastures become dormant duringthe dry summer and, not withstanding the high evaporative demand,transpiration may cease. This contrasts with forests which continue totranspire during summer.

Forest transpiration, however, declines with declining soil water supply. Dunin andAston (1984) showed a threshold level of soil moisture below whichtranspiration of E. maculata at Batemans Bay, N.S.W., declined at a linear ratewith available soil moisture. Jackson et al. (1972) reported a somewhat similarrelationship between transpiration and soil moisture for P. radiata in NewZealand. They also reported some differences in transpiration rates betweendifferent P. radiata clones. Forests with major differences in rooting depth arelikely to experience different transpiration changes during periods of decliningsoil moisture.

Dunin and Mackay (1982) compared daily transpiration rates for E. maculata atBateman's Bay, N.S.W. with rates for P. radiata plantations at Canberra(Denmead, 1969) and Mt. Gambier (Moore, 1976) under conditions ofunrestricted soil moisture and comparable solar radiation inputs (October orMarch) and similar leaf area indices. Rates were shown to be similar and

'varied from 3.2 mm dai l (P. radiata, Mt. Gambier), 3.8 - 7:5 mm dai l (P.radiata, Canberra) and 3.5 - 7.1 mm dai l (E. maculata, Bateman's Bay)although net radiation was greater in the pines because of lower albedo (seelater Section on energy supply).

Dunin and Mackay (1982) concluded their comparison of transpiration differencesbetween coniferous and eucalypt forests as follows:-

"Similarity in transpiration between communities (ofpine and eucalypt) wasobservedbothon an hourly and annualbasis. However, on the annualscale, differences canexist in thepatterns oftranspiration even though annualtotals were comparable. "

~t appears that, under the same site conditions, pine and eucalypt forests inAustralia are likely to transpire at similar yearly rates, but seasonal rates maydiffer due to differences in energy input and different transpiration limitations4IJring periods of water stress. Pine is likely to transpire at a greater rate thaneucalypt in the cooler months, all other factors being equal.

(d) Interception. The interception of rainfall by vegetation has been studied bymany workers and reviewed by Kittredge (1948), Penman (1963) and Blake(1975). Kittredge (1948), in reviewing the topic at that time, considered totalinterception during 'a storm to co;nsist of two parts as proposed by Horton(1919): '.


• 7 7 The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouality- A Review

I S + atwhereIS

at

.................................................................. Equation (2)

total interception,maximum amount of water retained on vegetative surfaces,or interception capacity,rate of evaporative loss from wetted surfaces, andduration of storm.

stemflow, precipitation channelled to the forest floor down tree stemsthroughfall, precipitation reaching the forest floor after falling through theforest canopy.

Despite many more recent interception studies equation (2) can still be reliablyused to describe the process, although a may not be constant (Gash, 1979).O'Shaughnessy and Moran (1983) concluded that interception capacity waslarger in a forest canopy than for pasture or other short crops. Dunin andMackay (1982), in an analysis of interception differences between P. radiataand eucalypt at Lidsdale, N.S.W., showed that the interception capacity of thepines was three to four times that of the eucalypts. The rate of evaporation ofintercepted water is considerably greater from forest canopies than frompasture species, and water evaporates from a forest canopy during, as well asafter, a storm. First proposed by Rutter (1967), numerous subsequent studieshave verified that water evaporates from a wet forest canopy at two to three

. times the potential transpiration rate based upon net radiation input (Stewartand Thorn, 1973; Moore, 1976;Stewart, 1977). These studies suggest that (Y islikely to be similar for different forest types, but Dunin and Mackay (1982)reported a values 30% greater in a P. radiata forest than in a nearby eucalyptforest, and suggested the difference there could be due to a rougher pinecanopy which favoured turbulent exchange of water vapour.

Studies indirectly determining interception in forests from field measurementshave been conducted by many workers. These often report interception as a(generally linear) function of storm rainfall and determine interception: grossprecipitation ratios for longer, often annual, periods. These studies utilize thefact that interception (I) is the unknown in the following equation thatdescribes the partitioning of gross precipitation (P) on its way to the forestfloor:

P = I + Sf + Ts Equation (3)whereSfTr

Numerous studies provide comparisons of interception between different foresttypes that have been determined in this manner from measurements of P, Srand Tr. Table 1 presents the results of some of these studies from Australiaand New Zealand where interception byP. radiata is compared with that byindigenous forests. Stemflow is a minor term in these studies, generallyaccounting for less than 5% of gross precipitation. Crockford and Richardson(1987), however, reported that stemflow constituted 11% of gross precipitationduring a three year stemflow study in an unthinned 16-19year old P. radiatastand near Canberra. Stemflow was strongly influenced by gross storm rainfall,only averaging about 5% of gross precipitation for storms less than 10 mm intotal.


~--~----------

The Effects of Radiata Pine Plantation Establishment and • 8 •Management on Water Yields and Water Quality. A Review

Table 1. Interception (I) as an overall percentage of gross precipitation (P) fordifferent forest types and ages as reported by various workers.

SPECIES liP (%) REFERENCE

E. regnans (mature)(Vic) 23.2 Langford and O'Shaughnessy(1978)

E. regnans (30 yrs)(Vic) 18.7 "

P. radiata (17 yrs)(Vic) 21.4 "

E. rossii - E. maculosa - 10.6 Smith (1974)

E. dives (mature)(NSW)

P. radiata (33 yrs)(NSW) 18.7 "

E. regnans (38 yrs)(Vic) 18.5 Feller (1981)

E. obliqua (38 yrs)(Vic) 15.0 "

P. ~adiata (38 yrs)(Vic) 25.5 "

Neopanaxarboreum scrub (NZ) 27.0 Wells and Blake (1972)

P. radiata (NZ) 33.0 "

Stemflow per tree increased following thinning from 1700 stems ha-1 to 680 stemsha-1 as more precipitation was intercepted by exposed trunks, particularlyduring rain events in which the rain fell with a high angle of incidence.

There is a tendency for interception by P. radiata to exceed that of native forests,and Dunin and Mackay (1982) considered the differences between pine andeucalypt interception to be about 10% of annual rainfall. As Blake (1975)points out, however, interception varies with storm rainfall and rainfallintensity during the storm. Annual values of interception expressed as apercentage of gross precipitation are likely to vary from year to year dependingon the disposition and nature of storms during the year. Interception by shortcrops and pasture can exceed 40% of gross precipitation during the growingseason (Penman, 1963), but the annual percentage is frequently much less thanthis.

Turner and Lambert (1987) reviewed numerous interception studies in P. radiataand concluded that interception was a function of gross precipitation and basalarea of the stand, these two factors accounting for 67% of the variation ininterception. They estimated that thinning a P. radiata stand in Belanglo S.P.,N.S.W. at age 17 would reduce interception by 121 mm yr-1 in an average yearwhen gross precipitation was 880 mm.

(e) Evapotranspiration differences between vegetation types. The principaldifferences in evapotranspiration (ET) between plant communities withsimilar leaf areas can be accounted for by differences in transpiration (T) and

Technical Paper No, 49 Forestry Commission of New South Wales

• 9 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouallty - A Review

interception (1). These differences between pastures, eucalypts and pine aresummarized in Table 2. A combination of contributions from both (1) and (T)suggests that the ranking for (ET) should be pine > eucalypt > > pasture.

Table 2. Ranking of the processes transpiration (T) and interception (I) forpasture, eucalypt forest and pine forest.

PROCESS

T

I

RANKING

Eucalypt = Pine > Pasture

Pine > Eucalypt > > Pasture

FACTORS PRIMARILYRESPONSIBLE FORDIFFERENCES BE1WEENVEGETATION TYPES

Rooting depth

a, Rate of evaporationfrom wet surfaces

So far it has been shown that (P) is independent of, and (ET) is directly dependenton, vegetation type. The other components of the water balance (Equation 1),which include runoff (RO), deep seepage (U) and soil moisture changes (AS),are therefore indirectly dependent on vegetation type. Over a lengthy period(AS) is generally small relative to the other terms. Water yield comprises thesum of (RO) and (U).

(f) Energy supply. One further factor needs consideration when comparingevapotranspiration rates from different vegetation types. All vegetationrequires a supply of energy for the evaporative process. This is provided bysolar radiation and a great deal of research has taken place into partitioningthis radiation to account for evapotranspiration from different vegetationtypes. These energy considerations are well reviewed by Stewart (1984). Theleaf area available to intercept solar radiation is an important factor in foresttranspiration (Dunin and Mackay, 1982).

Two physical characteristics account for most of the energy differences betweentall vegetation (forests) and short vegetation (crops and pastures). The albedo,or surface reflectance, offorests (conifers 0.10) (Denmead, 1969), (eucalypts0.14-0.20) (Wu, 1976) is less than that of short crops, (pasture 0.20-0.30) (Lee,1980) resulting in more energy being available for the evaporation of water. Inaddition rough vegetation such as forests has a different temperature profile tothat of shorter and aerodynamically smoother crops. The combined effect onenergy input is summarized by Stewart (1984):

"The emission of bothshort andlong-wave radiation will be less during the dayfromforests thanfrom agricultural crops andso theradiation inputto aforest isgreater thanfor smoother surfaces",

It should be noted, however, that the albedo of forests is likely to be altered bylogging. McCaughey (1978) reported increases in albedo following theclearfalling of a 60 year old balsam fir stand in Canada that resulted in a 10%reduction in nett radiation. .



Surface roughness and the height of the forest canopy result in greater airturbulence (lower canopy aerodynamic resistance) over forests than over shortsmooth vegetation. This greater turbulence provides access to a continualsupply of relatively dry air which helps to provide the energy to drive theprocess. The advection of energy, which is relatively common in Australia,further assists this process. Therefore forests may intercept more radiantenergy at times due to lower albedo, but they are also more efficient atharvesting thermal energy from the surrounding air. On the other handpastures and short crops, with a smooth surface and hence higher aerodynamicresistance, are less able to take advantage of this energy resource because ofthe lower leaf to air gradient in and immediately above the canopy. Fromenergy considerations alone forests could therefore be expected to have higherevapotranspiration rates than short crops and pastures.

2. Differences in Water Yields Between Conifers, Eucalypts and Pasture

As a consequence of a great deal of research which has been reported since 1970 allthree of the conclusions of Boughton (1970) concerning water yield from differentvegetation types (quoted earlier) need amendment today. This section will discuss thoserecent water yield studies that have more relevance to Australia. It will also considerother associated hydrologic differences between vegetation types, and amend theBoughton (1970) conclusions.

(a) Water yield differences between eucalypt and pasture. The difference betweenthe evapotranspiration of forested and pastured catchments in Victoria isdramatically depicted in Fig. 2, which is based on Fig. 3 of Holmes and Sinclair(1986). These workers determined mean annual evapotranspiration fromprecipitation and discharge data over a common ten year period for a range ofcatchments with differing precipitation and vegetation. The two envelopecurvesin Fig. 2 depict the mean annual evapotranspiration of fully afforestedcatchments (upper curve) and completely pastured catchments (lower curve)for a large range of precipitation values. Evapotranspiration differencesbetween the two vegetation types (indicated by the vertical line labelled 'A' inFig. 2) ranfed from 265 mm yr-1 when mean precipitation was 2,400 mm yr-1 to40 mm yr- for a mean precipitation of 600 mm yr-1. It was argued that theseevapotranspiration differences were responsible for water yield differences ofthe same magnitude. The upper envelope' curve in Fig. 2 has been derivedfrom data from eucalypt catchments. It is interesting to note that theseauthors also include evapotranspiration data from P. radiata plantations inSouth Australia which were determined independently. These data lie abovethe envelope curve. Although the authors admit that greater error probablyexists at lower precipitation values, Fig. 2 suggests that these P. radiataplantations have higher evapotranspiration values (and lower water yieldvalues) than eucalypt forests experiencing the same rainfall and situated at asimilar latitude.

Ruprecht and Schofield (1989) have recently reported the results of a study inWestern Australia in which a 94 ha catchment was cleared of native forest, anopen stand of mixed eucalypts, and converted to pasture. An increase in wateryield of 96 mm was recorded in the year following clearing, a value consistent

Technical Paper No, 49 Forestry Commission of New South Wales


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600

25001000 1500 2000

ANNUAL PRECIPITATION (mm)

400+ .... ... ...,r-__.-.,500

Fig. 2. Envelope curves depicting mean evapotranspiration for fully afforested catchmentsand completly pastured catchments in Victoria (after Holmes and Sinclair, 1986).Evapotranspiration differences between pasture and eucalypt for particular valuesof annual precipitation are given by the vertical difference 'A' . Evapotranspiration of P. radiata plantations in South Australia is indicated by the closed circles.

with the change in interception. This increase in yield, which occurred in ayear in which precipitation totalled 877 mm, was also consistent with theVictorian data of Holmes and Sinclair (1986).

Ruprecht and Schofield (1989) further showed, however, that annual water yields(as a percentage of precipitation) increased linearly for the next six yearsbefore stabilizing at 30%-32% of precipitation. This increase was attributed toa corresponding linear rate of expansion of the groundwater discharge areaand continued until groundwater recharge attained a new equilibrium. Thischange in the groundwater system has resulted in an ultimate water yieldincrease in this catchment of 340-350 mm for an annual rainfall of about 1,100mm, which is considerably greater than the water yield difference betweeneucalypt and pasture in Victoria for an equivalent annual rainfall ('A' inFig. 2).

This study demonstrates that, in circumstances where shallow groundwater systemsare significantly affected by vegetation reductions, water yield changes can bemuch greater than may have been expected from interception and root depth


The Effects of Radiata Pine Plantation Establishment and - 12-Management on Water Yields and Water Quality· A Review

changes, and that they may take some years to establish a new equilibrium. Italso appears feasible that a pasture catchment, containing such a shallowgroundwater system, may experience larger than expected water yieldreductions if afforestation were to reduce the area of groundwater discharge inthe catchment by a substantial amount.

(b) Water yield and P. radiata in Australia. Water yield differences betweencatchments of P. radiata and catchments of eucalypt or pasture have beenobserved by numerous workers (e.g., Smith et al., 1974; Feller, 1981; Pilgrim etal., 1982), and generally equated with interception differences.

In studies at Lidsdale, N.S.W. (Smith et al., 1974; Pilgrim et al., 1982) water yieldswere in the order cleared catchments> eucalypt catchments> P. radiatacatchments, but actual differences were strongly influenced by the nature andquantity of rainfall.

Interception differences between vegetation types have been used in simulationstudies that have resulted in computations of water yield changes consistentwith observation for the conversion of eucalypt forest to pasture or to P.radiata plantation (Aston and Dunin, 1980; Dunin and Mackay, 1982). Thesetechniques have also been used in broad scale predictions of water yieldchanges that might result from possible land use changes in a large catchment(Costin et al., 1984).

Holmes and Colville (1970a; b) and Colville and Holmes (1972) studied thehydrology of pasture converted to P. radiata on deep sandy soils at Mt.Gambier, S.A. The studies showed that recharge to ground water was verymuch reduced under the pine plantation. In a somewhat similar study inWestern Australia, again on deep sandy soils, Carbon et al. (1982) reportedthat, on an annual basis, deep drainage under pastures was about four timesthat under a pine stand and about 1.5 times that under a native forest.

(c) Water yield and P. radiata in New Zealand. A number of small catchmentstudies reported by Herald (1979), Pearce and Rowe (1979), Duncan (1980)and Dons (1981) have shown that afforestation of areas of pasture, gorse andregenerating indigenous scrub by P. radiata has generally reduced streamflow.

Dons (1987) compared the hydrology of three small catchments at Purukohukohuvegetated by native forest, P. radiata and pasture. Evaporative losses from thenative and pine forest catchments were similar and 33% greater than from thepasture catchment. Peakflows from the catchments were ranked pasture >pine> native, with low flows ranked native> pasture> pine. During theperiod 1981-1984 the pasture catchment yielded 289 mm yr-1 and 204 mm yr-1

more than the pine and native forest catchments, respectively. During thisperiod the pines were 8- to Ll-years-old. However, 87% of the pine catchmenthad been thinned from c 1,900trees ha-1 to c 550 trees ha-1 in the period1979-1981 (Beets and Brownlie, 1987) resulting in a non-closed canopy overmost of the catchment during the study period.


·13· The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouality - A Review

Interception (and evapotranspiration) in these pines was therefore unlikely to havebeen a maximum during the study, and yield reductions greater than 289 mmyr-1 may be expected under full canopy closure conditions.

Dons (1986) reported the effect on river flows of large-scale afforestation of 28%of the 906 km2 catchment of the Tarawera River byP. radiata. The originalvegetation had been a mixture of sparsely scattered light scrub (60%) and lownative bush (40%). Pine was planted between 1964 and 1981 and mean annual,summer and winter flows for this period were all significantly reduced incomparison with flows pre-afforestation, Mean annual river flows in the period1964-1981 were reduced by 83 mm because of afforestation with pine. Thistranslates into an average reduction of about 300 mm yr-1 in the afforestedareas. By 1981 the annual decrease in flow was estimated to be 13% ofpre-afforestation mean streamflow, or 157 mm. This is equivalent to 550 mmyr-1 in the area afforested.

(d) Water yield and pine afforestation in South Africa. In South Africa there hasbeen a recognition for many years that land use changes involving forests havethe potential to alter water yields and research into the effects of afforestationof grassland and native scrub on water yields has been carried out in bothsummer and winter rainfall areas.

Bosch (1979) reports the results of afforestation of a native Themeda spp. grasslandwith Pinuspatula in a predominantly summer rainfall area of Natal.Afforestation of 74% of a catchment with P. patula reduced streamflow by amaximum 440 mm in the twenty second year after planting. The averagereduction over a period of 27 years amounted to 257 mm per year. If theunplanted sections of the catchment are considered to have unchangedevapotranspiration then the streamflow decreases from the actual area plantedconvert to a 600 mm maximum value and a 350 mm average over 27 years.

Bosch (1979) also reported that dry period flows declined sharply eight years afterplanting, becoming a minimum about 19years after afforestation. Dry periodflows then increased. Fig. 3 depicts trends for total and dry period streamflowsas presented in Fig. 4 and Fig. 6 of Bosch (1979), and includes the periods overwhich afforestation, and a first thinning which removed 50% of the trees from86% of the planted area (between ages 15 and 20 years), took place. Anincrease in water yields, particularly in dry periods, is evident after 1971 (age21). Bosch does not attribute this increase to the thinning, but rather arguesthat it may have been caused by the defoliation of 25% of the remaining treesby Euproctis terminalis in 1972. It might be expected, however, that the drasticcanopy interception and transpiration changes brought about by this thinningwould result in decreased evapotranspiration and increased streamflow whichwould be relatively greater in periods of lower rainfall. It seems reasonablethen that some of this demonstrated increase should be attributed to theeffects of the thinning operation. It should be pointed out, however, that inanother afforested catchment (also affected by E. terminalis) dry periodstreamflows did not rise after thinning. This thinning, however, only removed


The Effects of Radiata Pine Plantation Establishment and • 14 •Manl;l9E!menton Water Yields and Water Quality. A Review

500 30

..........,..., ....~.,

~..~t:a ~.,~., ~.. 25

400 q;9 ~., ~..~46 ~..

~ ~46E <:> ~46 ~

'Etlt46 ~g ~,~ .5,

20W , ~en , ~ wU)3oo I \,

~CC I ,Q : CC..

faw : 15Cl

I Cl;:~9 200 •••u. •• 10 u.-' •

~• fi:••

~ Cl

1005

o 0

o 5 10 15 20 25 30

YEARS SINCE PLANTING

Fig. 3. Trends in total and dry periods streamflows following afforestation of native grasslandwithP. patula in South Africa (after Bosch, 1979).

40% of the trees from about 60% of this catchment, and the average age of thetrees was 2-3 years younger when thinned. These factors may have beenresponsible for the differences observed between catchments.

Van Lill et al. (1979) showed that afforestation of a Themeda spp. grassland byE. grandis at Mokobulaan in South Africa resulted in a streamflow reduction of340 mm per year after six years and that strearnflow changed from perennial toephemeral. Another catchment planted to P. patula showed similar water yieldtrends, but the changes were slower-developing. The authors concluded that itwas too early to assess the ultimate effect of planting with P. patula.

Van Wyk (1987) reported the effects on water yield of afforestation of catchmentsof native shrubs with P. radiata at Jonkershoek in the winter rainfall area ofSouth Africa. Water yield was monitored over a full 40-year rotation in thecase of one catchment 57% planted with P. radiata, and for shorter times inothers that were afforested to greater or lesser degrees. Pines were plantedwith a stocking of 1,330 sterns ha-1 and thinned three to four times over 30years to a final density of 150 to 175 stems ha-I. The mean decreases instreamflow for the three catchments with the longest record are presented inTable 3. The author considered that the magnitudes of these streamflowreductions were related to percentage of area afforested, total biomass andrainfall. Table 3 also presents mean annual decreases in streamflow for thesecatchments calculated for the actual area planted, therefore representing fullafforestation.

Technical Paper No. 49 Forestry Oomrnlssron of New South Wales

- 15 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouality- A Review

Table 3. Summary of streamflow changes in three of the Jonkershoekcatchments, South Africa (after Van Wyk 1987)

CATCH· LENGTH AFFOR· MEAN MEAN MEAN ANNUALMENT OF ESTED ANNUAL ANNUAL DECREASE IN

RECORD RAIN· DECREASE IN STREAMFLOWFALL STREAMFLOW FOR FULL

(YRS) (%) (MM) (MM) AFFORESTATION(MM)

Bosboukloof 40 57 1296 197 346

Biesievlei 32 98 1427 313 319

Tierkloof 24 36 1810 171 475

Note that the decrease calculated for Tierkloof catchment is considerably greaterthan for the other two, but this catchment has significantly higher rainfall andmay possibly have been subjected to fewer thinnings.

(e) Water yield changes and annual precipitation. The relationship between wateryield decrease and mean annual precipitation for some southern hemispherestudies is examined in Fig. 4. Mean water yield differences, converted for fullafforestation, between Pinus spp. and pasture or native shrubs, from data ofHolmes and Colville (1970a; b) Bosch (1979) and Van Wyk (1987), are plottedversus mean annual rainfall.

WATER YIELD DECREASE vs ANNUAL PRECIPITATION

E.sw 400

~c:o~3oo

::Iw>200c:

~~

100

C\NG EUCp..\.'(?i •••••••••?\NE f\E?\.PI •••••••••••

......................................

8

....,....

0'1------...----.....-----...---_...o 500 1000 1500

ANNUAL PRECIPITATION (mm)

2000

Fig. 4. Suggested relationships between water yield decrease and mean annual precipitationfor the complete afforestation of eucalypt and pasture by pine. Evapotranspira-tion differences between pasture and eucalypt for particular values of annualprecipitation are given by the vertical difference 'B'.


The Effects of Radiata Pine Plantation Establishment and • 16 -Management on Water Yields and Water Quality - A Review

Data from Dons (1986) are not included as mean annual rainfall during theafforestation period is not quoted in this study. A straight line of best fit hasbeen drawn through these points. This relationship is entirely consistent withFig. 4 of Bosch and Hewlett (1982) which relates water yield increasesfollowing complete clearfalling of conifers (from numerous studies) to meanannual precipitation. Data in Fig. 4 span a considerable precipitation range, arange that would encompass allP. radiata areas in N.S.W. Also included is astraight (1:10) line representing the difference between pine and eucalyptinterception as a function of rainfall as proposed by Dunin and Mackay (1982).As interception is the only component of ET that differs markedly betweenpine and eucalypt, it is suggested that interception differences approximatewater yield differences between these two species.

If this proposition is accepted, the 1:10 line in Fig. 4 becomes the relationshipbetween differences in pine and eucalypt water yields and annual precipitation,and the vertical difference between the two curves (labelled 'B' in Fig. 4)becomes the water yield difference (for that annual precipitation) betweencompletely afforested eucalypt catchments and completely pasturedcatchments. Holmes and Sinclair (1986) provide data from Victoria thatenable such water yield differences to be estimated for a large range of meanannual precipitation values and these differences are labelled 'A' in Fig. 2.These two independently derived estimates of water yield differences betweeneucalypt and pasture are compared in Table 4. There is close agreementbetween the two sets of values over the complete range of annual precipitationvalues presented here. This suggests that the suppositions inherent in derivingthe 'B' values in Fig. 4 are not wildly in error, and that the two relationshipspresented in this diagram are reasonable representations of real effects.

(f) Water yield changes and plantation age. Knights (1983) examined annual runofffrom aP. radiata catchment at Lidsdale, N.S.W. between ages 3 and 13years inrelation to growth rates and pine morphology. These pines, planted at a 2.5 mx 2.5 m spacing, were found to have fully closed canopies and root systems thateffectively occupied the soil column by age 11 years. Maximum growth ratesreportedly occurred between ages 6.5- and 8.5-years-old. Using a regressionmodel relating monthly runoff from the pine catchment to monthly runoff in anearby eucalypt catchment and to measured differences in monthly rainfall, itwas concluded that annual pine runoff between ages 3 and 6 years was 111%above that expected using the period after canopy closure (ages 11-13 years) asa reference. Between the ages of 7 and 10 years, (the period of maximumgrowth) annual runoff was 44% below that expected. While these resultssuggest a relative increase in water yield in P. radiata after canopy closure,annual rainfalls varied markedly during the study period. The average annualrainfall in the reference period (c 1,050 mm) was much greater than in thedrought period preceding it (ages 7-10 years) where it was 800 mm. Soilmoisture levels, and consequent runoff/rainfall relationships, may have beendifferent in this dry period in comparison with those generally occurring in thereference period.


Table 4.

- 17 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality - A Review

Water yield differences between eucalypt and pasture for a range ofmean annual precipitation values. 'A' values derived from Holmesand Sinclair (1986), 'B' values from Figure 4.

MEAN ANNUAL WATER YIELD DIFFERENCE BETWEENPRECIPITATION EUCALYPT AND PASTURE (MM)(MM)

'A' 'B'(From Figure 2) (From Figure 4)

600 40 50

800 90 90

1300 215 190

1500 240 225

1800 250 285

This casts some doubt on the validity of the model used and the conclusions drawnregarding the period of maximum water use in these pines. The age ofmaximum pine growth reported in this study (6.5 - 8.5 years) is lower than thatgenerally observed in N.S.W. Home et al. (1986) reported maximum growthrates between ages 12 and 15 years for P. radiata grown in Green Hills S.F.,Batlow, while Carter (1981) showed that growth rates increase up to at leastage 14 years in a number of N.S.W. plantations. If, as Knights (1983) implies,water use is related to stand growth rates, then the age of maximum water usein P. radiata in N.S.W. generally may be considerably later than her studyconcludes.

Van Wyk (1987) presented South African data on water yield decreases followingP. radiata afforestation averaged over consecutive four year periods. Thesedata, converted for full afforestation, are presented in Fig. 5.

Data for three catchments planted at 8 year intervals are plotted versus plantationage. There is considerable scatter in these data (the Tierkloof catchment,which receives more rainfall, generally has greater decreases) but a curve ofbest fit to all points has been fitted by eye. This curve suggests a maximumannual decrease in water yield for full afforestation of around 400 mm between16 and 27 years of age. The annual decrease is reduced to about 300 mm justprior to clear falling at age 40. Also presented in Fig. 5 is the trend curve,converted for full afforestation, from Bosch (1979) which relates water yielddecreases to stand age following afforestation with P. patula. Maximumdecreases in yield are greater here but appear to occur at about the same age,that is around age 20. This apparently greater decrease in maximum yield for P.patula may be due to site, seasonal or species differences, but is more likely torelate to differences in evapotranspiration between the initial vegetation types.

Forestry Commission of New South Wales Technical Paper No, 49


WATER YIELD DECREASE vs PLANTATION AGE

403530

oP.RADIATA - S.AFRICA•

25

•

20

P.PATULA - S.AFRICA

15

PLANTATION AGE (yrs)

105

•

I#

I~41;

~.~•••••••

Water yield decrease versus plantation age for areas fully afforested with P. radiata(after Van Wyck, 1987) and P.patula (after Bosch, 1979): squares - P. patula;closed circles - Biesievlei catchment; open circles - Bosboukloof catchment;dotted circles - Tierkloof catchment; all P. radiata..

E..§..5oowen

~400oWCl

:3 300W>=a:~2oo

~100

0

0

Fig,S

P.patula was planted on Themeda spp. grassland while P. radiata was plantedon land previously vegetated by various native shrubs and these may have had ahigher initial evapotranspiration rate than grassland. Therefore completeafforestation of pasture by P. radiata in an area receiving 1,300-1,400 mm rainannually in Australia may result in maximum annual reductions in water yieldin excess of 400 mm. The maximum reduction may occur around age 20 butthis could depend on the thinning schedule and be related to the age ofmaximum growth rate. These propositions are supported by the New Zealandstudy of Dons (1986), where maximum water yield reductions of 550 mm wereobserved in an afforested area which included P. radiata from 1- to 17years-old.

The actual decrease in catchment water yield resulting from afforestation ofgrassland by P. radiata during a specific period is therefore likely to depend ona combination of the following factors:-

1. Percentage of catchment afforested.2. Pine growth rates.3. Particular age classes present and the relative proportion of each.4. Thinning schedule.5. Precipitation received during the period.

This last factor is very important in Australia where year-to-year climatic variationis frequently large. While the actual magnitude of water yield decreasesfollowing afforestation will be greater in wetter years, yield decreases are likely

Technical'Papen'Jo:-4~ . Forestry Commission of New South Wales


to have more local impact during drought years when stream baseflows may begreatly reduced and streams that were formally permanent may becomeephemeral. Average values of precipitation and water yield therefore havemuch less applicability in the Australian environment, and observed changes inannual water yield, while influenced by the factors above, will vary markedly inresponse to the actual precipitation received.

(g) Other hydrologic consequences of afforestation. Water yield is not the onlyaspect of stream hydrology that is affected by changes in catchment vegetation.Peak discharges during storms are influenced by the type and amount ofcatchment cover. In a comparison between a mature P. radiata and a eucalyptcatchment at Lidsdale, N.S.W., Tjenada (1982) showed that unit hydrographpeaks and storm loss rates were lower in the pine. As loss rates were stronglyrelated to antecedent soil moisture conditions, the study suggested that soilmoisture levels were, on the whole, lower in the pine catchment.

Numerous studies have quantified the differences between forests and grassland inrespect of peak flows (e.g., Burch et al., 1987) and the topic has beensatisfactorily discussed by Holmes and Wronski (1982). The storm hydrograph,or discharge - time relationship, for a grassed catchment would begin risingearlier in the storm and have a steeper rising limb than for an equivalentafforested catchment. Peak flows for the grassland catchment would begreater and the recession would persist longer. Fig. 6, after Holmes andWronski (1982), depicts a hypothetical storm hydrograph for grassed andafforested catchments. Total catchment yield would be greater from thegrassed catchment and the dry-weather flow of ephemeral streams in grasslandwould be more prolonged.

Afforestation of pastured land byP. radiata would, in general, result in some, or all,of the following effects»

1. Reduced peak streamflows during storms2. Slower rising streams during storms3. Lower water yields4. Generally lower dry-weather flows5. Shorter periods of dry-weather flow in ephemeral streams with the possibility

that some perennial streams may become ephemeral.

Stormflow differences between pasture and forest will be most apparent under dryantecedent catchment conditions, disappearing when soil moisture levelspre-storm are universally high.

(h) Transpiration efficiency of P. radiata and eucalypts and its relationship toproductivity. Dunin and Mackay (1982) compared the net primary productionrates of eucalypt and pine stands and suggested that t{'Pical annual productionvalues may be 15 t ha-1 yr-1 for eucalypts and 25 t ha- ya-1 for pine. They basedthese figures on the following studies:-


The Effects of Radiata Pine Plantation Establishment and • 20 •Management on Water Yields and Water Ouallty - A Review

STORM

wCJCl:«Io(/)(5

TIME

Fig. 6 Hypothetical storm hydrographs for grassed and afforested catchments (after Holmesand Wronski, 1982).

Attiwell (1979) - netrrimary production of 10.8 t ha-1 y(l for E. obliqua atage 51 and 11.9 t ha- yr-1 for E. obliqua at age 44.

Rogers and Westman (1981) - net primary production of 10.5 t ha-1 yr-l fora mature woodland dominated byE. signata..

Forrest and Ovington (1970) - net prima~production of 23.4 t ha-) yr-1 forP. radiata at Tumut at age 9 and 19.8 t ha- yr-1 for P. radiata at age 12. Notethat these figures do not include litterfall, which may have contributed anadditional 1.5 - 3.0 t ha-1 yr-1.

Madgwick et al. (1977) - net primary production of 23.7 t ha-1 yr- l for P. radiataat age 8 in New Zealand.

As can be seen, these comparisons cited by Dunin and Mackay make no allowancefor the different ages of the stands. The pine stands were young and theeucalypt stands much older. In fact, net primary production rates for differentspecies vary with site conditions and age as shown in Fig. 7 which is adaptedfrom Turner (1986). The E.grandis plantation has been fertilised in addition tohaving a higher natural site quality than the E. obliqua forest and,



consequently, has higher net primary production rates. Peak production ratesare reached at an earlier age in the plantation but both chronosequencesdemonstrate reduced production rates at later ages.

A lengthy comparable chronosequence that has been derived on the one site is notavailable for P. radiata. However, net primary production rates for P. radiata atvarious ages from a number of separate studies are plotted versus abovegroundbiomass in Fig. 7. While most of these values lie between the two eucalyptcurves, there is an obvious influence of site quality on the magnitude of the netprimary production rates. Rates reported by Beets and Pollock (1987) (whichdo not include annuallitterfall) are from the Puruki Catchment in NewZealand. They are at the upper limit of values reported worldwide for any treespecies and reflect an environment without nutrient or water limitations wherepines grow vigorously all year.

Few studies have compared the net primary production rates of pines and eucalyptsunder identical site conditions. Frederick et al. (1985) report dry mattercontent and litterfall values for stands of eight-year-old E. regnans and P.radiata grown on similar adjacent sites in New Zealand. Up to this age total drymatter production (including litterfall) was around 148 t ha-1 for the pine

BIOMASS vs NET PRIMARY PRODUCTION RATE500

30

C?400..c:~

en~:::2: 300oiDoz::::>0200er:ill~m« 100

o 5

15

100

10 15 20 25 30 35 40

NET PRIMARY PRODUCTION RATE (tlhalvr)

45

120

50

Fig. 7 Relationship between biomass and net primary productivity for E. obliqua (circles) andE. grandis (triangles) for the range of ages indicated (after Turner, 1986). Also included are P. radiata data from Australia (stars) taken from Forrest and Ovington(1970) (including an estimated 1.0-2.5 t ha _1 of litterfall), Baker and Attiwell(1985) and Lambert (1978) and from New Zealand (squares) taken fromMadgwick et al. (1977) and Beets and Pollock (1987). Stand ages are indicated.



stand and 210 t ha-l in the E. regnans. The pines, however, received nofertiliser while the eucalypts were fertilised at establishment and at age twoyears.

Feller (1983) compared aboveground biomass of P. radiata and mixed E. obliqua/E. dives stands at Maroondah, Victoria, at age 37 years. Total production of337.5 t ha-l (P. radiata) and 372.5 t ha-1 (eucalypts) were reported. Theeucalypt stand was unthinned but 100 t ha-l had been removed in thinning theP. radiata. Given some mortality and treefall in the eucalypt during the 37years, the total biomass production was probably similar in the two stands overthe period.

Timber yield data are available over long (and comparable) periods for P. radiataandE. delegatensis at sites about 10 km apart in Bago S.F. in southern N.S.W.These mean volume increments have been converted to mean dry matterincrements using a crude 1:1 conversion that ignores actual wood densities and'moisture contents and differences between species. In addition thecontribution of branches, foliage and fruits to total biomass has been omittedbut this omission is likely to introduce a similar error in both species as theproportion of these components in the total biomass falls to similar levels inboth pines and eucalypts after the early years.

Published data reviewed by Hutson and Veitch (1985) suggests litterfall rates of5.0-9.0 t ha-1 yr-1 can be expected in higher site quality eucalypt forests, whilelonger studies indicate values of 4.0-8.0 t ha-l yr-l depending on productivity(R. Bridges,pers. comm.). Lambert (1978) reports litterfall figures of7.0 t ha-1

yr-l at Bago S.F. for an E. radiatalli. dalrympleana stand. Although highervalues have been reported in New Zealand, litterfall in Australian P. radiataplantations appears to range between 2.5 and 4.0 t ha-1 yr-I . For the purposesofbiomass calculations at Bago, litterfall values of 4.0 t ha-1 yr-I for P. radiataand 6.0 t ha-l yr-l for E. delegatensis have been used and these values ofestimated biomass production are presented in Table" 5. These comparisons,and those of Feller (1983) and Frederick et al. (1985), suggest that, onequivalent sites, total biomass production of pines and eucalypts may besimilar, particularly over long periods. Production of usable wood may begreater in a P. radiata plantation, however, as the biomass contribution fromannuallitterfall production appears to be significantly higher in eucalypts.

The results of Lambert (1978), who reported current organic matter production of9.0 t ha-l yr-l in a 42-year-old P. radiata stand at Lidsdale S.F. near Lithgow,N.S.W., and much lower production rates (3.0 and 3.5 t ha-1 yr-l respectively)for nearby 30-45 year-old E. dives/E. rubida and E. rossi/E. rubida woodlands,are somewhat at variance with the notion that the productivity of pine andeucalypt is similar on equivalent sites. The woodland in this study was allowedto regenerate naturally from coppice and seed in a cleared paddock over a 15year period and may have been subjected to fires and grazing. The silviculturaland establishment procedures used were very different for the two stands andmay have played some part in the demonstrated differences in productivity.

Technical Paper No. 49 Forestry Commission of New South W,'J1es

Table 5.

• 23 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Oualtty - A Review

Comparison of timber yields and estimated biomass production forP. radiata andE. delegatensis at Bago S.F., N.S.W.

SPECIES AGE

(YRS)

MAI4 ANNUALLITTERFALL

ANNUAL BIOMASS(M3 HA·1YR"1) (T HA-1YR"1

ESTlMATEDMEAN

PRODUCTION(T HA-1 YR-1)

E. delegatensist 61

P. radiata1

P. radiata1

64 11.9(1921 age class)64 12.2(1922) age class)

9.9

39 15.4(1948) age class)

4.0

4.0

6.0

4.0

15.9

16.2

15.9

19.4

E. delegatensis3 40 14.4 6.0 20.4

P. radiata1 33 17.3(1953 age class)

E. delegatensis3 35 14.5

4.0

6.0

21.3

20.5

1 Forestry Commission of N.S.W. yield data (unpubl.)2 Home and Robinson (1990)3 Data of Lindsay, in Borough et al. (1978)4 Mean annual increment over entire period, including thinnings.

Dunin and Mackay (1982) reviewed the transpiration efficiency of pines andeucalypts and concluded that:-

"There appears to be little difference between eucalypts and P. radiata in theirrespectiverelationships between growth and transpiration rate. Wecontendthat observeddifferences in growth and by implication in transpiration, result from siteplus climaticfactors havinga differential impacton morphological development. Themagnitudeofdifferential canopydevelopment between communitiesis likely to be site-specific andgeneralisations for the relative levels of growth and transpiration must await a betterunderstanding ofthefactors involved. "

Although few studies have been carried out, reported transpiration efficiencies ofpines and eucalypts lie between 4 and 5 mg dry matter produced per gm ofH20 transpired. Based on the reported equivalence in transpiration betweenpines and eucalypts at Lidsdale (Smith, 1974; Pilgrim et al., 1982), Dunin andMackay (1982) calculated transpiration efficiencies of 2.5 and 1.0 mg a.M. pergm H20 respectively for these species and suggested that the differences maybe due to large leaf area index fluctuations in the eucalypt stands. The Lidsdalepine-eucalypt comparisons appear anomalous and it seems reasonable toassume that, as a general rule, long-term biomass production by pine andeucalypt will be similar on the same site. Transpiration efficiencies overallmay also be similar for pine and eucalypt but short-term and seasonalefficiencies may differ.



If transpiration efficiency remains relatively constant as a forest stand ages, (andthere is no evidence at present to suggest otherwise), then the stand is likely toexperience peak rates of transpiration at the age of maximum net productivity.Peak transpiration rates may therefore occur at different ages for differentspecies (c/Fig.7) and be reflected in differing temporal patterns of water yieldchange as these stands regenerate and develop. Such a hypothesis could explainthe observed pattern of water yield change in the Victorian Mountain Ashforests following regeneration (Kuczera, 1985) in which water yields duringrapid forest regrowth were lower than those in the previous old growth forest,and the water yield changes in P. radiata reported by Knights (1983). Differentforest types may exhibit different patterns and magnitudes of water yieldchange during the rotation depending on site conditions and on the age ofmaximum net productivity which, in turn, may be related to the age ofmaximum transpiration rate.

(i) Effects of fertilization on water yields in P. radiata. Fertilization of P. radiataplantations at planting increases productivity considerably, particularly on siteswith low - moderate nutritional status (Turner, 1984). Later age fertilizationplus cultural improvements at establishment can further increase productivityon all sites. These treatments are likely to affect water yields by reducing thetime taken to reach full canopy closure, thereby increasing interception at thesame age in comparison with unfertilized stands. This also applies after laterage fertilization and this is likely to further reduce water yields at that age.Assuming transpiration efficiencies remain unaffected by fertilization, it islikely that transpiration rates would be increased at a particular age incomparison with equivalent unfertilized stands (due to increased netproductivity rates), leading to additional reductions in water yield.

The overall effect of fertilization on water yields in pine will tend to be site and agespecific but it is possible that annual evapotranspiration increases of 10% ormore may occur in some circumstances. Such increases may result in decreasesin water yield of 50-100 mm or more per year in comparison with an equivalentunfertilized stand. .

U) Present understanding of P. radiata water use. The principal variables affectingwater use in pine (those relating to site, climate, season, age and silviculturaltreatment) have been studied to a greater or lesser degree enabling somegeneral predictions to be made. Data presented in Fig. 2 and Fig. 4, derivedfrom mean annual values, allow an estimate of the mean annualevapotranspiration of a fully stocked P. radiata stand in south eastern Australiato be made for a range of mean annual precipitation values (Table 6). It mustbe understood, however that actual values of pine evapotranspiration in anyyear will depend on precipitation (amount and disposition), and factorsaffecting energy receipt and growth rate (leaf area, age, nutrition, soilmoisture, silvicultural treatment), and that the evapotranspiration valuespresented in Table 6 are general estimates only.


Table 6.

• 25 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouallty- A Review

Estimated mean annual evapotranspiration of a fully stocked P. radiataplantation for a range of mean annual precipitation values.

MEAN ANNUAL MEAN ANNUALPRECIPITATION EVAPOTRANSPIRATION

(MM) (MM)

750 700

1000 930

1250 1050

1500 1170

1750 1230

A systematic examination of these factors in a study that measures pine water useand rates of growth and biomass accumulation over a number of years has yetto be made. The results of such a study; preferably conducted over theduration of a rotation, would result in more precise predictions of pine wateruse at any age and a better basis for modelling the effects of pine afforestation.

(k) Amendments required to Boughton's water yield conclusions. Amendmentsnow required to the conclusions of Boughton (1970) concerning water yields,resulting from recent research, are listed below:-

1. Water yield differences between tree covered and grass covered catchmentsare generally not small, and may be considerable in areas of higher rainfall.

2. Mature forests of different tree species (eg pine and eucalypt) may usedifferent quantities of water annually due to differences in canopyinterception. Seasonal differences in water use between pine andeucalypt may result from differences in canopy structure and responseto water deficit.

3. Canopy interception, including the evaporation of intercepted water, isconsiderably greater in tree species than in grasses. In addition forest areasconverted to grassland may suffer large reductions in water infiltration ratesover time as soil macropores disappear and hydraulic conductivity valuesdiminish.

Changes to soil macropores and hydraulic conductivity values following conversionof forest to grassland are discussed later in l(b) of the Section on water quality.



EFFECTSON WATER QUALITY

1. Soil Erosion and Sediment Production.

(a) Influence of rainfall and running water. Raindrops landing on the surface ofthe soil often have sufficient energy to dislodge soil particles which are thenavailable for transport by running water (Ekern and Muckenhirn, 1947;Chapman, 1948). A litter layer or low ground cover will reduce raindropimpact on the soil surface and make fewer soil particles available formovement (Lee, 1980). While the canopy of a forest may intercept rainfall,water in excess of that stored on the foliage will fall to the forest floor asgravity drops. These drops are frequently much larger than raindrops andMoss and Green (1987) showed that if the fall height of gravity drops was only0.8 m these drops contained three times the erosive power of raindrops.Raindrop splash under forest canopies is then likely to be an important factorin surface soil detachment and movement when the soil surface is notcompletely protected by litter or short vegetation.

All precipitation arriving at the soil surface will percolate into the soil unlesseither of the following conditions are present:-

1. The infiltration rate of water into the soil, which is determined by thesaturated hydraulic conductivity of the surface soil, is less than therainfall rate.

2. The soil moisture store is completely filled to the soil surface.

Under either of these conditions water will pond on the soil surface and, unless thesurface is perfectly horizontal, move downslope as surface runoff towardsdrainage lines and streams.

Dislodged soil particles may be carried in suspension in this surface runoff, thequantity carried depending on numerous factors including the size of theparticles and the velocity and depth of the water (Cornish, 1975). If thevelocity of the running water decreases (through, for example, obstruction or areduction in slope) then some of this suspended material may be deposited.Unless stabilized in some way in the meantime this deposited sediment wouldbe available for re-suspension and transport during a later storm whichgenerated surface runoff with sufficient velocity and volume.

Running water also has the energy to dislodge soil particles and make themavailable for transport, with higher velocities and greater depths being moreefficient in this regard (Cornish, 1975). Soil types in which the particles havelower bonding energies are generally more erodible, material being madeavailable for transport at lower water velocities. These bonding energies arerelated to soil texture, degree of aggregation of soil particles and their capacityto become dispersed in water, soil properties frequently related to soil parentmaterial. The relative erodibility of soils formed on different parent materialsis discussed in a later section. The smaller soil particles (clay and silt) aremore easily transported by water but are generally resistant to dispersion,

Forestry Commission of New South Wales. Technical Paper No, 49


unless sodium is a significant component of the soil cation exchange complex(Wallis and Stevan, 1961). The larger soil particles (sand) require greaterwater velocities for transportation. A doubling of water velocity results inparticles 64 times larger being transported (Cornish, 1975).

The principal types of soil erosion resulting from running water are sheet erosion,rill erosion, gully erosion and the erosion of stream banks (AlIen and Leonard,1966). The first three types can be influenced by forest practices that increasesurface runoff and disturb the soil surface, while stream bank erosion canresult from the destabilization of banks following logging machinerymovement and tree removal. However the reduction in the frequency andmagnitude of peak flows following afforestation of cleared land is likely to leadto reduced streambank erosion.

The magnitude of any accelerated soil erosion and increased suspended sedimentlevels in streams resulting from forestry operations can be very dependent onthe erodibility of the soil, which is frequently determined by the soil parentmaterial (Dyrness, 1967). It is generally agreed that soils formed on intrusiveigneous rocks (e.g., granite) are the most highly erodible, while soils formed onextrusive igneous rocks such as basalt are the least erodible (Anderson, 1954;Wallis and Willen, 1963). The erodibility of soils formed on most sedimentaryand metamorphic rocks lies between these extremes.

A knowledge of soil parent material is therefore important in the preliminaryassessment of potential soil erodibility, a factor which must be taken intoaccount when planning forest operations that ultimately result in minimalincreases in erosion.

(b) Factors influencing surface runoff. While undisturbed forests generallycontain soils with high hydraulic conductivity values, managed forests maycontain areas compacted by machinery movement. In these areas (roads, tracksand log landings) considerable surface compaction frequently occurs (Schuster,1979), and hydraulic conductivity values may be reduced by orders ofmagnitude (Riley, 1984). Bren and Leitch (1985) showed that a forest roadgenerated more stormflow than nearby undisturbed forest for small tomoderate storms. These effects may occur in both eucalypt forests and pineplantations, but pine plantations frequently have higher roading densities andwheeled pine logging machinery (such as forwarders) generally exert higherground pressures. This, together with the fact that thinning operations in pineplantations usually occur a number of times in a 40 year rotation, can result inincreasing compaction with stand age as demonstrated by Hamilton (1965) inP. radiata plantations near Canberra. Sands et al. (1979) showed increased soilcompaction following conversion of eucalypt woodland to P. radiata plantationin South Australia. These authors considered that increased compaction wasdue to loss of organic matter and the extent to which logging had occurred inwet soils. Soil compaction also has the potential to reduce pine plantationproductivity.

Technical Paper No. 49 Forestry Commission of New South Wales.

- 28 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Ouallty- A Review

The surface layers of forest soils frequently contain large macropores resultingfrom the decay of tree roots and from soil faunal activity. These macroporesform preferential pathways for rapid subsurface soil water movement (Harr,1977). Compaction ofsurface soil by logging equipment can destroy theselarger pores (Moore et al., 1986) resulting in greater surface runoff. Burch etal., (1987) showed that the near-surface hydraulic conductivities of grasslandsoils in central Victoria, that had been cleared of forest for 80 years, weremuch lower than those of nearby forest soils, resulting in greater runoff fromthe grassland. The authors attributed the hydraulic conductivity decline to theabsence of continuous macropore pathways in the grassland soils. If this is ageneral phenomenon then it might be expected that the initial near-surfacesoil hydraulic conductivities of P. radiataplantations planted onlong-established pasture would be lower than those of plantations establishedon nearby areas of recently cleared Eucalypt forest. Plantations established onpasture may therefore experience greater and more frequent surface runoff,and-higher stormflow peaks, than plantations established on equivalent landrecently cleared of eucalypt forest but these hydrologic effects will bediminished in comparison with pre-afforestation levels in pasture country. Asplantations age (and in the absence of cornpaetion from logging. equipment) itcould be expected that soil macropores would develop with a consequentincrease in hydraulic conductivity values.

As soon as anarea becomes waterlogged further rainfall may generate surfacerunoff. The identification of areas in a catchment that preferentially wet up istherefore desirable, and this has proved possible following the theoreticalanalysis of flow in hillslopes (O'Loughlin, 1981). An extension of this analysis(O'Loughlin, 1986) has resulted in a contour - based digital terrain model thatpredicts the boundaries of waterlogged zones as they expand or contract in thelandscape. The model is robust enough to accommodate certain naturallandscape differences such as vegetation type, and has been used to predict thegrowth of wet areas following forest clearing (O'Loughlin, 1986; Burch et al.,1987; Moore et al, 1988) and to suggest alternative strategies that avoidwaterlogged areas when harvesting P. elliotti plantations in Queensland(O'Loughlin, 1987a). Moore and Burcb (1986) have combined the digitalterrain model with the concept of unit stream power to determine the·distribution of more erosion-prone areas in a catchment. Although thesemodels are not yet developed to the point where they can be used as planningtools by land management agencies, this may be possible in the future(O'Loughlin, 1987b).

(c) Slope stability and landslips. P. radiata has commonly been planted on steepslopes in New Zealand and much research has been conducted in that countryinto factors affecting slope stability. Tree roots assist in stabilizing steepslopes, and studies have shown that tensile strengths of the root-wood of nativeNew Zealand tree species declined rapidly when cleared for P. radiataplantation (O'Loughlin and Watson, 1981). Shallow landslips often resultedfrom suchclearing (O'Loughlin and Watson, 1981; O'Loughlin and Ziemer,1982). A P. radiataplantation has been shown to increase slope resistance toshallow landslips and surface erosion substantially between 5 and 10 years afterestablishment (O'Loughlin, 1984). However live root-wood strength of

Forestry Commission of New South Wales, Technical Paper No, 49


P. radiata has been found to be low in comparison with native New Zealandtree species, and to decline exponentially after the trees have been felled(O'Loughlin and Watson, 1979). Reduced soil strengths were consideredresponsible for two large landslips which deposited 60,000 m3 of sediment instreams in a 2.7 km2 area ofyoungP. radiata plantation (Pearce and Watson,1983). Slope failure has also been shown to result from changed hydrostaticpressures associated with large blocked soil pipes which may have been formedwhen the roots decayed (Pierson, 1983). This New Zealand research suggeststhat the afforestation of grassland by P. radiata in Australia should improveslope stability. Steep areas presently planted to P. radiata are likely to suffersome loss of stability following each thinning operation, and perhaps a greaterreduction after clearfalling. Such reductions in stability would be accentuatedduring wet periods when soil moisture levels were high.

(d) Forest operations and stream sediment levels.

(i) General. Soil erosion resulting from running water is a natural process andoccurs in both undisturbed and disturbed forested catchments. Erosion ratesare generally low in undisturbed forests, resulting in low-moderatestreamwater suspended sediment and turbidity levels even during storms.Forestry operations may increase erosion rates and consequently increasestream sediment levels, as has been shown by many workers (e.g., Reinhart andEschner, 1962; Megahan and Kidd, 1972;Anderson et al., 1976). Disturbedareas such as roads that provide ready source areas for surface runoffgeneration have been demonstrated as major contributors to elevatedstreamwater suspended sediment levels (Fredrickson et al., 1975).Kochenderfer and Aubertin (1975), however, showed that when exceptionalcare was taken in the location, construction and maintenance of snigtracks,roads and landings, stream turbidity increases were small in comparison withunplanned operations. Table 7, taken from this study, indicates theimprovement in water quality that may result from appropriate environmentalplanning and execution of forest roading and harvesting operations.

(ii) Suspended sediment and P. radiata. Increased streamwater suspendedsediment and turbidity levels have been associated for many years with theestablishment and management of P. radiata plantations in Australia.Boughton (1970) considered that the critical periods during which increasedturbidity levels were likely to occur were at plantation establishment, duringthe early years and at clearfelling and re-establishment. Boughton also cites aninferential case of increased turbidity in the Shoalhaven River during a stormthat occurred 2-4 years after the establishment of a P. radiata plantation atMongarlowe. In a plot study in the Lower Cotter catchment, Gilmour (1968)concluded that eucalypt forests and well established P. radiata plantationsprovided excellent catchment protection. However soil losses were muchhigher from young pine plantations. On the other hand, Hopmans et al. ( 1987)reported that clearing a eucalypt catchment (with streamside buffer stripretention) at Cropper Creek in Victoria for a P. radiata plantation had littleeffect on stream sediment concentrations. Stream sediment exports weresubstantially increased, however, because of large increases in water yields.


• 30 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality - A Review

Cornish (1988) reported a streamwater turbidity study in the catchment of thewater supply of Adelong, N.S.W. 24% of this catchment was occupied by aP. radiata plantation (which was subject to thinning operations during thestudy) and the remainder was agricultural or pastoral land, much of it clearedand grazed. Turbidity levels in the plantation streams were found to be slightlyhigher than in nearby pastoral streams, and levels increased with increasingarea of pine. Turbidity levels were high at the offtake of the town water supplyfurther downstream, and this study demonstrated that the mix of land usespresent in the catchment resulted in unacceptable water quality requiringadditional treatment for domestic consumption.

In a comparative study of streamwater turbidity levels in widely separatedcatchments in N.S.W. containing differing proportions of native forest, clearedland and P. radiata plantation, Cornish (1983b) concluded that rainforestlogging had the least effect on levels and P. radiata plantations the most. Thestudy also showed that land cleared for agriculture or pasture generallycontributed more turbidity to streams than managed eucalypt forests, and thatcoastal rivers usually became more turbid toward their mouths as theproportion of forest diminished. Cornish (1990), in a lengthy study ofstreamwater turbidity in a P. radiata plantation at Sunny Corner near Bathurstconcluded that turbidity levels were highest in those catchments whererelatively greater areas had been thinned or clearfelled in the previous year.Levels were generally higher in streams within the plantation than in streamsdraining nearby grazed pastoral land, and always higher than in an adjoiningeucalypt nature reserve. Parent material and the resultant soils varied withinthe study area but no soils were highly erodible. The water enteringWinburndale Dam, of which Sunny Corner P. radiata plantation constituted56%, had turbidity levels of 5 Nephelometric Turbidity Units or greater during40% of the lO-year study period. These are undesirable levels requiring

Table 7.

TREATMENT

Effects of forest operations on stream turbidity at Fernow (afterKochenderfer and Aubertin 1975).

STREAM TURBIDITY (JACKSON TURBIDITY UNITS)

BASEFLOW MEAN

Commercial clearcut, water values not considered»

STORMFLOW MAXIMUM

During loggingFirst year after cutSecond year after cut

490382

56,0005,000

170

Silvicultural clearcut, care taken with roads:-

During loggingFirst year after cutSecond year after cut

Undisturbed control.-

Forestry Commission of New South Wales.

65·2

2

903523

25

Technical Paper No. 49

The Effects of Radiata Pine Plantation Establishment and • 31 •Management on Water Yields and Water Ouallty- A Review

treatment of such water for domestic consumption although WinburndaleDam, a former water supply for Bathurst, now only provides water for thewatering of parks and gardens.

Although few comparative studies have been carried out it appears likely that,other factors being equal, streams in P. radiata plantations subject to normalthinning operations may have generally higher suspended sediment andturbidity levels than streams in eucalypt forests, managed or undisturbed.However soil erosion and land degradation is widespread on pastoral land inAustralia (Rowan, 1986). Pastoral land afforested with P. radiata maytherefore suffer no reduction in water quality resulting from increasedsuspended sediment levels after afforestation, and for lengthy periods duringthe rotation may in fact enjoy improved water quality.

Streamwater suspended sediment concentrations were compared in P. radiata,native forest and grazed pasture catchments in New Zealand by Dons (1987).Maximum concentrations were considerably higher in the pasture catchment,and this was attributed to surface disturbance by stock and increased surfacerunoff. Both mean and maximum concentrations were lower in the pineplantation due to the stabilizing influence of luxurious grass growth in thestream channel which reduced the movement of unconsolidated streamsediments which are a feature of this pumice country. This study demonstratesthat a number of site specific factors generally play a part in the outcome ofcatchment erosion and sediment production studies in addition to the effectsof land management practices.

Fire is used as a tool in the establishment and re-establishment of P. radiataplantations. As fire may have an impact on the surface soil, on litter andground cover and on the local hydrology some discussion of its effects iswarranted here. Windrows of stacked timber are burnt during establishmentproducing high temperatures at the soil surface (Humphreys and Craig, 1981).Inwell structured soils with high clay contents soil physical properties aregenerally not altered significantly by high temperatures (Craig, 1968; Clinnickand Willatt, 1981), but in sandy soils the loss of organic matter during the fireresults in a reduction in both infiltration rates and in the proportion of largersoil pores (Tarrant, 1956). Certain soils become hydrophobic following fires(DeBano and Rice, 1973; Tunstall et al., 1976) resulting in reduced infiltrationrates and greater surface runoff. Slash burning of logging debris prior tosecond rotation establishment will remove litter and surface cover leaving anexposed soil surface subject to the effects of raindrop impact and increasedsurface runoff. Soils with a relatively low inherent permeability are likely to bemost affected by slash burning (Walker et al., 1986). The effects on surfacerunoff will be much greater if widespread hydrophobicity is induced during theslash burn, resulting in a potentially serious erosion situation. Slash burning,unless well controlled, may also consume ground cover and litter in thestreamside buffers resulting in a reduction in effectiveness at a critical time.Windrow burning in streams and drainage lines may lead to increased erosionin these sensitive areas. Large quantities of ash with high nutrient levels maybe produced under burnt windrows (Humphreys and Craig, 1981) and this ashis much more likely to be transported to streams if windrows are in drainage


- 32 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water QIJa,lity - A, R~view

lines. Increased nutrient concentrations in streamwater derived from ash mayresult in at least short term eutrophication of streams (Wood, 1975) and apermanent loss of nutrient capital from the site.

(iii) Erosion mitigation measures in P. radiata plantations. Following numerousNorth American studies that demonstrated that control over various forestpractices could significantly reduce soil erosion and stream sediment levels(e.g., Reinhart and Eschner, 1962; Kochenderfer and Aubertin, 1975), thepublication and implementation of guidelines to good forestry erosionmitigation practices became widespread (e.g., Rothwell, 1971; Packer, 1976).Guidelines more specifically written for Australian forests and forestrypractices followed (Cameron and Henderson, 1979; Cornish, 1983a). Many ofthese guidelines were enforced in N.S.W. in 1977 as conditions attached tologging licences in native forests, with subsequent application in pineplantations. (Anon., 1984a; Cornish, 1985a).,Other States enforced their owncodes of forest practices that were designed to reduce soil erosion andminimise environmental impacts (e.g. Anon., 1987; Anon., 1989b).:'< •

The. introduction of new types of harvesting equipment in P. radiata plantations inthe early 1970's resulted in unfavourable soil-machinery interactionsparticularly when soils were very wet and soil strengths were low. Deep ruttingalong.extraction tracks, which was sometimes associated with soil compaction,resulted when machinery was operated in adverse moisture conditions. Accessroads in the plantations were also damaged in these conditions, and a system ofwet weather forest closures was introduced in N.S.W P. radiata plantations tominimise rutting depths and sedimentdischarge tostreams (Anon... 1984b;Cornish, 1985b). Many of the P. radiata plantations in southern N.S.Warelocated in predominantly winter rainfall areas where soils may remain nearsaturation for lengthy periods (Cornish et al., 1981). As industries generallyrequire 'a continuous supply of timber the imposition of forest closures leads to

the necessity to stockpile timber and to consequent increases in costs. Hencethere is 'an understandable industry pressure to keep forest closures to aminimum. Wet weather closures can be reduced to some degree by theplanning of harvesting operations. In N.S.W. Forestry Commissionplantations, operational harvesting plans Classify areas as suitable for winter,summer orintermediate seasonal logging (Anon., 1984b) and this .classification is an important factor in reducing water quality impacts in winter.

As noted earlier roads are an important potential source of sediment to streams.Roading densities in P. radiata plantations are relatively high, particularly inolder-plantations established before the introduction of specific erosionmitigation measures. While good practice today suggests a reduced roadingpattern in which roads are located, if possible, on ridges and well away fromstreams, older plantations frequently have considerable lengths of streamsideroading. Even if these roads are closed in wet weather they are likley to

increase stream turbidity and suspended sediment levels during rain because, there is frequently little opportunity for sediment to be deposited in the short

distance between road and stream. Consideration should be given to therelocation of roads away from streams during second rotation establishment.

Forestry Commission of New South Wales. Technical Paper No; 49


The provision of streamside buffer strips of undisturbed vegetation is an importantcomponent of erosion mitigation measures in forests, as discussed by Clinnick(1985). Numerous problems arise with the provision, management andeffectiveness of buffer strips inP. radiata plantations. Older plantations wereestablished on areas completely cleared of native forest, with pine planted tothe streambanks. The retention of a buffer strip in such a plantation results ina strip of pine on either side of the stream which is possibly unthinned andsubject to potential windthrow in the later years of the rotation. Managementof the strip at time of clearfelling and re-establishment also poses problems. Ifclearfelled the question of what type of vegetation to encourage in the strip inthe second rotation may not be easy to answer but catchment protectioneffectiveness should be the prime consideration. The provision of streamsidebuffer strips in plantations established on cleared pastoral land also introducessimilar management problems. Although the exclusion of fire is preferred, itmay not always be possible to prevent buffer strips being burnt, as discussedearlier.

Contour ploughing is an example of a preferred erosion mitigation practice(Rowan,1986) not universally employed in P. radiata plantations because of thelimitations of existing machinery and the impact on harvesting economics.Some reduction in water quality is always possible when such compromises aremade.

2. ChemicalWaterQuality

(a) Nutrients and major ions. In relatively humid environments the chemicalcomposition of streamwater is determined by precipitation chemistry and bycatchment geology (Gibbs, 1970;Cornish, 1987). The cationic composition ofstreamwater in eucalypt forested catchments in south eastern N.S.W. wasfound to be greatly influenced by the mineralogy of the principal bedrock inthe catchment by Cornish and Binns (1987). Talsma and Hallam (1982), in awater quality study in eucalypt and P. radiata catchments in the Cotter Valleynear Canberra, concluded that water quality and composition depended moreon geological and soil properties than on vegetation type. In this study thechemical composition of streamwater from undisturbed pine plantations wasfound to be similar to that from geologically similar catchments of nativevegetation.

Following the site exposure that accompanies logging, some forest soils experiencean accelerated release of certain ions from the mineralisation of organicmatter and from mineral weathering (Aubertin and Patric, 1974). The extentto which those ions released by exposure are removed from the site by leachingto streams is a function of the uptake of those ions by vegetation and the ionexchange properties of the soil. A suppression of forest regrowth may result inincreases in ions in streamwater (Likens et at., 1970) but this rapidly reversesas forest growth proceeds (Likens et al., 1978). Biologically active ions are theones most affected by these processes.


• 34 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality - A Review

Cooper et al. (1987) studied streamwater nitrogen and phosphorus levels incatchments of native forest, pasture and P. radiata in New Zealand over a 14year period. The P. radiata catchment, previously improved pasture with aregular history of fertilizer application, was planted at the beginning of thestudy. No fertilizer was administered after afforestation. Streamwaterphosphorus levels were high in the pasture catchment throughout the study.Phosphorus levels were initially high in the P. radiata catchment also butdeclined after 5 years, becoming considerably lower than in the pasturecatchment. Nitrate nitrogen levels in streamwater were initially similar inpasture and pine catchments, but increased after 5 years in the pine catchmentrelative to levels in the' pasture catchment. This rather surprising result wasattributed to a more complete removal of nitrate nitrogen by the thick grassmat in the pasture stream channel. Initially present in the P. radiata streamchannel also, it had become much reduced in extent due to channel shadingafter 5 years. Nitrate nitrogen levels therefore increased as this nitrogen sinkdiminished. This study, although a rather special case, shows that previousfertilizer history can influence stream water nutrient levels for some yearsafter afforestation of pasture byP. radiata, 'and that instream processes mayplay an independent role in modifying these levels.

Fire in a forested catchment may have a considerable effect ,on ionicconcentrations ira streamwater. Lee (1980) summarises the factors responsiblefOJ; fire affecting water quality as follows:-

'"Th~ ~jjects'Jjfi~e on catchment water quality depend on thefrequency, extent; and severity of, burning, watershedphysical conditions, postfire weather, and therapidity of reforestation".

stash burning is frequently used during second rotation establishment of P. radiataplantations. Kimmins and Feller (1984) reported that the broadcast burning ofIogging slashfollowing partial clearfelIing of a catchment in British Columbiaresulted in ageneral pattern of increased concentrations and fluxes of ions instreamwater for the first 2-3 years following treatment. Values then declinedto, and sometimes below, pretreatment levels. The most pronounced increasesw,ere observed for potassium and nitrate. Similar effects were observedfollowing wildfire in a eucalypt catchment near Eden by Mackay and Robinson(1987).

Hart (1974) has reviewed the potential impacts of elevated nutrient concentrationsin streamwater in relation to water use. Elevated water nitrate levels can effecthumans and livestock and are toxic to infants, Increased nutrient levels canproduce eutrophication resulting in algal blooms which may lead todeoxygenation of the streamwater and a deterioration in stream fauna habitat.Concentrations necessary for these effects are rarely the product of forestmanagement.

(b) Applied forestry chemicals. Various chemicals may be applied during theestablishment and growth of P. radiata plantations. These chemicals(principally fertilizers, herbicides and pesticides) may end up in streamwaterdepending on a number of factors including mobility, rate of fixation by the



soil, rate of uptake by vegetation, application method and the proximity ofapplication sites to streams and drainage lines. Nambier and Cellier (1985)concluded that the broadcast application of nitrogenous fertilizer was likely to

increase nutrient uptake through increased root contact in a P. radiataplantation established on sandy soils in South Australia. These authorssuggested that broadcast, as opposed to spot, applications would reduceleaching losses of nitrogen. Nambier and Bowen (1986) stressed the need todesign fertilizer application strategies to maximise uptake and minimiseleaching losses. Research overseas (e.g., Moore, 1975) indicates that the aerialapplication of nitrogenous fertilizers to forests does not result in streamwaterN concentrations above public health standards or at levels likely to result ineutrophication if applications are well removed from streams.

Herbicides are frequently used in Australia to limit weed competition whenP.radiata plantations are established. Leitch and Flinn (1983) reported thatlevels of hexazinone in streamwater were barely above detection levelsfollowing aerial spraying of a P. radiata catchment in northern Victoria. Thelow residues detected were attributed to care during the operation and thepresence of a 30 m wide buffer strip of native eucalypt forest on either side ofthe stream. Leitch and Fagg (1985), in a similar study in the same area inwhich clopyralid herbicide was applied aerially to a P. radiata plantation, foundonly low concentrations of this herbicide in streamwater. The contaminationthat did occur was due to rainfall washing herbicide deposits from streamsidevegetation that had intercepted minor amounts of spray drift. This generallyfavourable result was again attributed to the presence of unsprayed streamsidereserves, and to accurate spraying techniques and the pattern of rainfall(low-intensity storms followed by high-intensity storms) after spraying. InBritain, however, Hornung and Newson (1986) reported that hand applicationof herbicide is preferred to reduce the risk of stream contamination fromaerial spraying in that country. In general wind speed and droplet size are themajor factors influencing spray drift during aerial applications.

Aerial spraying of copper oxychloride to a P. radiata plantation at Nundle in N.S.W.to combat Dothistroma did not result in streamwater copper concentrationsabove background levels during a sixweek period following application (M.Lambert,pers. comm.). Research generally indicates that water quality isunaffected if applications of herbicides and pesticides are made well away fromstreams (Gibbons and Salo, 1973), but more care should be taken withpesticides because very lowconcentrations in streamwater may still have anadverse effect on some aquatic fauna (Lee, 1980).

(c) Bacterial water quality. Bacterial contamination of streamwater by organismssuch as faecal coliforms occurs in even the most pristine forested ecosystemsbecause of the influence of native and feral animals (e.g., Waiter and Jezeski,1973). This contamination is frequently slight but gives rise to backgroundlevels that are greater than zero and may vary seasonally. Mixed catchmentsthat are grazed by cattle may contribute pathogens such as salmonella to

streamwater (Fair and Morrison, 1967).


·36 • The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Oualltv- A Review

Little study has been made of the effects of forestry on bacteria levels in Australianstreams. Langford et al., (1982) concluded that neither E. coli nor totalcoliforms were increased as a result of forestry activities in the Coranderrkcatchments in Victoria. Lee (1980) considers that recreational activities inforests pose the greatest potential threat to streamwater from elevatedbacteria levels. Of most concern in this respect is the provision of adequatesewage treatment facilities in what are often remote forested areas. In additiona continuous supply of nutrient (primarily phosphorus and nitrogen) tostreamwater from sewage may lead to eutrophication and fish mortality.

Forests are becoming used more frequently for the disposal of sewage sludge andeffluent from municipal sewage treatment plants (Pierce and Keller, 1980).While there is a potential for heavy metal pollution and increased soil salinity,research generally shows that, provided the disposal is well planned, waterquality is unaffected by this practice (Sopper and Kardos, 1973). The capacityof a forest to assimilate particular quantities of effluent varies with site andhydrologic characteristic.s, and these must be fully appreciated before longterm forest disposal is commenced (Pierce and Keller, 1980).

(d) Salinity. Ground water tables rise following forest clearing (Sadler andWilliams, 1981). If the lower horizons of the soil profile or the groundwateritself contain appreciable quantities of salt, some may be transported towardsthe soil surface with the 'rising watertable, reaching the streams from areas ofgroundwater discharge. Recognised as a longstanding problem in WesternAustralia (Wood, 1924), large areas in both Victoria and Western Australiahave now been affected with severe degradation of water quality in streams andrivers (Boughton, 1970; Sadler and Williams, 1981; Schofield et al., 1988). It isgenerally considered that the time scales involved in dryland salinitydevelopment on the one hand and those for forest regeneration followinglogging on the other are quite different, and that salinity is not a product ofnormal forest management in salinity susceptible areas. However, Stokes andLoh (1982) showed that streamwater salinity levels increased in the yearfollowing the complete clearing of a eucalypt catchment in Western Australia,suggesting that in certain circumstances salinity increases may be moreimmediate and that further research is required into the effects of logging andregeneration on salinity levels in these susceptible environments. The waterbalance of logged jarrah and karri stands in south-western Western Australiahas been shown to return to the pre-logged state in about 10 years (Stonemanet al., 1989). This was considered to be a consequence of the rapid recovery ofthe vegetation cover, suggesting that in the areas examined any increasedsalinity following logging was insufficient to impede this revegetation.

The likelihood of the subsoils of long-cleared pasture catchments containingpotentially harmful accumulations of salt is low in areas of moderate - highrainfall in N.S.W. Such catchments are unlikely to have developed drylandsalinity, although water tables may now be high and topograpbically lowerareas saturated preventing easy pine establishment. Water tables in such areaswill fall when pine becomes well established. Drier cleared areas of pasture

Forestry Commission of New South Wales. Technical Paper No,' 49


(rainfall perhaps < 750 mm yr-l ) may have developed some salinity problemsif subsoils are saline, and this would constitute another factor preventing easyafforestation of such areas by P. radiata.

Although eucalypt species have been preferred to date (Schofield et al., 1989),P. radiata may have a role in the re-afforestation and reclamation of somecleared areas now affected by dryland salinity.


- 38 - The Effects of Radiata Pine Plantation Establishment andManagement on Water Yields and Water Quality· A Review

CONCLUSIONS

1. Water Use and Water Yield

(a) The principal processes that determine variations in water use betweendifferent types of vegetation are transpiration, canopy interception, energyinput, and possibly, growth rate.

(b) Under similar conditions tree species such as pine and eucalypt transpire atabout the same overall rate, and at a greater rate than more shallow rootedspecies such as unirrigated grasses subject to seasonal dormancy.

(c) Tree species intercept much more precipitation than pasture species because ofgreater interception capacities and higher rates of evaporation from wettedsurfaces.

(d) P. radiata intercepts more precipitation than eucalypt species because of adifferent canopy architecture, the overall difference being up to 10% of annualrainfall.

(e) Forests receive more energy for the evaporation process than grassland, and thisprocess in forests is itself more efficient.

(f) Land cleared of eucalypt forest and planted to P. radiata is likely to suffer along-term reduction in water yield up to about 10% of annual precipitation.

(g) The complete afforestation of a cleared pasture catchment in Australia byP.radiata is likely to result in maximum reductions in water yield in excess of400 mm per annum where precipitation exceeds 1,300mm annually. Whileactual annual reductions will depend on the precipitation received, such anoverall change would be major, at least at the local level.

(h) The actual decrease in annual water yield resulting from afforestation ofgrassland by P. radiata will depend on a combination of the following factors,each having an important bearing on the final outcome:-

(i) Water yield changes will be in direct proportion to the percentage of thecatchment afforested.

(ii) Until maximum water use is established at about the age of maximum growthrate, water yields in a P. radiata plantation will decrease before increasing.Plantation water yields overall will therefore depend on the particular ageclasses present and the relative proportion of each.

(iii) As thinning a plantation reduces canopy interception and changes growth rates,water yields will increase following each thinning operation, with first thinningpossibly responsible for a yield increase in excess of 100 mm in the followingyear.

Forestry Commission of New South Wales. Technical Paper No. 49


(iv) Water yield decreases in plantations also depend on the amount and nature ofthe precipitation received, greater decreases occurring when annualprecipitation is greater and average rainfall intensities lower.

(i) Afforestation of grassland by P. radiata results in additional hydrologicconsequences in the catchment which may be summarized as follows:-

(i) Peak streamflows during storms may be reduced.

(ii) Streams may take longer to rise following rainfall.

(iii) Baseflow in streams during dry weather may decline.

(iv) Periods of dry-weather flow in ephemeral streams may become shorter, andsome perennial streams may become ephemeral.

(j) Overall biomass production on an equivalent site will be similar for pine andeucalypt, although biomass production rates are strongly influenced by siteconditions and vary with the age of the stand. Maximum production ratesoccur at different ages in different species. Although little data are available(and more research is required), transpiration efficiencies are probably similarin pine and eucalypt. Maximum transpiration rates for any species may occurat the age of maximum biomass production possibly leading to a specificspecies (and site) pattern of water yield change during the rotation.

(k) Fertilization of pine plantations may result in site dependent decreases in wateryield of 50-100 mm or more per year in comparison with an equivalentunfertilized stand.

(1) The dependence of water use on growth rate is not well understood for P.radiata, and a systematic examination of relevant factors needs to beundertaken in a lengthy study of growth rates and water use to refinepredictions of P. radiata water use with age.

(m) Dryland salinity is unlikely to be present in pasture catchments in moderate high rainfall areas suitable for afforestation by P. radiata in N.S.W.

2. Water Quality

(a) Suspended sediment and turbidity are the most widespread pollutants ofstreamwater in P. radiata plantations.

(b) Principal factors likely to be responsible for increased turbidity levels inplantation streams include:-

(i) A high roading density, particularly in older plantations.

(ii) The frequent occurrence of streamside roading networks.


-------------------------


(iv) General reliance on wheeled harvesting equipment and downhill extractionpatterns.

(v) Harvesting throughout the year, notwithstanding the fact that P. radiata areasin southern Australia experience a wet winter when soils frequently remainnear saturation for long periods.

(vi) Lack of suitable machinery that would allow the complete introduction of somepractices such as contour ploughing that are universally accepted as less likelyto result in increased erosion.

(vii) Streamside buffer strips, if present, may be ineffective at certain timesdepending on their management, their composition and their treatmentduring the re-establishment of the plantation.

(c) Areas in catchments that wet up first and remain wet for longer periods arelikely to be more prominent in the supply of suspended sediment and turbidityto streams following disturbance by machinery, and therefore require moresensitive management if water quality problems are to be avoided.

(d) While the imposition of logging and planning controls such as erosionmitigation. measures, wet weather closures and the seasonal planning oflogging areasin plantations are designed to reduce the impact on streamwaterfrom suspended sediment, and turbidity, these measures appear less successfulin pine plantations than in hardwood forests. Factors responsible for thisinclude the greater roading density and intensity of operations in pine, thewinter rainfall maximum and year-round logging.

(e) Afforestation of grassland by P. radiata should generally increase slope stability.Steep P. radiata areas are likely to suffer some loss of stability following eachthinning operation, with the possibility of a greater reduction in stabilityoccurring after clearfalling.

(f) Grassland areas with a long history of grazing since clearing may have developedlower hydraulic conductivity values over time. Afforestation Of such areas maypresent a higher erosion risk at establishment than equivalent recently clearedeucalypt land because of increased and more frequent surface runoff.

(g) Cleared pastoral areas suffer widespread soil erosion problems in Australia.Afforestation of pasture lands generally may therefore result in a reduction insoil erosion overall, and improved water quality during much of the rotation.

(h) Slash burning during re-establishment of the plantation may result in increasedraindrop impact and surface runoff, particularly if the fire is hot and the soilhydrophobic, leaving the area vulnerable to erosion until adequate surfacerevegetation occurs.

Forestry Commission of New South Wales. Technical Paper, No. 49


(h) Slash burning during re-establishment of the plantation may result in increasedraindrop impact and surface runoff, particularly if the fire is hot and the soilhydrophobic, leaving the area vulnerable to erosion until adequate surfacerevegetation occurs.

(i) Fire, particularly a hot fire, has the potential to consume ground cover and litterin unprotected streamside buffer strips thereby considerably reducing theireffectiveness as an erosion mitigation measure,

(j) Any elevated streamwater nitrogen and phosphorus levels in pasture catchmentsresulting from a history of fertilizer application or introduced legumes arelikely to decline gradually when afforested byP. radiata.

(k) Hot slash burning, and windrow burning, may result in increased concentrationsand fluxes of ions such as nitrate and potassium in streamwater, the increasesdisappearing after a few years.

(1) Windrow burning in streams and drainage lines is likely to allow considerablequantities of ash to be washed into streams during rain, resulting in possibleshort term eutrophication of the stream and in a permanent loss of nutrientcapital from the site.

(m) Broadcast, rather than spot,applications of nitrogenous fertilizers to P. radiataplantations may reduce the likelihood of leaching losses to streams, at least insandy soils such as in South Australia where surface runoff is minimal.

(n) The aerial application of herbicides and pesticides to plantations of P. radiata isunlikely to effect streamwater quality if streams, streamside buffers and majordrainage lines all remain unsprayed. This requires proper control over dropletsize and the avoidance of windy days.

(0) Recreational activities and the associated provision of sewage disposal facilitiesare the greatest potential threats to the pollution of streamwater from elevatedbacteria levels in forested catchments.

(p) While the disposal of sewage sludge and effluent in forests generally does noteffect streamwater quality, care must be taken in planning to tailor disposalrates to the hydrologic characteristics of the particular site.



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