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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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Check Woody Herb Total
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Competition control (herbicides)
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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02468
1012141618
0 100 200 300
N rate (pounds per acre)
Volu
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resp
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(tons
per
acr
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Loblolly (+ 50 pounds per acre P)
Loblolly (no phosphorus)Slash (+ 50 pounds per acre P)
Slash (no phosphorus)
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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73
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
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ean
volu
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(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.
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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.
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