Plant and Soil217: 39–47, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Root strength, growth, and rates of decay: root reinforcement changes oftwo tree species and their contribution to slope stability

Alex Watson1,∗, Chris Phillips1 and Michael Marden21Landcare Research, P.O. Box 69, Lincoln, Canterbury, New Zealand and2Landcare Research, P.O. Box 445,Gisborne, New Zealand

Key words:root decay, root growth, root reinforcement, root site-occupancy, root strength, slope stability

Abstract

Information on live root-wood strength, rates of root decay and root growth of both radiata pine (Pinus radiataD.Don) and kanuka (Kunzea ericoides(A. Rich.) Joy Thomps. var.ericoides) are combined to form a generalizedconceptual model of changes in nett root reinforcement. The model provides an initial opportunity to rank theplant species having specific below-ground rooting habits that can be used to control erosion, and when linkedwith extreme flood probability can be used to indicate the risk of a storm likely to cause slope instability in theperiod between clear-felling and regrowth. Erosion-susceptible slopes planted 1 year after clearfelling in radiatapine at 1250 stems ha−1 regain root site-occupancy in 4.7 years, an interval during which there is an 80% chanceof experiencing an extreme flood. Similarly for radiata planted at 800 and 400 stems ha−1, root site-occupancyis regained in 5.6 and 7.5 years, and the probability of occurrence of an extreme event within these periods is 85and 90%, respectively. For erosion-susceptible slopes on which kanuka has become established, the probabilityof a significant event within the 2.8 years prior to root site-occupancy is 60%. Slopes felled of radiata pine arepotentially more vulnerable to the stresses promoting slope instability, at least in the earlier years.

Introduction

Information on rates of root growth, live root woodstrength, rates of decay, and site-occupancy has beenobtained from ongoing studies over the last decade orso, which have been aimed at improving our under-standing of the role vegetation plays in contributing toslope stability through root reinforcement (Ekanayakeet al., 1997; Marden et al., 1991; O’Loughlin andWatson, 1979; Phillips and Watson, 1994; Watson andO’Loughlin, 1990; Watson et al., 1995, 1997). Stud-ies involving root strength loss after clear-felling andrates of root growth of replacement vegetation indicateinadequate root–soil reinforcement for a period of upto 6 years after logging. This roughly corresponds tothe observed period of increased frequency of slopeinstability.

The rate at which a tree planting site is occupied byroots depends on the lateral root growth rate and theplanting density. Root site-occupancy is based on the

∗ FAX No: + 64 3 325 2418. E-mail: watsona@landcare.cri.nz

use of these parameters to estimate the length of timerequired to obtain root system overlap within the uppersoil layers, and is taken as a theoretical starting pointfor plantation forests to begin contributing to slope sta-bility. The concept projects the tree root system ontothe ground as a circle. Trees, i.e. circles, are grownat the selected planting density, increasing circle dia-meter over time until root site-occupancy is attained.At site-occupancy the nett root reinforcement of thesoil by roots, i.e. the sum of the reinforcement by theresidual decaying roots of the cut trees and that by thenew roots of the regenerating trees, is considered to be100%. If the nett root reinforcement is less than 100%,areas devoid of roots will remain between adjacenttrees.

Much of the hill country of the East Cape regionof the North Island of New Zealand is prone to ser-ious erosion. The original podocarp-hardwood forestwas cleared to create farmland during the latter halfof the nineteenth century. Increased erosion, coupledwith productivity decline following 70–100 years ofintensive grazing, has resulted in several cycles of

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Figure 1. A generalized representation of a part-forested slope. The forces resisting failure depend on the soil strength plus the additional soilreinforcement supplied by root soil cohesion. The forces promoting failure depend on the weight of soil and trees, slope angle, and wind force.Diagram modified from Wu and Swanston (1980).

abandonment and natural reversion over much of thearea.

Over recent decades, large areas of farmland havereverted to indigenous scrub, in which Kanuka (Kun-zea ericoides(A. Rich.) Joy Thomps. var.ericoides)forms the major component. These areas have eitherbeen re-cleared for continued pastoral developmentor are now being considered for forestry conversion.The former situation has often been associated withan increase in the frequency of translational landslides(Figure 1).

This region also has a history of extreme floodswhich, when combined with the inherently unstablegeology and the original forest removal, have been ma-jor contributors to erosion of the East Cape hill coun-try. In response to a particularly severe event in 1988,the New Zealand Government established a scheme

to convert marginal pastoral farmland in the moreerosion-susceptible areas to radiata pine (Pinus radi-ata D. Don) dominated plantation forestry (Ministryof Forestry, 1994).

On the erosion-prone hill country in the East Caperegion, the recommended minimum initial plantingdensity for radiata pine is 1250 stems ha−1. Overrecent years there has been some concern at an in-creasing trend towards lower planting densities, duein part to genetically improved and therefore more ex-pensive planting material, and increased labour costs.The stand density and the size of the individual rootsystems are major factors that influence the degree towhich trees enhance slope stability. The rate at whichtrees contribute to stability is largely dependent on thetime taken to reach site-occupancy, the magnitude ofthe root–soil reinforcement, and their modification of

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the soil-water regime by evapotranspiration. At lowerplanting densities trees provide a much reduced levelof stability for several years after planting, as theirability to promote the stronger, drier soils needed forslope stabilization is seriously diminished. If the treesare removed by clear-felling, the beneficial effects ofevapotranspiration are lost immediately, and thereafterslope stability is likely to decline at a rate largelydetermined by the magnitude of the live root-woodstrength and the rate at which this strength is lost afterthe death of the parent trees.

In this paper we present data on live root-woodstrength, rates of root decay and root growth of bothradiata pine and kanuka. A comparative model ofrates of root reinforcement changes that occur betweenclear-felling and regrowth of vegetation is also presen-ted. The implications for slope stability in relationto the probability of an extreme storm event occur-ring on slopes covered by an undisturbed naturallyregenerating tree species (kanuka) and three plantingregimes of a plantation forestry species (radiata pine)are evaluated and discussed.

Site characteristics and sources of data

Radiata pine root samples used to generate liveroot/root decay information (O’Loughlin and Watson,1979) were collected from trees growing on dissectedhill country, approximately 30 km north of Christ-church (43◦32′S, 172◦35′E), New Zealand. Here theannual rainfall is 774 mm (Ashley Forest, Station No.H32252, altitude 107 m) (Tomlinson and Sansom,1994).

The balance of the data were collected from twostudy sites in the Gisborne region, east of the Rauku-mara Range in the East Cape hill country. The radiatapine stands were at Mangatu Forest in the headwatersof the Waipaoa and Mangatu Rivers. Mean annualrainfall at the forest headquarters is 1350 mm (NZMet. Service, 1984). The kanuka stands were grow-ing north of Gisborne (38◦30′S, 178◦04′E) on poorlyutilized farmland that, in part, had been allowed torevert to indigenous scrub. Mean annual rainfall at thissite is 1409 mm. Descriptions of the topography andsoils for the radiata pine and kanuka sites are availablein Watson and O’Loughlin (1990) and Watson et al.(1997), respectively.

The climate of the East Cape is temperate marine,with warm dry summers and cool wet winters. Ex-treme floods, generally resulting from high-intensity

rainfall during tropical cyclones have been a predom-inant factor contributing to the unstable nature of thehill country east of the Raukumara Range. Althoughsuch storms are relatively infrequent, there were fourmajor events during the 1980s, the largest of whichoccurred in March 1988, when up to 900 mm fell within 5 days. In all there have been 29 extreme floods ofthe Waipaoa River (catchment area 1580 km2) duringthis century (Kelliher et al., 1995).

Methods

Root strength and decay rates

Tensile strength of individual tree roots, collectedfrom live trees or stumps of known cutting date, isthe most common parameter measured to determinelive root strength and rates of root strength loss withdecay (O’Loughlin and Watson, 1979). Root samplesfrom selected trees were sealed in plastic bags and,when necessary, kept in cool storage to preserve root-moisture content prior to testing. Detailed descriptionof sampling techniques and laboratory testing are de-scribed in Watson et al. (1997). Briefly, the root isclamped at both ends in the testing device and the res-istance to an applied force necessary to break it undertension is taken as the measure of the tensile strength.

Rates of root growth

Root growth information comes from two larger stud-ies designed to investigate temporal changes in rootmorphology and biomass of radiata pine (Watson andO’Loughlin, 1990) and kanuka (Watson et al., 1995).A total of 28 root systems, 13 radiata pine and 15kanuka, were excavated using high-pressure water.The technique required selection of trees growing onslopes steep enough to allow an easy flow of sluicedmaterial away from the excavation site. Sluicing beganfar enough downhill from the stump to ensure that asthe excavation proceeded up slope it was of sufficientdepth to expose the deeper vertical roots. As the rootsystems were exposed, they were propped in positionand their architecture mapped and measured (Table 1).

The excavated radiata pine were taken from threeeven-aged planted stands. Age classes for kanuka weredetermined by taking the mean of the annual ringcounts of the excavated trees within each stand.

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Table 1. Site parameters, age, number and size of sampled trees and dimensions of their root systems. Figures in parentheses represent thestandard error of means

Radiata pine Kanuka

Mean age (years) 8 (0) 16 (0) 25 (0) 6 (0.7) 16 (1.4) 32 (2.7)

Stand density(a) (stems/ha−1) 240 250 270 16 000 12 800 3900

No. of trees 5 5 3 5 5 5

Mean tree height (m) 9.5 (–) 21.1 (–) 30.4 (–) 6.1 (0.5) 6.7 (0.3) 13.6 (0.3)

Mean DBH(b) (mm) 170 (10.7) 400 (25.9) 550 (27.7) 45 (4.4) 66 (7.2) 127 (15.3)

Mean max. root length (m) 3.5 (0.3) 4.2 (0.5) 9.1 (1.0) 1.5 (0.2) 3.0 (0.4) 3.6 (0.7)

Max. root length (m) 4.7 6.4 10.4 1.9 4.5 6.1

Mean max root depth (m) 1.8 (0.1) 2.4 (0.1) 2.9 (0.1) 1.3 (0.3) 1.6 (0.2) 1.3 (0.1)

Max. root depth (m) 2.1 2.6 3.1 2.2 2.1 1.5

aData for radiata pine are final stand densities and are from different stands.bDBH=Diameter at Breast Height (height measured to 1.4 m on up hill side of tree).

Figure 2. The relationship of time (years) between extreme floods of the Waipaoa River at Kanakanaia (discharge>1500 m3 s−1) and thechance of their occurrence. Analysis is based on time interval between 29 extreme floods this century. The step function represents fieldobservations, the curve gives their expected chance of occurrence as represented by the cumulative exponential probability function describedin Kelliher et al. (1995).

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Landslide risk estimation

The analysis of landslide risk is based on the proposalthat extreme river floods provide a surrogate for severerain storms associated with slope instability. The pro-posal is supported by historical landslide data from theWaipaoa catchment. In this approach, the catchmentarea of the Waipaoa (1580 km2), a major regionalriver flowing southward from the Raukumara Range,was treated as a large areal rain gauge (Kelliher etal., 1995). Extreme flood events were defined as hav-ing discharge rates exceeding the average annual flow(15 m3 s−1) by 2 orders of magnitude. In all, therehave been 29 events since 1900. A standard probab-ility analysis was used to produce a relationship oftime between extreme floods and the risk of their oc-currence (Figure 2). As all data were used to definethe probability distribution function, independent errorestimates could not be computed.

Results and discussion

Root strength and rates of decay

The mean maximum live tensile strength of 188small-diameter (1.3–13.9 mm) radiata pine roots was17.6±0.9 MPa at a mean diameter of 5.3±0.2 mm(Table 2). The range in tensile strength was 7.7–37.5MPa (O’Loughlin and Watson, 1979). After felling theparent trees, the root-wood tensile strength declined onaverage 5.9 MPa per year (Figure 3).

The mean maximum live tensile strength of 64small-diameter (1.1–13.5 mm) kanuka roots was32.5±3.2 MPa at a mean diameter of 4.7±0.3 mm(Table 2). The range in tensile strength was 15.5–80.0 MPa. Mean tensile root-wood strength increasedby 33% to reach a maximum of 43.1±2.8 MPa, 12months after the cutting of the parent trees, and did notfall below the mean live root-wood strength until about24 months after tree felling (Watson et al., 1997). Themean rate of loss of tensile strength of kanuka root-wood over the 3-year period after attaining maximumtensile strength, was 9.2 MPa per year (Figure 3).

The initial increase in kanuka root tensile strengthand the delay in the onset of root decay is discussed inWatson et al. (1997). To summarize, loss of root-woodmoisture after felling is assumed to be accompaniedby a decrease in root diameter. As tensile strengthis a function of the inverse square of the radius, asmall decrease in root diameter can give an appre-ciable increase in the calculated root tensile strength.

The length of time between the death of the parenttree and the onset of root decay is species dependent.In radiata pine, a soft-wood species, the time will beshort, possibly a few weeks (O’Loughlin and Watson,1979), but in more durable timbers it will be muchgreater. For kanuka, the time between felling and theonset of root decay is about 12 months (Figure 3).

Root growth and site-occupancy

In the 8 years following planting, the mean lateralgrowth rate of radiata pine roots was 0.44 m year−1

(Table 1). For trees planted at 1250 stems ha−1 (arecommended slope stabilization regime), 800 stemsha−1 (a commercial forestry regime) and 400 stemsha−1 (an agro-forestry regime), root site-occupancy atthis average root growth rate would occur in 3.7, 4.6,and 6.5 years, respectively.

Uniform canopy heights are a feature of kanukastands and indicate a relatively short colonization orestablishment period. The average lateral growth rateof kanuka roots during the first 6 years was 0.25 myear−1 (Table 1), and at a natural establishment dens-ity of 16 000 stems ha−1 site-occupancy by the rootsystems would be achieved in 1.8 years. Although ka-nuka root systems are considerably smaller than thoseof radiata pine, the difference in individual biomass ismore than compensated for by the higher kanuka standdensity.

Information on root strength and rates of decaywhen combined with root growth data enable the con-struction of a generalized conceptual model of thechanges in nett root reinforcement provided by ka-nuka and radiata pine following felling through theearly phase of the subsequent planting cycle (Fig-ure 4). Nett root reinforcement is assumed to attaina maximum value at root site-occupancy. If tree re-planting is delayed, say 1 year, nett root reinforcementof both tree species is at a minimum between 2 and2.5 years after felling. The root reinforcement by ra-diata pine, established at 1250, 800, and 400 stemsha−1, will decline by between 70 and 80% before theroots of the replacement trees start to contribute tosoil strength. As the onset of decay of kanuka roots isdelayed 1 year (Figure 3), the maximum decline in nettroot reinforcement after felling is about 25%. Slopesfelled of radiata pine are potentially more vulnerableto the stresses promoting slope instability, as theirlower root contribution to soil strength indicates slopefailure could be triggered by storm events of lower

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Table 2. Root-wood strength and root diameter of radiata pine and kanuka. Figures in parentheses represent the standard error of means

Months since Radiata pine Kanuka

cutting Mean tensile Mean root Mean tensile Mean root

strength (MPa) diameter (mm) strength (MPa) diameter (mm)

[n] [n]

0 17.62 (0.46) 5.3 [188], (0.2) 32.45 (1.59) 4.7 [64], (0.3)

3 14.37 (0.54) 5.6 [105], (0.2) – –

6 – – 36.36 (1.36) 5.6 [74], (0.4)

9 12.31 (0.56) 6.2 [134], (0.2) – –

12 – – 43.13 (1.42) 5.9 [80], (0.3)

14 11.04 (0.39) 6.8 [140], (0.2) – –

18 – – 34.94 (1.56) 5.2 [89], (0.3)

24 – – 31.64 (1.38) 6.1 [100], (0.3)

29 3.33 (0.47) 8.3 [51], (0.5) – –

30 – – 28.30 (1.41) 6.1 [79], (0.3)

36 – – 24.20 (1.67) 7.0 [67], (0.4)

42 – – 19.15 (1.04) 5.3 [68], (0.2)

48 – – 15.53 (0.94) 6.3 [87], (0.2)

Figure 3. Plot of mean maximum tensile strength against time elapsed since tree felling for kanuka roots<20 mm diameter and radiata pineroots<18 mm diameter.

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Figure 4. Comparative model of relative root reinforcement changes after clear felling (at year 0) and planting (at year 1) of kanuka and radiatapine. Growth curve (1) represents an initial natural kanuka establishment density of 16 000 stems ha−1. Growth curves (2), (3) and (4) representinitial radiata pine planting densities of 1250 stem ha−1 (a recommended slope stabilisation regime), 800 stems ha−1 (a commercial forestryregime), and 400 stems ha−1 (an agro-forestry regime), respectively.

rainfall duration and intensity than those required tofail a slope cleared of kanuka.

If we now assume that the probability of an ex-treme flood on the Waipaoa River can be used toindicate the likelihood of a storm to cause slope in-stability, the curve in Figure 2 suggests that there isabout a 30% chance of at least one extreme eventevery year (observed data suggests 25%) and a greaterthan 99% chance every 10 years. Erosion-susceptibleslopes replanted 1 year after felling in radiata pineat 1250 stems ha−1 regain root site-occupancy in4.7 years, an interval during which there is an 80%chance of experiencing an extreme flood. Similarly

for radiata replanted at 800 and 400 stems ha−1,root site-occupancy is regained in 5.6 and 7.5 yearsduring which the probability of occurrence of an ex-treme event is 85 and 90%, respectively. For erosion-susceptible slopes on which kanuka has become es-tablished, the probability of a significant event withinthe 2.8 years prior to root site-occupancy is 60%.Although the difference in time that either specieswould leave a slope vulnerable to a major storm isbetween 2 and 5 years, for slopes growing radiatapine at the planting densities suggested, the increasein probability of such an event happening is in theorder of 20–30%. This increase in risk probability ap-

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proximates the 12-month delay in the onset of kanukaroot decay – a delay which may in part account forthe greater level of stability provided by kanuka onclearcut slopes.

Conclusions

The general understanding of how vegetation con-tributes to slope stability is relatively well advanced.However, a detailed appreciation of the parametersthat could be used in selecting species that have spe-cific below-ground qualities that can be used to controlshallow mass movement is not as well developed. InNew Zealand, the choice of plant species for erosioncontrol are often chosen by default. Species selectionis frequently decided on features such as availabil-ity, drought tolerance, altitudinal range, or perceivedbelow-ground characteristics based on above-groundgrowth, rather than on their performance attributes interms of stabilization. Our conceptual model providesan initial opportunity to rank plant species that havespecific below-ground rooting habits that can be usedto control erosion. In particular, as the model allowsthe use of more detailed information on below-groundplant characteristics coupled with an opportunity tomanipulate planting density, it opens the possibility fora more cost-effective targetting of plant materials forlandslide control and prevention.

The underlying objective of this paper was to ex-plore the concept of root site-occupancy in relation tostand density and by using a simple model and a floodfrequency probability curve, give forest managers anappreciation of the risk involved in leaving erosion-prone slopes without the benefit of adequate rootreinforcement; in another words, supply a workabletool that tended more to the practical than academic. Itwas not our intention to provide a rigorous risk predic-tion analysis. To be able to estimate the risk to±10%was considered a reasonable goal, given the diversityof input parameters.

Many studies have shown that tree roots improvesoil strength and slope stability (O’Loughlin andZiemer, 1982; Phillips et al., 1990; Sidle 1991; Wuand Erb, 1988), and many models have also beendeveloped to assess vegetation effects on slope stabil-ity. A limited number of models allow comparisonsbetween vegetation types or management regimes.Of these, the most recent group tend to be distrib-uted physically based models that can be coupledwith GIS (Sidle and Wu, in press). However, sev-

eral non-distributed and time-oriented models havebeen developed to evaluate effects of timber harvest-ing or vegetation management on landslide occurrence(Benda and Zhang, 1990; Sidle, 1992; Ziemer et al.,1991). Our approach has been similar to the lattergroup. This study demonstrates that our conceptualmodel when coupled with data on root strength, rootdecay, and root growth is useful for evaluating veget-ation conversion or clearance issues on unstable hillcountry such as found in the East Cape region of NewZealand.

Acknowledgements

We thank our colleagues Dr Jagath Ekanayake andDonna Rowan for assistance during the data collectionphases of the various projects upon which this paperis based; and the Wood Technology Dept., Schoolof Forestry, Canterbury University for the generoususe of their Instron Universal Testing Machine. Thisstudy was funded by the Foundation for Research,Science and Technology (Contracts C09406, C09606,C09807).

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