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Chapter 8.

The Evolution of

Pine Plantation Silviculturein the Southern United States

Thomas R. Fox, Eric J.Jokela, and H. Lee Allen1

AbstractIn the 1950s, vast acreages of cutoverforest land and degraded agricultural land existedin the South. Less than 2 million acres of southernpine plantations existed at that time. By the endof the 20th century, there were 32 million acres ofsouthern pine plantations in the Southern UnitedStates, and this region is now the woodbasketof the world. The success story that is southernpine forestry was facilitated by the application ofresearch results generated through cooperativework of the U.S. Department of Agriculture ForestService, southern forestry schools, State forestryagencies, and forest industry. This chapter reviewsthe contributions of applied silvicultural researchin land classification, tree improvement, nurserymanagement, site preparation, weed control, andfertilization to plantation forestry in the South.These practices significantly increased productivityof southern pine plantations. Plantations establishedin the 1950s and 1960s that produced < 90 cubicfeet per acre per year have been replaced byplantations established in the 1990s that areproducing > 400 cubic feet per acre per year.Southern pine plantations are currently amongthe most intensively managed forests in the world.Growth of plantations managed using modern,integrated, site-specific silvicultural regimes rivalsthat of plantations of fast-growing nonnativespecies in the Southern Hemisphere. Additionalgains in productivity are likely as clonal forestryis implemented in the South. Advances in forestbiotechnology will significantly increase growthand quality of future plantations. It appears likelythat the South will remain one of the majorwood-producing regions of the world.

INTRODUCTION

P ine (Pinus spp.) plantation silviculture in theSouthern United States is one of the majorsuccess stories for forestry in the world.In 1952, there were only 1.8 million acres of pineplantations in the South (fig. 8.1), containing 658million cubic feet of timber (U.S. Departmentof Agriculture, Forest Service 1988). At the turnof the 21st century, there are 32 million acres ofpine plantations in the South that contain 23.9billion cubic feet of timber (Wear and Greis 2002).Perhaps more remarkable is the significantincrease in productivity that occurred duringthis period (fig. 8.2). Mean annual increment ofpine plantations has more than doubled, androtation lengths have been cut by > 50 percent.The success of pine plantation silviculture hasturned the South into the woodbasket of theUnited States (Schultz 1997).

These remarkable changes in the last 60 yearswere the result of a variety of factors that cametogether at the end of World War II. Economicfactors, including a declining agricultural economycoupled with a rapidly expanding pulp and paperindustry based on southern pine, combined toprovide the impetus for the large increase insouthern pine plantations. The success of thiseffort was due in large part to the cooperativeresearch and technology transfer efforts of manyorganizations, including the U.S. Department ofAgriculture Forest Service (Forest Service), Stateforestry agencies, forestry programs at southernuniversities, and forest industry.

The objectives of this chapter are to describethe evolution of southern pine plantationsilviculture over the last 50 years and to outlineour view of the current state of the art of pineplantation silviculture in the South. Rather thanpresent an exhaustive review of the literature,

1 Associate Professor of Forestry, Virginia Polytechnic Instituteand State University, Department of Forestry, Blacksburg, VA24061; Professor of Forestry, University of Florida, School ofForest Resources and Conservation, Gainesville, FL 32611;and C.A. Schenck Distinguished Professor of Forestry, NorthCarolina State University, Department of Forestry, Raleigh,NC 27695, respectively.

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we will highlight what we believe are the majoradvances during the last 50 years and illustratetheir contribution to the productivity gains thathave been observed during this time (fig. 8.3). Aspart of this, we hope to demonstrate the significantcontributions that applied coop-erative researchhas made to this success story.

SETTING THE STAGE FOR PLANTATIONFORESTRY IN THE SOUTH

C learing of forests for crop production occurredthroughout the Coastal Plain and Piedmontfrom the colonial period until the beginningof the Civil War (Williams 1989). In Virginia > 25million acres, or 47 percent of the total land areain the State, had been cleared by 1860. Soil erosionwas a serious problem associated with production

of cotton and tobacco, which were the mostimportant agricultural crops throughout theSouth (Bennett 1939). Declining soil productivitydue to erosion, accompanied by low prices for cashcrops and pest problems such as the boll weevil(Anthonomus grandis grandis), caused largeamounts of agricultural land to be abandonedthroughout the South between the end of theCivil War and World War II.

The South has been an important source oftimber and forest products since colonial times(Williams 1989). Other than timber for local use,the first major products from southern forestswere naval stores from longleaf pine (P. palustrisMill.) and ship timbers from live oak (Quercusvirginiana Miller) (Butler 1998, Williams 1989).

Figure 8.1Number ofacres of pine plantationsin the Southern UnitedStates from 1952 to1999 (data from U.S.Department ofAgriculture 1988,Wear and Greis 2002).

Figure 8.2Estimatedtotal yield and pulpwoodrotation age in pineplantations in theSouthern United Statesfrom 1940 through 2010.

Figure 8.3Estimatedcontributions of intensivemanagement practices toproductivity in pineplantations in theSouthern United Statesfrom 1940 through 2010.

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The production of lumber in the South increasedgradually following the Civil War and moredramatically beginning in the 1880s and 1890s,when available timber in the Lake States wasdepleted. Between 1890 and 1920, the South wasthe major lumber-producing region in the country.Production peaked at approximately 140 billionboard feet in 1909, when the South produced 46percent of all timber cut in the United States(Williams 1989). After 1909, lumber productiondeclined gradually until the start of the GreatDepression in 1929, when production fell sharply.

The discovery by Charles Herty that acceptablepulp and paper could be made from southernpine had a dramatic impact on southern forestrybeginning in the 1930s (Reed 1995). A rapidincrease in the pulp production in the Southfollowed this discovery (Josephson and Hair 1956).Numerous pulp and paper mills were constructedthroughout the South during the 1930s, increasingthe demand for smaller diameter southern pinetimber. Pulp and paper companies purchased largetracts of timberland during this period to providepulpwood for these new facilities (Williams 1989).

At the start of the 20th century, almost no effortwas devoted to reforestation following timberharvest (Williams 1989). Destructive fires oftenfollowed logging, killing much of the naturalregeneration that might otherwise have becomeestablished on many cutover tracts. During the1920s, the Forest Service recognized the needfor large-scale tree planting in the South andbegan a research program to address reforestationissues. The first large-scale planting of southernpine occurred between 1920 and 1925 when theGreat Southern Lumber Company plantedapproximately 7,000 acres near Bogalusa, LA(Wakeley 1954). During the 1920s, the ForestService also began its reforestation program inthe South with the planting of 10,000 acres in theSumter National Forest in South Carolina. Duringthe 1930s, the Civilian Conservation Corps planted> 1.5 million acres across the South. The successof these early efforts demonstrated the feasibilityof establishing pine plantations.

THE ADVENT OF PLANTATION FORESTRY

A t the end of World War II, the legacy ofabusive agricultural practices that haddegraded soil productivity to the point wherecrop production was no longer profitable, coupledwith exploitative timber harvesting withoutprovision for regeneration, left the South with asubstantial acreage of land requiring reforestation.

Commenting on the situation in the 1950s,Wahlenberg (1960) stated, Much land suitablefor loblolly pine that has been made unproductivethrough heavy cutting, wildfire, naturalcatastrophe, or abandonment of agriculture is inneed of planting. Wakeley (1954) estimated thatthere were 13 million acres of land requiringplanting in the South in 1950.

Tree planting in the South, which had nearlyceased during World War II, rapidly increased inthe years immediately following the war (U.S.Department of Agriculture, Forest Service 1988).A large percentage of this planting occurred onfarmland associated with the Soil Bank Programof the 1950s. The successful reforestation ofabandoned and degraded agricultural landillustrated the conservation value of trees andtheir role in reducing soil erosion and improvingwater quality (Bennett 1939). The rapid expansionof the pulp and paper industry in the South duringthe 1930s increased the demand for pine pulpwoodand stimulated planting on forest industry land.By this time, the superior growth and yield ofpine plantations relative to naturally regeneratedstands had become evident. For example, theoriginal plantations established by Great SouthernLumber Company clearly showed the potentialvalue of fully stocked plantations compared to thepoorly stocked naturally regenerated stands thatwere the norm at the time (Wakeley 1954).

NURSERY PRACTICES ANDSEEDLING HANDLING

A rtificially regenerating the largeacreages found in the South required anabundant supply of high-quality seedlings. Aconcerted research effort of the Forest Service onreforestation in the South began in the 1920s andculminated with the publication of AgriculturalMonograph 18 Planting the Southern Pine(Wakeley 1954). This classic publication providedforesters detailed information on seed collectionand processing, seedling production, and plantingpractices needed to successfully establish southernpine plantations. With its publication, the stagewas set for the rapid expansion of southernpine seedling production. In 1950, the ForestService, the Soil Conservation Service, theTennessee Valley Authority, and all States inthe South operated forest nurseries to producepine seedlings for reforestation activities onpublic and private land (U.S. Department ofAgriculture, Forest Service 1949). Many industrialorganizations also began to establish or expandnurseries to meet their seedling needs at this time.

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Wakeley (1954) developed a widely usedgrading system for southern pine seedlings basedon seedling height, root-collar diameter, and stemand needle characteristics that were correlatedwith seedling survival. However, seedling survivalwas a continuing problem throughout the Southduring the 1950s, 1960s, and 1970s (Dierauf 1982).Although many of the factors affecting seedlingsurvival, such as weather, insects, and disease,were thought to be difficult to control, the problemreceived considerable attention because of therelative scarcity and high cost of geneticallyimproved seed. The formation of the AuburnSouthern Forest Nursery ManagementCooperative in 1970 highlights the importanceplaced on improving nursery practices andseedling quality. Root characteristics of seedlings,including root:shoot ratio and the number of first-order lateral roots, were demonstrated to beimportant factors affecting seedling performance(Carlson 1986). Improved nursery practices, suchas sowing seed by size class and single familygroups, reducing nursery bed density, top pruning,root pruning, increasing nitrogen (N) fertilization,and mycorrhizal inoculation, were incorporatedinto standard operating procedures at most pineseedling nurseries, substantially improving thesize and quality of the seedlings produced (Mexaland South 1991). Although seedling survival isstill probably best correlated with root-collardiameter (South 2000), physiological criteria suchas root growth potential were also developed tobetter evaluate seedling quality (Johnson andCline 1991). Proper care and handling of seedlingsduring lifting and transport to the planting sitewere found to be the critical factors ensuringinitial survival and growth of seedlings (Dierauf1982, U.S. Department of Agriculture 1989). Theuse of refrigerated vans for seedling storage andtransport, now widespread throughout the South,was probably the single most important factorin making certain that seedlings arrive at theplanting site in good condition. Improved survivaland growth also occurred when larger seedlingswere planted deeper and earlier in the season;i.e., prior to December (South 2000). Today,improved nursery practices, together withproper care and handling of seedlings duringtransport, storage, and planting, have increasedsurvival rates for planted seedlings to levelscommonly > 90 percent.

Tree Improvement and Genetic GainA major limitation on seedling production in

the 1950s was the absence of reliable supplies ofhigh-quality seed from desirable sources (Squillace

1989). Geographic variation in seed sources wasknown to affect growth of southern pine, with localsources outgrowing more distant sources (Wakeley1944). Therefore, use of local seed, collected within100 miles of the planting site, was recommendedfor reforestation (McCall 1939). At that time, mostseed was obtained from cones collected from treesfelled during logging of natural stands (Wakeley1954). In order to provide a more consistentsupply of cones, seed production areas were oftenestablished in natural stands containing goodphenotypes (Goddard 1958).

The seed orchard concept was proposed asearly as the late 1920s as means of producinggenetically improved seed (Bates 1928). The highcost of establishing and managing seed orchardswas initially a major obstacle to their widespreaduse (Perry and Wang 1958), because it was notwidely accepted that genetic improvement throughselection and breeding would lead to significantgains in the growth of southern pine (Wakeley1954). This view began to change in the 1950sas evidence supporting the value of geneticimprovement in forest trees started to emerge(Lindquist 1948, Schreiner 1950). The valueof genetically improved seed was finallyrecognized when it was demonstrated that thecosts associated with seed orchards could beeconomically justified (Perry and Wang 1958).Bruce Zobel, on behalf of the Texas ForestService and in cooperation with 14 forestproducts companies, formed the first treeimprovement program in the South (Zobel andTalbert 1984). The formation of this industry-university-Government applied researchcooperative was a major event in southern pineplantation forestry. The future success of southernpine plantation forestry was in large part a directresult of the applied research conducted throughcooperative programs at universities throughoutthe South. Additional tree improvement researchcooperatives were soon founded at the Universityof Florida in 1953 and North Carolina StateUniversity in 1956 (Southern Industrial ForestResearch Council 1999).

The seed orchard concept quickly gained favorand became the preferred method of producingsouthern pine seed (Zobel and others 1958). Thefirst southern pine seed orchard was establishedby the Texas Forest Service in 1952 to producedrought-hardy loblolly pine (P. taeda L.) (Zobel1953). Industrial members of the University ofFlorida Cooperative Forest Genetics ResearchProgram began establishing slash pine (P. elliottiiEngelm. var. elliottii) seed orchards in 1953 (Wang

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and Perry 1957). By 1987, > 9,700 acres of seedorchards had been established in the South, and> 85 percent of the trees planted in the Southoriginated from improved seed produced in seedorchards (Squillace 1989).

Tree improvement programs in the Southfocused primarily on improving volume growth,tree form, disease resistance, and wood quality(Dorman 1976, Zobel and Talbert 1984). Because ofthe length of time required for tree breeding andtesting, the gains in wood production due to treeimprovement were not fully realized for severaldecades (Todd and others 1995, Zobel and Talbert1984). Seed from first-generation seed orchardsbecame available in large quantities in the 1960sand early 1970s. When these plantations maturedin the 1980s, they produced 8 to 12 percent morevolume per acre at harvest than trees grownfrom wild seed (Squillace 1989). The increasedfinancial value of plantations established withfirst-generation improved seed probably exceeded20 percent when gains from other traits such asstem straightness, disease resistance, and wooddensity were included (fig. 8.4) (Todd and others1995). Continued breeding and testing led to thedevelopment of second-generation orchards in1980s. Second-generation seed orchards currentlyproduce more than 50 percent of the seed inthe South. It is estimated that volume growthin current plantations will be 14 to 23 percentgreater than in plantations established using first-generation material (fig. 8.4) (Li and others 1997).

MECHANICAL SITE PREPARATION

Before the 1950s, planting was generallylimited to old fields and grassy savannasthat originated on cutover sites followingfrequent wildfires. Most cutover pine sites in theSouth were regenerated after harvest by leavingsix to eight seed trees per acre (Duzan 1980).Unfortunately many of these stands failed toregenerate pine adequately due to competitionfrom hardwoods. The inconsistent resultsobtained with natural regeneration led to trialswith clearcutting and planting. Foresters facedconsiderable obstacles in their attempt to convertthese natural stands of mixed pine and hardwoodsto plantations after harvest. Lack of markets forlow-grade hardwoods often led to poor utilizationthat left large numbers of nonmerchantable stemsand heavy logging slash on the site. This inhibitedplanting and, coupled with the rapid regrowth ofhardwoods, led to poor survival and growth ofseedlings planted in the rough.

Initially, little site preparation was done becauseof the cost (Shoulders 1957). However, the needfor site preparation was highlighted by the failureof many plantations established on cutover sites,which was in stark contrast to the success ofplantations established on old agricultural fieldsand grassy savannas. The old-field effect onimproved survival and growth was attributed tovarious factors, including low levels of competinghardwood vegetation, improved soil physicalproperties, and improved soil fertility due toresidual fertilizer and lime. Therefore, the aimof site preparation was to re-create these old-field conditions on cutover sites using variousmechanical means such as anchor chaining,chopping, burning, root raking, shearing, anddisking. Mechanical site preparation practicesoften evolved more rapidly through trial anderror by field foresters and equipmentmanufacturers than through formal researchand development efforts.

The most consistent thread in thedevelopment of site preparation practiceson upland cutover sites in the South was theneed to control competing hardwood vegetation(Haines and others 1975). Roller-drum chopperswere introduced as a site preparation toolin the middle 1950s and quickly gainedpopularity. Chopping, especially when followedby prescribed fire, reduced logging slash andresidual nonmerchantable stems and, thus,improved access to the site for planting (Balmerand Little 1978). However, chopping did noteffectively control competing hardwoodvegetation. Disk harrows were first employedin the late 1950s to provide soil tillage similar tothat found in old fields and to control hardwoodsprouting. However, the level of hardwoodcontrol achieved following harrowing was often

Figure 8.4Growth increases in southern pineplantations due to tree improvement practices in theSouthern United States (adapted from Li and others1997, Todd and others 1995).

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disappointing (Duzan 1980). The intensity ofmechanical site preparation continued to increaseduring the 1960s and 1970s in pursuit of thedesired old-field conditions, culminating in thewidespread use of shearing, windrowing, andbroadcast disking as the standard practicethroughout much of the Piedmont and upperCoastal Plain (Haines and others 1975, Wells andCrutchfield 1974). Large bulldozers were used inthis three-pass system. Residual stems and stumpswere first sheared near the groundline using a KGblade. The slash and logging debris were rakedinto piles and windrows. Unless great care wastaken, the forest floor and topsoil were often rakedinto the piles and windrows along with the slash.The area was then broadcast disked with a largeharrow. In many cases, the windrows and pileswere then burned after the debris dried. Theimproved survival and early growth of seedlingsplanted on these intensively prepared sites,coupled with the greatly reduced hardwoodsprouting, suggested that foresters had finallyachieved the holy grail of site preparationturning cutover sites into old fields.

Foresters in the lower Coastal Plain faced adifferent set of problems than their counterpartsin the Piedmont. In addition to the concerns withthe control of competing vegetation, the presenceof poorly drained soils with high seasonal watertables greatly affected survival and growth ofplanted seedlings. The widespread conversion ofswamps into productive agricultural lands throughintensive drainage clearly demonstrated the valueof removing excess water from wet sites for cropproduction (Wooten and Jones 1955). The firstlarge-scale drainage project for forestry in theSouth occurred in the Hofmann Forest in easternNorth Carolina in the late 1930s. By the 1950s theimproved growth of loblolly and slash pine plantedadjacent to drainage canals was clearly evident(Maki 1960, Miller and Maki 1957, Schlaudt 1955).The phenomenal growth response of plantedpines following drainage reported in a numberof studies, ranging from 80 percent to almost1,300 percent (Terry and Hughes 1975), led to thewidespread drainage of forested wetlands in theAtlantic and Gulf Coastal Plain in the late 1960sand early 1970s. Large draglines were used toconstruct sophisticated drainage systems includingprimary, secondary, and third-stage ditches thatremoved excess water and, thus, improved access,reduced soil disturbance during harvesting, andimproved survival and growth of planted seedlings(Terry and Hughes 1978).

As on upland sites, reducing logging debrisand controlling competing hardwood vegetationwere major objectives of site preparation on wetsoils in the Coastal Plain. Chopping, burning,KG shearing, windrowing, and root-rakingpractices evolved much as they had on uplandsites. However, seasonally high water tables andflooding limited the survival and growth of plantedseedlings on poorly drained soils, even whenharrowing was combined with intensive debrisclearing (Cain 1978). Even on drained sites,reduced evapotranspiration rates in youngplantations led to extended periods when the soilswere saturated during the winter, which decreasedseedling survival and growth (Burton 1971).The improved growth of seedlings on elevatedmicrotopography with improved soil aeration(McKee and Shoulders 1970) led to thedevelopment of bedding in the Coastal Plain.The first bedding was done with fire plowsmodified to produce a raised planting site forseedlings (Bethume 1963, Smith 1966). Specializedbedding plows were introduced in the 1960s,and bedding soon became the standard sitepreparation practice on poorly drained soils,based on the superior growth observed on beddedcompared to flat-planted sites (McKee andShoulders 1974, Terry and Hughes 1975, Wellsand Crutchfield 1974). Because slash interfereswith bedding and decreases the quality and heightof the beds, intensive land clearing, often involvingKG shearing and windrowing, was usuallyconducted on sites requiring bedding to ensurethat quality beds were formed (Duzan 1980).Effective bedding treatments improved surfacesoil tillage and soil aeration, and reduced shrubcompetition. In some cases double bedding, usingtwo passes of the bedding plow, was required toachieve the conditions needed to ensure superiorsurvival and growth of planted seedlings.

CONCERN OVER SUSTAINABILITY ANDENVIRONMENTAL IMPACTS OF INTENSIVELYMANAGED PLANTATIONS

The intensity of site preparation conducted inboth the Piedmont and the Coastal Plain tosimulate old-field conditions soon generatedconcern about long-term site productivity. A reportby Keeves (1966) on second-rotation productivitydeclines in radiata pine (P. radiata D. Don) onintensively prepared sites in Australia, apparentlycaused by heavy windrowing, stimulated greatinterest in the South. Subsequent work withradiata pine in New Zealand confirmed that

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windrowing on sandy soils induced severe nutrientdeficiencies that would degrade site quality(Ballard 1978). Foresters throughout the Southobserved the wavy height growth pattern inwindrowed plantations where trees adjacent tothe windrows were considerably taller than treesbetween the windrows. A large windrow effecton growth of loblolly pine was documented in theNorth Carolina Piedmont (Fox and others 1989,Glass 1976). Windrowing decreased site index by11 feet in this loblolly pine plantation. As in NewZealand and Australia, it was demonstrated thatdeclines in growth observed on windrowed siteswere caused by nutrient deficiencies due todisplacement of the forest floor and topsoil fromthe interior of the stand to the windrows (Morrisand others 1983, Vitousek and Matson 1985).These observations led to the search foralternative, less intensive site preparationtreatments that would maintain site quality(Burger and Kluender 1982, Tippin 1978).

Nutrient losses associated with intensivewhole-tree harvesting also generated muchconcern during this period. Nutrient budgetcalculations seemed to suggest that whole-tree harvesting would deplete soil nutrientreserves, particularly such elements as calcium,and consequently degrade site quality (Ballardand Gessel 1983, Mann and others 1988).Numerous studies comparing conventional bole-only harvests with whole-tree harvests wereinstalled in response to this concern. Long-term analysis of these studies eventually revealedthat whole-tree harvesting had no detrimentaleffects on soil nutrient levels or site productivityon most sites if the slash and logging debris wereleft on site (Johnson and Todd 1998). Whereexcessive soil disturbance during harvest and sitepreparation did have negative effects, ameliorativetreatments such as soil tillage and fertilizationrestored productivity in nearly all cases (Fox 2000,Nambiar 1996).

Because long-term site productivity wasclosely tied with organic matter and N availability,harvesting and site preparation treatments weremodified during the 1980s to leave as much organicmatter on site as possible. The goal was to obtainthe amount of soil tillage required to achieveacceptable seedling survival while leaving mostof the logging slash and forest floor on site (Morrisand Lowery 1988). The link between improvedharvest utilization and site preparation led tomore integrated harvesting and site preparation

regimes (Burger and Kluender 1982). In thePiedmont, the desire to minimize soil disturbanceduring site preparation, concerns over nutrientlosses and long-term site productivity, andthe availability of newly developed herbicidesthat effectively controlled hardwood sproutscombined to shift most of the site preparationfrom mechanical to chemical treatments. In theCoastal Plain, mechanical treatments weremodified so that sites could still be bedded withlarger amounts of slash and logging debris lefton site. V-blades were developed that pushedaside logging debris and cleared a path forbedding plows without removing organic matterand nutrients from the site. Also, larger beddingplows were developed that cut through thick rootmats and residual slash while still creating well-formed beds that elevate seedlings above highwater tables, thus reducing the need forwindrowing on poorly drained sites.

The impacts of intensive forest managementon water quality have long been an importantissue in the South. The large amount of bare soilexposed following harvest and site preparationoften led to increased erosion on steeply slopingland in the Piedmont (Nutter and Douglass1978). The work of Coile and Schumaker (1964)demonstrated the correlation between topsoildepth and site quality in the Piedmont. Giventhe degraded site quality in most of the Piedmontcaused by the past agricultural practices,additional losses of topsoil by erosion followingharvest and site preparation were a concern.There were also concerns about the offsiteenvironmental impacts of intensive harvesting andsite preparation. Increased erosion and movementof sediment that increased turbidity in streamsbecame a major issue with the amendment of theFederal Water Pollution Control Act in 1972, whichfor the first time regulated forestry activities asnonpoint sources of pollution. Best ManagementPractices (BMP) were developed in all theSouthern States in response to this legislationto minimize soil erosion and offsite movementof sediment from forestry activities (Cubbageand others 1990). These BMPs have proven tobe very effective at reducing nonpoint sources ofpollution from forestry activities when properlyimplemented (Aust and others 1996). Althoughvoluntary in most States, compliance with BMPs isuniformly high today in forestry operations acrossthe South (Ellefson and others 2001).

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CONTROLLING COMPETING VEGETATION

The detrimental effects of hardwoodcompetition on growth and yield of southernpines were recognized from the earliestdays of plantation forestry (Cain and Mann1980, Clason 1978, Duzan 1980). One of the mainobjectives of site preparation was to create old-field conditions where hardwood competitionwas absent. However, chemical site preparationwas not widely used during this period, generallybecause the poor utilization during harvestrequired mechanical methods to provideacceptable access to the site (Lowery andGjerstad 1991). Unfortunately on most cutoversites, mechanical site preparation alone did noteffectively control hardwood sprouting. In theabsence of follow-up release treatments, manyplantations turned into low-quality hardwoodstands with scattered, poorly growing pines(Duzan 1980). During the 1960s and 1970s,2,4,5-T was widely used to release young pineplantations from competing hardwoods, becauseit was inexpensive to apply and effective on manyspecies of hardwoods, and pines were resistantto the herbicide (Lowry and Gjerstad 1991).

The registration of 2,4,5-T for forestry useswas cancelled in 1979. At that time, both hardwoodrelease treatments and chemical site preparationessentially ceased for a number of years in theSouth. However, concerns about sustainabilityof the long-term productivity of sites that wereintensively prepared mechanically, and concernsabout hardwood sprouting on less intensivelyprepared sites, fostered the search for herbicidesthat could replace 2,4,5-T (Fitzgerald 1982).The Auburn University Silvicultural HerbicideCooperative was formed in 1980 to identify andtest herbicides suitable for use in forestry.Numerous trials were established to evaluateherbicide efficacy and document the growthresponse of pines following herbicide application.

Several alternative herbicides such as glyphosate(Roundup, Accord), hexazinone (Velpar),imazapyr (Arsenal), sulfometuron methyl(Oust), and triclopyr (Garlon) were soonregistered for forestry uses. The newercompounds were more environmentally benign,with low mammalian and fish toxicity, rapiddegradation, and minimal offsite movement(Neary and others 1993). Hardwood controlin pine plantations using these newer herbicideswas generally more successful than previoustreatments with herbicides such as 2,4,5-T(Minogue and others 1991).

The use of herbicides for site preparationbegan to increase as results from studies of thenewer herbicides revealed that these compoundseffectively controlled hardwood sprouting(Fitzgerald 1982, Miller and others 1995) and,thus, increased pine growth (fig. 8.5). Chemicalsite preparation expanded rapidly when it wasdiscovered that similar or better growth occurredat a lower cost on chemically prepared sitescompared to mechanically prepared sites (Knoweand others 1992). By the 1990s, chemical sitepreparation had replaced mechanical sitepreparation on most upland sites (Lowery andGjerstad 1991) and is currently the dominantform of site preparation in the Piedmont andupper Coastal Plain.

Although the effect of hardwood competitionon pine growth was well documented (Cain andMann 1980, Clason 1978), the effect of herbaceousvegetation in young pine stands was not wellknown in the 1960s, because herbicides thateffectively controlled grasses and otherherbaceous vegetation without damaging pineseedlings were not available. However, mechanicalweeding experiments in young pine plantationsshowed that height growth of seedlings increasedsignificantly following control of grass andherbaceous vegetation (Terry and Hughes 1975).With the advent of newer herbicides such ashexazinone in the 1970s that effectively controlledherbaceous weeds without damaging young pineseedlings, large and consistent growth responsesfollowing herbicide applications were widelyobserved (Fitzgerald 1976, Holt and others 1973,Nelson and others 1981). By the late 1980s, itwas clear that herbaceous weed control had along-term impact on pine growth (fig. 8.5) (Gloverand others 1989, Smethurst and Nambiar 1989).Control of herbaceous weeds during the firstgrowing season was soon a widespread practicein pine plantations throughout the South (Minogueand others 1991).Figure 8.5Effect of competition control on growth of loblolly

pine at age 8 (adapted from Miller and others 1995).

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Slash

ACCELERATING GROWTH BY FERTILIZATION

Even though a considerable body of research onforest soil fertility, tree nutrition, and responseto fertilizers existed showing that growthincreases following fertilization were possible(Walker 1960), forest fertilization did not developas an operational silvicultural practice until the1960s. Operational deployment was hampered byan inability to accurately identify sites and standsthat consistently responded to fertilization. Amajor breakthrough occurred with the discoveryof spectacular growth responses in slash pinefollowing phosphorus (P) additions on poorlydrained clay soils, locally called gumbo clay, inthe flatwoods of Florida (Laird 1972, Pritchettand others 1961). Volume gains of up to 5 tons peracre per year over 15 to 20 years were observedon similar soils throughout the Coastal Plain(Jokela and others 1991a). The long-term growthresponse following P fertilization on these gumboclays translated into 5- to 15-foot increases in siteindex. When foresters learned to identify theseP-deficient sites and prescribe appropriatefertilizer applications, fertilization emerged asan operational treatment (Beers and Johnstono1974, Terry and Hughes 1975). Typically, optimalgrowth responses were achieved on these siteswhen approximately 50 pounds per acre ofelemental P was added at the time of planting(Jokela and others 1991a).

Results from fertilizer trials on other soiltypes in the Coastal Plain and Piedmont wereencouraging, but they remained somewhatinconsistent (Pritchett and Smith 1975). Thisinconsistency limited further expansion of forestfertilization programs. The Cooperative Researchin Forest Fertilization (CRIFF) Program at theUniversity of Florida and the North Carolina StateForest Fertilization Cooperative were formedin 1967 and 1970, respectively, to address thisproblem. Researchers in these two programsand the Forest Service worked to identify reliablediagnostic techniques to identify sites and standsthat responded to fertilization. Diagnostictechniques including soil classification, soil andfoliage testing, visual symptoms, and greenhouseand field trials were developed to help forestersdecide whether or not to fertilize (Comerfordand Fisher 1984; Wells and others 1973, 1986).The soil classification system developed by theCRIFF Program proved to be an effective toolfor determining the likelihood of obtaining aneconomic growth response following fertilizationand was adopted widely (Fisher and Garbett

1980). Critical foliar concentrations for N and Pwere identified for slash and loblolly pine thatwere well correlated with growth responsefollowing fertilization (Comerford and Fisher1984, Wells and others 1973).

Field trials conducted by both the NorthCarolina State Forest Fertilization Cooperativeand the CRIFF Program, initiated in the 1970sand 1980s, revealed that growth of most of theslash and loblolly pine plantations in the Southwere limited by the availability of both N andP (Allen 1987, Fisher and Garbett 1980, Gentand others 1986, Jokela and Stearns-Smith1993, North Carolina State Forest NutritionCooperative 1997). This work confirmed that alarge and consistent growth response followingmidrotation fertilization with N (150 to 200pounds per acre) and P (25 to 50 pounds per acre)occurred on the majority of soil types (fig. 8.6).Growth response following N plus P fertilizationaveraged 75 cubic feet per acre per year on poorlydrained soils and 69 cubic feet per acre per yearon well-drained soils, which represents a growthincrease of approximately 25 percent (NorthCarolina State Forest Nutrition Cooperative1997). These responses have typically lasted forat least 6 to 10 years, depending on soil type,fertilizer rates, and stand conditions. Basedon these results, the number of acres of southernpine plantations receiving midrotation fertilizationwith N and P increased from 15,000 acres annuallyin 1988 to approximately 975,000 acres in2000 (North Carolina State Forest NutritionCooperative 2001). By the end of 2000, > 11.1million acres of southern pine plantations hadbeen fertilized in the United States since 1969.

Figure 8.6Growth response of loblolly and slash pineon a variety of soil types following midrotation applicationof nitrogen (N) and phosphorus (P) fertilizer (adapted fromNorth Carolina State Forest Nutrition Cooperative 1997).

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DEVELOPMENT OF FORESTSITE CLASSIFICATION

S ite quality is perhaps the single mostimportant factor affecting growth and yieldof plantations. Merchantable yield tends toincrease in an exponential fashion as site qualityincreases. This relationship became moreimportant in the early 1950s as managementshifted from natural stands to plantations becausethe financial returns from the investment inplantation forestry were insufficient on poor-quality sites. Unfortunately in the early yearsof plantation forestry in the South, it was oftendifficult to assess the quality of many sitesscheduled for planting because they were cutoverand poorly stocked (Coile 1960). This led to a largeeffort in the 1950s and 1960s to correlate soilproperties, understory vegetation characteristics,geology, and landform with forest site quality(Carmean 1975). Soil properties such as drainageclass, depth to the subsoil, and texture of thetopsoil and subsoil were correlated with loblollyand slash pine site index (Barnes and Ralston1955, Coile and Schumaker 1964). The Coilesystem of land classification was widely used byindustrial landowners throughout the South toidentify and prioritize sites suitable for planting(Coile 1960, Thornton 1960).

The need for detailed soil information increasesas management practices become more intensive(Stone 1975). Growth responses to silviculturaltreatments such as drainage, site preparation,fertilization, thinning, and weed control werefound to be strongly affected by soil properties(Fox 1991). For example, growth response to Pfertilization was large and sustained on poorlydrained Ultisols in the lower Coastal Plain, butwas small and inconsistent on sandy Spodosolsin the same landscape (Comerford and others1983). Soil properties were also found to stronglyinfluence the efficacy and offsite movement ofherbicides, such as hexazinone, and had to betaken into account to develop appropriateprescriptions (Lowery and Gjerstad 1991).Equipment limitations and the potential forerosion, compaction, and puddling during harvestand site preparation were also affected by soiltype (Morris and Campbell 1991).

Specialized soil classification programswere developed to provide managers with theinformation needed to make silvicultural decisionsin intensively managed plantations. The CRIFFsystem was created to identify soils most likely tobe nutrient deficient based on soil morphologicalproperties (Fisher and Garbett 1980). Many

organizations initiated detailed soil mappingprograms to provide foresters with site-specificinformation on soil properties consideredimportant for intensive forest management(Campbell 1978, Thornton 1960). These soilsurveys were developed specifically for forestrypurposes and have generally proven more usefulthan the multipurpose soil maps produced by theNatural Resources Conservation Service (Morrisand Campbell 1991).

The development of sophisticated GeographicInformation Systems during the 1990s provideda powerful tool to assist with the implementationof intensive silvicultural regimes. Spatial analysisof the growth responses to silvicultural practiceson different soil types enables foresters to makedetailed silvicultural recommendations on a site-specific basis. Use of Global Positioning Systemsallowed foresters to very accurately determinetheir exact location. Armed with these tools,foresters are now able to make detailedsilvicultural prescriptions on a site-by-site basis.These site-specific prescriptions are a greatimprovement over the general recommendationspreviously used.

REALIZING THE GROWTH POTENTIALOF SOUTHERN PINE

When planted in the Southern Hemisphere,slash and loblolly pine were found to growsignificantly faster than in their naturalrange (Sedjo and Botkin 1997). Foresters in theSouth were puzzled by this phenomenon, andover the years numerous explanations were putforward to explain the observed differences ingrowth potential between the two regions. Forexample, climatic differences, especially lowernighttime temperatures leading to lowerrespiration rates, were often proposed asexplanations for the differences (Harms andothers 1994). In addition, diseases endemic tothe Southern United States, such as fusiform rust[Cronartium quercuum (Berk.) Miyabe ex Shiraif. sp. fusiforme (Hedge. & N. Hunt) Burdsall & G.Snow] and those caused by root pathogens, werenot found in the Southern Hemisphere.

It was also noted that plantation managementpractices in the Southern Hemisphere wereusually more intensive than those in the SouthernUnited States (Evans 1992). Complete removalof weeds, especially during the first few years ofthe rotation, was a standard practice. Fertilizerswere used to correct nutrient deficienciesthroughout the rotation. This was in contrastto the operational silvicultural practices used in

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the Southern United States through the 1980sthat focused on reducing costs per acre. Earlyherbicide applications, whether for chemicalsite preparation, herbaceous weed control, orhardwood release, usually did not completelycontrol competing vegetation. Even thoughgrowth response was found to be proportionalto the amount of competing vegetation controlled(Burkhart and Sprinz 1984, Liu and Burkhart1994), operational herbicide treatments wereusually based on application rates that achieveda threshold level of control at the lowest cost.Similarly, fertilization treatments were generallylimited to a single application during the rotationto minimize costs (Allen 1987). Perhaps moreimportantly, silvicultural treatments weregenerally applied as individual, isolated treatmentsrather than as part of an integrated system.Notable in this respect for many organizationswas the debate over the relative value of geneticimprovement and silvicultural treatments forincreasing stand productivity. In the SouthernHemisphere, it was recognized early on that toachieve high levels of productivity in southernpine plantations, genetics and silvicultural factorsmust be considered as equal components of anintegrated management system.

Several forward-looking research projectsestablished during the 1980s provided directevidence of the growth potential of intensivelymanaged southern pine within its nativerange. Most notable among these were studiesestablished by the Plantation ManagementResearch Cooperative at the University ofGeorgia and the Intensive Management PracticesAssessment Center at the University of Florida.

Empirical results from these studies demonstratedspectacular growth responses of both slash andloblolly pine following complete and sustainedweed control in combination with repeatedfertilization (Borders and Bailey 2001, Colbert andothers 1990, Neary and others 1990, Pienaar andShiver 1993). These results demonstrated that thegrowth potential of southern pines was not beingachieved in most operational plantations in theSouth, and that growth rates rivaling those in theSouthern Hemisphere could be achieved in theSouth through intensive management (table 8.1).

PREDICTING GROWTH AND YIELD INSOUTHERN PINE PLANTATIONS

Throughout the 1950s and early 1960s,forest managers were forced to rely onyield predictions developed for natural stands.Miscellaneous Publication 50 (U.S. Departmentof Agriculture, Forest Service 1929) was themost widely used source of southern pine volumepredictions at that time. However, it was soonapparent that stand growth and yield inplantations differed fundamentally from thatin natural stands. Growth-and-yield models forsouthern pine plantations began to appear inthe 1960s in response to the need for improvedgrowth-and-yield information (Bennett 1970,Bennett and others 1959, Burkhart 1971, Clutter1963, Coile and Schumaker 1964). Initially,plantation growth-and-yield models were whole-stand models that simply predicted current standyield (Bennett 1970, Bennett and others 1959).However, more sophisticated models were soondeveloped that were able to predict total yieldas well as the diameter distribution of the stand(Bennett and Clutter 1968, Burkhart and Strub1974, Smalley and Bailey 1974). These diameterdistribution models, although more complicatedand data intensive, proved to be substantiallymore useful tools for forest managers, becausevolume of specific products could be estimatedwhich provided a more accurate estimate ofstand value. In the 1970s, distance-dependentindividual-tree growth models were developedthat incorporated the effects of neighboringcompeting trees on growth (Daniels and Burkhart1975). Distance-dependent tree growth modelsshould provide better estimates of the impact ofsilvicultural practices such as thinning. However,it has generally been found that diameterdistribution models give results very similarto those of individual-tree growth models inmost cases with less effort and lower cost(Clutter and others 1983).

Table 8.1Growth rates of pines throughoutthe Worlda

Location Species Age MAI

years ft3/ac/yr

Costa Rica Pinus caribaea 8 449New Zealand P. radiata 25 457Brazil P. taeda 15 429South Africa P. taeda 22 412Australia P. taeda 20 302United States

Florida P. elliottii 20 207Georgia P. taeda 14 374

MAI = mean annual increment.a Data from Arnold (1995), Evans (1992), Borders and Bailey(2001), Yin and others (1998).

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Growth-and-yield research in the Southwas enhanced tremendously by the work of thePlantation Management Research Cooperativethat formed at the University of Georgia in 1976and the Virginia Polytechnic Institute and StateUniversity Growth and Yield Cooperative thatformed in 1979. These two programs haveproduced sophisticated and very accurate modelsof growth and yield in southern pine plantations.Models have been developed that accuratelypredict the impact of silvicultural practices suchas site preparation (Bailey and others 1982,Clutter and others 1984), thinning (Amateis andothers 1989, Cao and others 1982), fertilization(Amateis and others 2000, Bailey and others 1989),and the impact of hardwood competition on standstructure and yield (Burkhart and Sprinz 1984,Liu and Burkhart 1994). Modern growth-and-yield models, whether individual tree growthmodels or diameter distribution models, canaccurately predict stand-level timber productionin intensively managed pine plantations with aremarkable degree of precision (Pienaar andRheney 1995).

As plantations replaced natural stands,foresters strove to create a fully regulated forestthat optimized financial returns from the overallland base under management (Davis 1966). Theintroduction of linear programming as a forestplanning tool in the 1960s was a major advancein this effort (Chappelle 1966, Curtis 1962, Leak1964). Improvements in computers in the 1960smade it possible to use linear programmingtechniques to solve realistically sized forestharvest scheduling problems for the first time(Clutter and others 1983). The development ofthe MAX-MILLION linear programming-basedharvest scheduling program (Clutter 1968)fundamentally changed pine plantationmanagement throughout the South. For thefirst time organizations were able to use thistechnique to manage timberland in an organizedand quantitative manner that optimized thepresent value of future cash flows. Forestmanagers were also able to use these harvestplanning tools to predict the financial returnsfrom alternative silvicultural regimes thatimproved plantation growth. It was soon widelyrecognized that increased survival and growthof plantations resulting from improved genetics,site preparation, weed control, fertilization, anddensity management could significantly increasethe financial returns from forest management.This realization was the driving factor in thewidespread implementation of intensive

silviculture that occurred in the 1980s and1990s. The descendants of these original harvestscheduling models have been revised andimproved to the point where they are now able tosolve the extremely complex harvest schedulingproblems presented by the adjacency and harvestblock size restrictions now imposed on industrialplantations in the South (Van Deusen 1999).

CURRENT STATE-OF-THE-ART: INTEGRATED,SITE-SPECIFIC SILVICULTURE

Management of southern pine plantationsin the United States is being transformedfrom a relatively extensive system ofplanting coupled with isolated individualtreatments to a much more intensive system inwhich genetic and site resources are manipulatedin concert to optimize stand productivity.Heretofore, site quality was viewed as a staticproperty, and individual treatments were appliedin isolation with little understanding of theirinteractions and synergies. Today, management ismoving toward a more fully integrated approach inwhich improved genotypes are matched to specificsoil types, and silvicultural treatments, includingsite preparation, weed control, and fertilization,are integrated to maintain optimal water andnutrient availability throughout the rotation (Allenand others 1990, Neary and others 1990). Withthis approach, site quality is no longer fixed, butcan be improved greatly by proper management.

In the past, most silvicultural decisions werebased primarily on the results of empirical fieldtrials. An important feature of current state-of-the-art silvicultural regimes is that they arenow based on both empirical results and anunderstanding of the physiological processescontrolling forest productivity. It is now widelyrecognized, not only by research scientistsbut also by operational foresters, that forestproductivity is determined by the ability of theforest to capture incoming solar radiation andconvert it to stemwood biomass (Cannell 1989).Productivity of southern pine plantations is relatedto stand leaf area (fig. 8.7), which is controlledby the genetic potential of the trees and theavailability of light, water, and nutrients (McCradyand Jokela 1998, Vose and Allen 1991, Vose andothers 1994). Recent research has shown thatnutrient availability, rather than availabilityof light or water, most strongly affects leafarea development and, consequently, controlsproductivity on most sites in the South (Albaughand others 1998, Colbert and others 1990,Dalla-Tea and Jokela 1991).

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Leaf area index

Ann

ual v

olum

e gr

owth Nutrients

Water

Genotype

05

101520253035404550

0 10 20 30 40

Site mean volume (tons per acre)

Fam

ily m

ean

volu

me

(ton

s pe

r ac

re)

Increasing site quality

A

B

C

In intensively managed plantations, interactionsamong silvicultural treatments and genetics arenow recognized. There are also large differencesin growth efficiency among families of both loblollyand slash pine, and these differences can now beexploited to improve stand productivity (Li andothers 1991, McCrady and Jokela 1998, Samuelson2000). The combined effect on growth potentialthat results from the use of improved genotypesand intensive silviculture appears to be at leastadditive (McKeand and others 1997). Recentresults from progeny tests demonstrated thatthe growth of some better families increasedmore than the growth of poorer families as sitequality or silvicultural inputs, or both, increased(fig. 8.8). Foresters are now using this informationto deploy better genotypes to higher quality sitesthat will be managed more intensively.

Foresters now modify silvicultural practices totake advantage of interactions among treatmentsbased on a better understanding of their impactson site resource availability (Allen and others1999). As an example, both chemical sitepreparation and disking treatments are used tocontrol competing hardwoods. Although diskingalso improves soil physical properties, it is likelythat the combined growth response followingdisking coupled with herbicide treatment would beless than additive. Therefore, chemical treatmentsare now substituted for mechanical treatments onsites where hardwood competition is a severeproblem. In contrast, the growth responsefollowing fertilization coupled with herbicidecontrol of competing hardwoods might be morethan additive since hardwoods responding tofertilizer compete more vigorously with the pinecrop tree for light and water (Borders and Bailey2001, Swindel and others 1988). Weed control plusfertilization is the most widespread treatmentused to accelerate growth in pine plantations inthe South (Albaugh and others 1998, Colbert andothers 1990, Jokela and others 2000). Fertilizationregimes have been developed that enable forestersto match nutrient supply with the demand of thestand. Depending on the soil type, various typesand amounts of fertilizer may be added four ormore times during a 20-year rotation to augmentnative soil fertility and maintain high nutrientavailability. These fertilizer applications arecoordinated with site preparation treatmentsand weed control as needed during the rotation toameliorate soil physical limitations and eliminatecompetition for soil water and nutrients, thusinsuring optimal growing conditions for thedesignated crop trees throughout the rotation.

Current growth rates in intensively managedplantations in the South may exceed 350 cubicfeet per acre per year (Borders and Bailey 2001),which puts them on par with fast-growingplantations in other parts of the World (table 8.1).These intensively managed plantations offerlandowners attractive financial returns (Yin andSedjo 2001, Yin and others 1998). Although thecosts associated with intensive management arehigher, financial returns from such plantationsare higher because the growth rates are muchgreater and the rotation lengths shorter. Generalrealization of this fact is causing a paradigm shiftin the philosophy of forest landowners in theSouth. Current management of pine plantationsis moving away from the traditional focus onminimizing cost per acre to a new emphasis ondecreasing cost per ton of wood produced. Becausewood costs are usually the single largest cost inpulp, lumber, and engineered wood production,minimizing wood cost through intensivemanagement may be the best way for forestindustry in the South to remain competitive inglobal markets.

Figure 8.8Performance of loblolly pine families[identifications are (A) 0756, (B) 0859, and(C) 0164] as site quality increases (adaptedfrom McKeand and others 1997).

Figure 8.7Relationship between leaf area indexand growth rate in southern pine plantations.

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Tissue cultureplantlets

Clonalhedges

ClonaloutplantingsClones selected

Cryopreservation

Embryogenic tissue

Rootedcuttings

THE FUTURE: CLONAL FORESTRY ANDTHE PROMISE OF BIOTECHNOLOGY

Because of the continued increase in the worldspopulations, demand for forest products isincreasing, while large amounts of forestland are being lost to other land uses such asurbanization (Wear and Greis 2002) or degraded(Food and Agriculture Organization of the UnitedNations 1997). In addition, timber harvesting innative forests in many parts of the world is beingrestricted. The use of intensively managedplantations for timber production will have toincrease in the future to meet the increasingdemand for wood and fiber and still reservelarge areas of native forests for conservationand preservation uses (Sedjo and Botkin 1997).

Implementing integrated site-specificsilvicultural management regimes that optimizewater and nutrient availability throughout therotation will remain the paradigm of plantationforestry in the future. However, at some point thegrowth response to some silvicultural treatmentswill probably level off. Once a site is weed-free,no additional growth gains are likely fromadditional herbicide application until the weedsgrow back. Current management regimes areapproaching this level of competition controlin some plantations (Yin and others 1998).However, the future of fertilization may besomewhat different. As growth rates of foreststands increase, the demand for nutrients will alsoincrease. The nutrient supply in most forest soils isnot high enough to meet these increased demands.Current fertilization regimes focus on maintainingN and P supply. It is likely that as growth ratesand nutrient demand increase, deficiencies ofnutrients other than N and P will develop in theSouth as they have in other parts of the World(Evans 1992, Gonalves and Benedetti 2000, Jokelaand others 1991b, Will 1985). Fertilization regimesin the South will have to be modified to supplyboth macronutrients and micronutrients in amanner that matches nutrient demand of thestand throughout the rotation. Mechanistic modelsof soil nutrient supply, tree demand, and uptakeare being developed for southern pines so thatfertilizer regimes can be optimized for specificsoil types across the region. Significant growthincreases in the future are likely to occurfrom this more sophisticated managementof nutrient availability.

The potential gains in future plantationsthrough genetic manipulation of southern pineare large. At the turn of the 21st century, mostplantations were still planted with open-pollinated,

half-sib families. Many organizations are movingtoward the use of seed produced by controlledpollination of elite parents, because this canincrease growth significantly (fig. 8.4). Onedrawback of controlled pollination is the additionalexpense and time required to produce this seed.Consequently, the quantity of control-pollinatedseed now available is not sufficient to meetlarge-scale reforestation needs. To overcomethis obstacle, rooted cuttings are being used tomultiply the limited number of seedlings availablefrom controlled pollination (Foster and others2000). This technology is widely used in otherparts of the world with species such as radiata pineand eucalyptus (Eucalyptus spp.) and will soon beoperational with southern pine in the UnitedStates.

Clonal forestry holds the greatest promisefor increasing the productivity of southern pineplantations in the near term. Clonal forestryrelies on vegetative propagation proceduresto mass produce identical copies of selectedindividual trees that possess excellent geneticpotential (Gleed and others 1995). Clonaleucalyptus plantations are widely planted inthe Southern Hemisphere and have dramaticallyimproved productivity (Arnold 1995). Growthrates exceeding 600 cubic feet per acre peryear have been documented in clonal eucalyptusplantations in Brazil (Evans 1992). In addition,clones with specific wood properties have beendeveloped to optimize pulp production. Thetechnology to mass produce clones of southernpine is still under development and includes theuse of rooted cuttings and somatic embryogenesis.In the near term, it is likely that some combinationof somatic embryogenesis and rooted cuttingswill prove to be the most economical and efficientway to produce adequate numbers of southernpine clones (fig. 8.9). Based on results from clonal

Figure 8.9Integration of rooted cuttings and somaticembryogenesis into a clonal forestry program for southernpines in the United States.

77

plantations in other parts of the world, it willlikely be possible to increase productivity ofsouthern pine plantations by at least 50 percentby deploying appropriate clones to specific soiltypes and then implementing integrated, intensivesilvicultural regimes. Mean annual increments> 500 cubic feet per acre per year may soon bewithin our reach on selected sites in the South.

In the longer term, prospects for newdevelopments in forest biotechnology arebright. Research is revealing the genetic basisof disease resistance, wood formation, and growthin southern pine. Molecular markers are beingdeveloped that will substantially increase theefficiency of conventional tree breeding programsbecause they will no longer have to rely onphenotypic expression of desired traits in long-term field trials (Williams and Byram 2001). Theuse of molecular markers is particularly valuablewith complex traits that have low heritability,which is usually the case in southern pine.

Genetic engineering accomplished by directlyintroducing foreign DNA into trees has beenreported in a number of species, including radiatapine and hybrid poplar (Bauer 1997). The potentialfor this technology to dramatically improve woodproperties, disease resistance, and growth ratesof forest trees has been reported widely in boththe technical and popular press. Unfortunately,although the first successful transgenic trees wereproduced in the 1980s (Fillatti and others 1987),it remains difficult to produce transgenic trees,especially the southern pines. Numerous hurdlesremain to be overcome before the promise ofgenetic engineering in trees is fulfilled (Sederoff1999). Even with the concerted research effortscurrently underway in this area, it seems likelythat several decades will elapse before transgenictrees are a feature of operational southernpine plantations.

CONCLUSIONS

Management practices in southern pineplantations have undergone a dramaticevolution over the last 50 years. By applyingresearch results to operational plantations,foresters have more than doubled the productivityof operational southern pine plantations over thisperiod (fig. 8.3). For example, older managementpractices that produced plantations with growthrates of < 90 cubic feet per acre per year havebeen replaced by new practices that create standsthat are currently producing 400 cubic feet peracre per year on some sites. Pine plantations inthe South are among the most intensively

managed forests in the world (Schultz 1997).Site-specific, integrated management regimesthat incorporate the genetic gains available fromtree improvement along with silvicultural practicesthat optimize resource availability throughout therotation are now the norm. Growth rates in manypine plantations in the South are now approachingthose in the Southern Hemisphere. Additionalgains in productivity are likely as managementregimes are refined further. In the near term,implementation of clonal forestry holds thegreatest promise to dramatically increaseproductivity in southern pine plantations.As a result, the South is likely to remainthe woodbasket of the United States for theforeseeable future.

LITERATURE CITEDAlbaugh, T.J.; Allen, H.L.; Dougherty, P.M. [and others]. 1998.

Leaf area and above- and belowground growth responses ofloblolly pine to nutrient and water additions. Forest Science.44: 112.

Allen, H.L. 1987. Fertilizers: adding nutrients for enhancedforest productivity. Journal of Forestry. 85: 3746.

Allen, H.L.; Dougherty, P.M.; Campbell, R.G. 1990.Manipulation of water and nutrientspractices andopportunities in Southern U.S. pine forests. ForestEcology and Management. 30: 437453.

Allen, H.L.; Weir, R.J.; Goldfarb, B. 1999. Investing in woodproduction in southern pine plantation. In: Opportunitiesfor increasing fiber supply for the paper industry in theSouthern United States: a university perspective. Raleigh,NC: North Carolina State University: 3138.

Amateis, R.L.; Burkhart, H.E.; Walsh, T.A. 1989. Diameterincrement and survival equations for loblolly pine treesgrowing in thinned and unthinned plantations on cutover,site prepared lands. Southern Journal of Applied Forestry.13: 170174.

Amateis, R.L.; Liu, J.; Ducey, M.J.; Allen, H.L. 2000.Modeling response to midrotation nitrogen and phosphorusfertilization in loblolly pine plantations. Southern Journalof Applied Forestry. 24: 207212.

Arnold, R.B. 1995. Investment in fast-growing trees offersfuture wood procurement advantages. Pulp and Paper:135137.

Aust, W.M.; Shaffer, R.M.; Burger, J.A. 1996. Benefits andcosts of forestry best management practices in Virginia.Southern Journal of Applied Forestry. 20: 2329.

Bailey, R.L.; Burgan, T.M.; Jokela, E.J. 1989. Fertilizedmidrotation-aged slash pine plantationsstand structureand yield projection models. Southern Journal of AppliedForestry. 13: 7680.

Bailey, R.L.; Pienaar, L.V.; Shiver, B.D.; Rheney, J.W. 1982.Stand structure and yield of site-prepared slash pineplantations. Res. Bull. 291. Athens, GA: University ofGeorgia, College of Agriculture Experiment Station. 83 p.

Cha

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8.

Pin

e Pl

anta

tion

Silv

icul

ture

Sout

hern

For

est

Scie

nce:

Past

, Pre

sent

, and

Fut

ure

Prod

ucti

vity

78

Ballard, R. 1978. Effects of slash and soil removal on theproductivity of second rotation radiata pine on a pumice soil.New Zealand Journal of Forest Science. 8: 248258.

Ballard, R.; Gessel, S.P., eds. 1983. I.U.F.R.O. symposium onforest site and continuous productivity. Gen. Tech. Rep.PNW163. Portland, OR: U.S. Department of Agriculture,Forest Service, Pacific Northwest Research Station.[Number of pages unknown].

Balmer, W.E.; Little, N.G. 1978. Site preparation methods.In: Tippin, T., ed. Proceedings: a symposium on principlesof maintaining productivity on prepared sites. Atlanta:U.S. Department of Agriculture, Forest Service,Southeastern Area State and Private Forestry: 6064.

Barnes, R.L.; Ralston, C.W. 1955. Soil factors related to growthand yield of slash pine plantations in Florida. Florida Agric.Exp. Tech. Bull. 559. [Place of publication unknown]:[Publisher unknown]. 23 p.

Bates, G.G. 1928. Tree seed farms. Journal of Forestry.26: 969977.

Bauer, L.S. 1997. Fiber farming with insecticidal tree. Journalof Forestry. 95: 2023.

Beers, W.L., Jr.; Johnstono, H.E. 1974. Intensive culture ofslash pine. In: Williston, H.L.; Balmer, W.E., eds.Proceedings of a symposium on management of young pines.Atlanta: U.S. Department of Agriculture, Forest Service:201211.

Bennett, F.A. 1970. Yields and stand structural patterns for old-field plantations of slash pine. Res. Pap. SE60. Asheville,NC: U.S. Department of Agriculture, Forest Service,Southeastern Forest Experiment Station. 81 p.

Bennett, F.A.; Clutter, J.L. 1968. Multiple-product yieldestimates for unthinned slash pine plantationspulpwood,sawtimber, gum. Res. Pap. SE35. Asheville, NC: U.S.Department of Agriculture, Forest Service, SoutheasternForest Experiment Station. 21 p.

Bennett, F.A.; McGee, C.E.; Clutter, J.L. 1959. Yields of old-field slash pine plantations. Stn. Pap. 107. Asheville, NC:U.S. Department of Agriculture, Forest Service,Southeastern Forest Experiment Station. 19 p.

Bennett, H.H. 1939. Soil conservation. New York: McGraw-HillBook Co., Inc. 993 p.

Bethume, J.E. 1963. Ridging: low-cost techniques forincreasing growth on wet flatwoods sites. Forest Farmer.23(3): 68, 18.

Borders, B.E.; Bailey, R.L. 2001. Loblolly pinepushing thelimits of growth. Southern Journal of Applied Forestry.25: 6974.

Burger, J.A.; Kluender, R.A. 1982. Site preparationPiedmont.In: Symposium on the loblolly pine ecosystem (east region).Raleigh, NC: North Carolina State University: 5874.

Burkhart, H.E. 1971. Slash pine plantation yield estimatesbased on diameter distributions: an evaluation. ForestScience. 17: 452453.

Burkhart, H.E.; Sprinz, P.T. 1984. A model for assessinghardwood competition effects on yields of loblolly pineplantations. FWS384. Blacksburg, VA: VirginiaPolytechnic Institute and State University, Schoolof Forestry and Wildlife Resources. [Number ofpages unknown].

Burkhart, H.E.; Strub, M.R. 1974. A model for simulation ofplanted loblolly pine stands. In: Fries, J., ed. Growth modelsfor tree and stand simulation. Stockholm, Sweden: RoyalCollege of Forestry: 128135.

Burton, J.D. 1971. Prolonged flooding inhibits growth ofloblolly pine seedlings. Res. Note SO124. New Orleans:U.S. Department of Agriculture, Forest Service, SouthernForest Experiment Station. 4 p.

Butler, C.B. 1998. Treasures of the longleaf pines naval stores.Shalimar, FL: Tarkel Press. 269 p.

Cain, M.D. 1978. Planted loblolly and slash pine response tobedding and flat disking on a poorly drained sitean update.Res. Note SO237. New Orleans: U.S. Department ofAgriculture, Forest Service, Southern Forest ExperimentStation. 6 p.

Cain, M.D.; Mann, W.F., Jr. 1980. Annual brush controlincreases early growth of loblolly pine. Southern Journalof Applied Forestry. 4: 6770.

Campbell, R.G. 1978. The Weyerhaeuser land classificationsystem. In: Balmer, W.E., ed. Proceedings of the soilmoisture-site productivity symposium. Atlanta: U.S.Department of Agriculture, Forest Service, SoutheasternArea, State and Private Forestry: 7482.

Cannell, M.G.R. 1989. Physiological basis of wood production:a review. Scandanavian Journal of Forest Research.4: 459490.

Cao, Q.V.; Burkhart, H.E.; Lemin, R.C., Jr. 1982. Diameterdistributions and yields of thinned loblolly pine plantations.FWS182. Blacksburg, VA: Virginia Polytechnic Instituteand State University, School of Forestry and WildlifeResources. 62 p.

Carlson, W.C. 1986. Root system considerations in the qualityof loblolly pine seedlings. Southern Journal of AppliedForestry. 10: 8792.

Carmean, W.H. 1975. Forest site quality evaluation in theUnited States. Advances in Agronomy. 27: 209269.

Chappelle, D.E. 1966. A computer program for schedulingallowable cut using either area or volume regulation duringsequential planning periods. Res. Pap. PNW33. Portland,OR: U.S. Department of Agriculture, Forest Service,Pacific Northwest Forest Experiment Station. [Numberof pages unknown].

Clason, T.R. 1978. Removal of hardwood vegetation increasesgrowth and yield of a young loblolly pine stand. SouthernJournal of Applied Forestry. 2: 9697.

Clutter, J.L. 1963. Compatible growth and yield modelsfor loblolly pine. Forest Science. 9: 354371.

Clutter, J.L. 1968. MAX-MILLIONa computerized forestmanagement planning system. Athens, GA: Universityof Georgia, School of Forest Resources. [Not paged].

Clutter, J.L.; Fortson, J.C.; Pienaar, L.V. [and others]. 1983.Timber management: a quantitative approach. New York:John Wiley. 333 p.

Clutter, J.L.; Harms, W.R.; Brister, G.H.; Rheney, J.W. 1984.Stand structure and yields of site prepared loblolly pineplantations in the Lower Coastal Plain of the Carolinas,Georgia, and north Florida. Gen. Tech. Rep. SE27.Asheville, NC: U.S. Department of Agriculture, ForestService, Southeastern Forest Experiment Station. 173 p.

79

Coile, T.S. 1960. Summary of soil-site evaluation. In: Burns,P.D., ed. Southern forest soils. Baton Rouge, LA: LouisianaState University: 7785.

Coile, T.S.; Schumaker, F.X. 1964. Soil-site relations, standstructure, and yields of slash and loblolly pine plantationsin the Southern United States. Durham, NC: T.S. Coile, Inc.296 p.

Colbert, S.R.; Jokela, E.J.; Neary, D.G. 1990. Effects of annualfertilization and sustained weed control on dry matterpartitioning, leaf area, and growth efficiency of juvenileloblolly and slash pine. Forest Science. 36: 9951014.

Comerford, N.B.; Fisher, R.F. 1984. Using foliar analysis toclassify nitrogen-responsive sites. Soil Science Society ofAmerica Journal. 48: 910913.

Comerford, N.B.; Fisher, R.F.; Pritchett, W.L. 1983. Advancesin forest fertilization on the Southeastern Coastal Plain. In:Ballard, R.; Gessel, S.P., eds. I.U.F.R.O. symposium on forestsite and continuous productivity. Gen. Tech. Rep. PNW163.Portland, OR: U.S. Department of Agriculture, ForestService, Pacific Northwest Forest Experiment Station:370378.

Cubbage, F.W.; Kirkman, L.K.; Boring, L.R. [and others].1990. Federal legislation and wetland protection in Georgia:legal foundations, classification schemes, and industryimplications. Forest Ecology and Management. 33/34:271286.

Curtis, F.H. 1962. Linear programming the managementof a forest property. Journal of Forestry. 60: 611616.

Dalla-Tea, F.; Jokela, E.J. 1991. Needlefall, canopy lightinterception, and productivity of young, intensively managedslash and loblolly pine stands. Forest Science. 37: 12981313.

Daniels, R.F.; Burkhart, H.E. 1975. Simulation of individualtree growth and stand development in managed loblollypine plantations. FWS575. Blacksburg, VA: VirginiaPolytechnic Institute and State University, Divisionof Forestry and Wildlife Resources. [Number ofpages unknown].

Davis, K.P. 1966. Forest management: regulation and valuation.2d ed. New York: McGraw Hill Book Co. 519 p.

Dierauf, T.A. 1982. Planting loblolly pine. In: Proceedings ofthe symposium on the loblolly pine ecosystem (east region).Raleigh, NC: North Carolina State University: 24135.

Dorman, K.W. 1976. The genetics of breeding southern pines.Agric. Handb. 471. Washington, DC: U.S. Department ofAgriculture, Forest Service. [Number of pages unknown].

Duzan, H.W., Sr. 1980. Site preparation techniques for artificialregeneration. In: Mann, J.W., ed. Proceedings of a seminaron site preparation and regeneration management. Clemson,SC: Clemson University: 5463.

Ellefson, P.V.; Kilgore, M.A.; Phillips, M.J. 2001. Monitoringcompliance with BMPs. The experience of State forestryagencies. Journal of Forestry. 98: 1117.

Evans, J. 1992. Plantation forestry in the tropics. 2d ed. Oxford:Clarendon Press. 403 p.

Fillatti, J.J.; Sellmer, J.; McCown, B. [and others]. 1987.Agrobacterium mediated transformation and regenerationof Populus. Molecular and General Genetics. 206: 192199.

Fisher, R.F.; Garbett, W.S. 1980. Response of semimature slashand loblolly pine plantations to fertilization with nitrogen andphosphorus. Soil Science Society of America Journal. 44:850854.

Fitzgerald, C.H. 1976. Post-emergence effects of Velparin a Piedmont pine plantation. Proceedings of the SouthernWeed and Science Society of America. 29: 299.

Fitzgerald, C.H. 1982. Chemical site preparation and releasein loblolly pine stands. In: Symposium on the loblolly pineecosystem (east region). Raleigh, NC: North Carolina StateUniversity: 7585.

Food and Agriculture Organization of the United Nations.1997. State of the worlds forests1997. Rome. 465 p.

Foster, G.S.; Stelzer, H.E.; McRae, J.B. 2000. Loblolly pinecutting morphological traits: effects on rooting and fieldperformance. New Forests. 19: 291306.

Fox, T.R. 1991. The role of ecological land classification systemsin the silvicultural decision process. In: Mengel, D.L.; Tew,D.T., eds. Ecological land classification: applications toidentify the productive potential of southern forests. Gen.Tech. Rep SE68. Asheville, NC: U.S. Department ofAgriculture, Forest Service, Southeastern ForestExperiment Station: 96101.

Fox, T.R. 2000. Sustained productivity in intensively managedforest plantations. Forest Ecology and Management. 138:187202.

Fox, T.R.; Morris, L.A.; Maimone, R.A. 1989. The impact ofwindrowing on the productivity of a rotation age loblollypine plantation. In: Miller, J.H., ed. Proceedings of the fifthbiennial southern silvicultural research conference. Gen.Tech. Rep. SO74. New Orleans: U.S. Department ofAgriculture, Forest Service, Southern Forest ExperimentStation: 133140.

Gent, J.A., Jr.; Allen, H.L.; Campbell, R.G.; Wells, C.G.1986. Magnitude, duration, and economic analysis of loblollypine growth response following bedding and phosphorusfertilization. Southern Journal of Applied Forestry. 10:124128.

Glass, G.R. 1976. The effects from rootraking on an uplandPiedmont loblolly pine (Pinus taeda L.) site. Tech. Rep. 56.Raleigh, NC: North Carolina State Forest FertilizationCooperative. 44 p.

Gleed, J.A.; Darling, D.; Muschamp, B.A.; Nairn, B.J. 1995.Commercial production of tissue cultured Pinus radiata.Tappi Journal. 78(9):147150.

Glover, G.R.; Creighton, J.; Gjerstad, D.H. 1989. Herbaceousweed control increases loblolly pine growth. Journal ofForestry. 87: 4750.

Goddard, R.E. 1958. Additional data on the cost of collectingcones from a seed production areas. Journal of Forestry.56: 846848.

Gonalves, J.L.; Benedetti, V., eds. 2000. Nutriaoe fertilizaaoflorestal. Piracicaba, SP, Brazil: Instituto de Pesquisas eEstudos Florestais. 427 p. In Portuguese.

Haines, L.H.; Maki, T.E.; Sandeford, S.G. 1975. The effect ofmechanical site preparation treatments on soil productivityand tree (Pinus taeda L. and P. elliottii Engelm. var.elliottii) growth. In: Bernier, B.; Winget, C.H., eds. Forestsoils and forest land management. Quebec: Les Presses deLUniversite Laval: 379396.

Harms, W.R.; DeBell, D.S.; Whitesell, C.D. 1994. Standand tree characteristics and stockability in Pinus taedaplantations in Hawaii and South Carolina. CanadianJournal of Forest Research. 24: 511521.

Cha

pter

8.

Pin

e Pl

anta

tion

Silv

icul

ture

Sout

hern

For

est

Scie

nce:

Past

, Pre

sent

, and

Fut

ure

Prod

ucti

vity

80

Holt, H.A.; Voeller, J.E.; Young, J.F. 1973. Vegetation controlin newly established pine plantations. Proceedings of theSouthern Weed Science Society of America. 26: 294.

Johnson, D.W.; Todd, D.E., Jr. 1998. Harvesting effects on long-term changes in nutrient pools of mixed oak forests. SoilScience Society of America Journal. 62: 17251735.

Johnson, J.D.; Cline, M.L. 1991. Seedling quality of southernpines. In: Duryea, M.L.; Dougherty, P.M, eds. Forestregeneration manual. Dordrecht, The Netherlands: KluwerAcademic Publishers: 143159.

Jokela, E.J.; Allen, H.L.; McFee, W.W. 1991a. Fertilization ofsouthern pines at establishment. In: Duryea, M.L.;Dougherty, P.M., eds. Forest regeneration manual.Dordrecht, The Netherlands: Kluwer Academic Publishers:263-277.

Jokela, E.J.; McFee, W.W.; Stone, E.L. 1991b. Micronutrientdeficiency in slash pine: response and persistence of addedmanganese. Soil Science Society of America Journal. 55:492496.

Jokela, E.J.; Stearns-Smith, S.C. 1993. Fertilization ofestablished southern pine stands: effects of single and splitnitrogen treatments. Southern Journal of Applied Forestry.17: 135138.

Jokela, E.J.; Wilson, D.S.; Allen, J.E. 2000. Early growthresponses of slash and loblolly pines following fertilizationand herbaceous weed control treatments at establishment.Southern Journal of Applied Forestry. 24: 2330.

Josephson, H.R.; Hair, D. 1956. The United States. In:Haden-Guest, S.; Wright, J.K.; Teclaff, E.M., eds. A worldgeography of forest resources. New York: The Ronald PressCo.: 149200.

Keeves, A. 1966. Some evidence of loss of productivity withsuccessive rotations of Pinus radiata in the south-east ofSouth Australia. Australian Forests. 30: 5163.

Knowe, S.A.; Shiver, B.D.; Kline, W.N. 1992. Fourth-yearresponse of loblolly pine following chemical and mechanicalsite preparation in the Georgia Piedmont. Southern Journalof Applied Forestry. 16: 99105.

Laird, C.R. 1972. Fertilization of slash pine on poorly drainedsoils in northwest Florida. Circ. 378. Gainesville, FL:University of Florida, Florida Cooperative ExtensionService. [Number of pages unknown].

Leak, W.B. 1964. Estimating maximum allowable timberyields by linear programming. Res. Pap. NE17. Broomall,PA: U.S. Department of Agriculture, Forest Service,Northeastern Forest Experiment Station. [Numberof pages unknown].

Li, B.; McKeand, S.E.; Allen, H.L. 1991. Genetic variationin nitrogen use efficiency of loblolly pine seedlings. ForestScience. 37: 613626.

Li, B.; McKeand, S.E.; Hatcher, A.V.; Weir, R.J. 1997. Geneticgains of second generation selections from the NorthCarolina State University-Industry Cooperative TreeImprovement Program. In: Proceedings of the 24th southerntree improvement conference. [Place of publicationunknown]: [Publisher unknown]: 234238.

Lindquist, B. 1948. Forstgenetik in der SchwedischenWalbaupraxis (forest genetics in Swedish forestry practice).Berlin: Neuman Verlag Radebeul. [Not paged].

Liu, J.; Burkhart, H.E. 1994. Modelling inter- and intra-specificcompetition in loblolly pine (Pinus taeda L.) plantations oncutover, site-prepared lands. Annals of Botany. 73: 429435.

Lowery, R.F.; Gjerstad, D.H. 1991. Chemical and mechanicalsite preparation. In: Duryea, M.L.; Dougherty, P.M., eds.Forest regeneration manual. Dordrecht , The Netherlands:Kluwer Academic Publishers: 251261.

Maki, T.E. 1960. Improving site quality by wet-land drainage.In: Burns, P.Y., ed. Eighth annual forestry symposium:southern forest soils. Baton Rouge, LA: Louisiana StateUniversity Press: 106114.

Mann, L.K.; Johnson, D.W.; West, D.C. [and others]. 1988.Effects of whole-tree and stem-only clearcutting onpostharvest hydrologic losses, nutrient capital and regrowth.Forest Science. 34: 412428.

McCall, M.A. 1939. Forest seed policy of the U.S. Departmentof Agriculture. Journal of Forestry. 37: 820821.

McCrady, R.L.; Jokela, E.J. 1998. Canopy dynamics, lightinterception, and radiation use efficiency of selected loblollypine families. Forest Science. 44: 6472.

McKeand, S.E.; Crook, R.; Allen, H.L. 1997. Genetic stabilityon predicted family responses to silvicultural treatments inloblolly pine. Southern Journal of Applied Forestry. 21:8489.

McKee, W.H., Jr.; Shoulders, E. 1970. Depth of water tableand redox potential of soil affect slash pine growth. ForestScience. 16: 399402.

McKee, W.H., Jr.; Shoulders, E. 1974. Slash pine biomassresponse to site preparation and soil properties. SoilScience Society of America Proceedings. 38: 144148.

Mexal, J.G.; South, D.B. 1991. Bareroot seedling culture. In:Duryea, M.L.; Dougherty, P.M., eds. Forest regenerationmanual. Dordrecht, The Netherlands: Kluwer AcademicPublishers: 89115.

Miller, J.H.; Zutter, B.R.; Zedaker, S.M. [and others]. 1995.A regional framework of early growth response for loblollypine relative to herbaceous, woody, and complete competitioncontrol: the COMProject. Gen. Tech. Rep. SO117. NewOrleans: U.S. Department of Agriculture, Forest Service,Southern Forest Experiment Station. 48 p.

Miller, W.D.; Maki, T.E. 1957. Planting pine in pocosins.Journal of Forestry. 55: 659663.

Minogue, P.J.; Cantrell, R.L.; Griswold, H.C. 1991. Vegetationmanagement after plantation establishment. In: Duryea,M.L.; Dougherty, P.M., eds. Forest regeneration manual.Dordrecht , The Netherlands: Kluwer Academic Publishers:335358.

Morris, L.A.; Campbell, R.G. 1991. Soil and site potential.In: Duryea, M.L.; Dougherty, P.M., eds. Forest regenerationmanual. Dordrecht, The Netherlands: Kluwer AcademicPublishers: 183206.

Morris, L.A.; Lowery, R.F. 1988. Influence of site preparationon soil conditions affecting stand establishment and treegrowth. Southern Journal of Applied Forestry. 12: 170178.

Morris, L.A.; Pritchett, W.L.; Swindel, B.F. 1983. Displacementof nutrients into windrows during site preparation of aflatwoods forest soil. Soil Science Society of AmericaJournal. 47: 951954.

Nambiar, E.K.S. 1996. Sustained productivity of forestsis a continuing challenge to soil science. Soil ScienceSociety of America Journal. 60: 1629164