Chapter 7 · 2014. 10. 14. · Chapter 7 Uneven-Aged Silviculture of Longleaf Pine James M. Guldin...

Chapter 7

Uneven-Aged Silviculture ofLongleaf Pine

James M. Guldin

Introduction

The use of uneven-aged silviculture has in-creased markedly in the past 20 years. This isespecially true in the southern United States,where the use of clearcutting and planting isoften viewed as a practice whose emphasis onfiber production results in unacceptable con-sequences for other values, such as those thatbenefit from maintenance of continuous for-est cover over time. Public lands in general,and national forest lands in particular, havebecome the focal point for the replacementof clearcutting and planting with even-agedand uneven-aged reproduction cutting meth-ods that rely on natural regeneration, and thatcan better achieve management goals that aredefined by residual stand structure and condi-tion rather than by harvested volume.

Land managers in the southern UnitedStates are keenly interested in a renaissancefor longleaf pine (Pinus palustris Mill.) (Landerset al. 1995; Barnett 1999). Of the four south-ern pines, longleaf pine has experienced thegreatest percentage loss in forest area, from37 million hectares prior to European colo-nization to approximately 2.2% of that cur-rently (Frost this volume). That scarcity has in-

James M. Guldin � Arkansas Forestry Sciences Laboratory, Southern Research Station, USDA Forest Service,Monticello, Arkansas 71656.

creased the ecological value of the stands thatremain. The scattered tracts of remnant un-managed longleaf pine stands have high emo-tional and physical appeal. Managed stands oflongleaf pine, especially those in which pre-scribed fire has been regularly applied, pro-vide exceptional values for endangered speciessuch as the red-cockaded woodpecker (Pi-coides borealis Vieillot), game species such asbobwhite quail (Colinus virginianus L.), andfire-dependent species such as wiregrass (pri-marily Aristida beyrichiana Trin. & Rupr.). Thentoo, many foresters fondly recall the memoryof the exceptional quality of lumber that ma-ture stands of longleaf pine were, and are, ca-pable of producing.

The perception exists that many of thesevalues can be provided by management oflongleaf pine especially through the use ofuneven-aged silviculture. In public debates,this may be supported by little other thanthe layperson’s view that uneven-aged silvi-culture is the opposite of clearcutting, andthus innately has something to recommendit. Foresters have been a bit more reluctantto wholly embrace the application of uneven-aged silviculture in longleaf pine, citing amongother reasons the intolerance to shade of the

217

218 III. Silviculture

species, the difficulty in obtaining natural re-generation, and the cost.

These factors may in part explain why thefocus of discussion regarding uneven-aged sil-viculture in southern pines is especially promi-nent on public lands such as national and stateforests, and private lands managed as gameplantations. These ownership entities share anumber of attributes, including a diversity ofownership objectives, and the capability, di-rectly or indirectly, to subsidize timber pro-duction with other resource values. For ex-ample, the National Forests of Florida havemade a commitment to manage longleaf pineusing both even-aged and uneven-aged sys-tems. While this is admirable from the per-spective of using a diversity of reproductioncutting methods to meet a diversity of forestmanagement objectives, the proposed scale ofthe practice may outstrip the research that sup-ports widespread application.

There is no reason to suspect that the prin-ciples of uneven-aged silviculture cannot besuccessfully adapted to longleaf pine standsin the lower Atlantic and Gulf Coastal Plains.The method has been successfully applied overtime in other southern pines—most notably,mixed loblolly (P. taeda L.)–shortleaf (P. echinataMill.) pine stands in the upper West GulfCoastal Plain (Baker et al. 1996). A review ofthat history of success and failure will be ofvalue in providing perspective regarding theapplication of the method in longleaf pine.

Uneven-aged silviculture has been success-ful in different forest types (Guldin 1996). Suc-cess with the method depends on the ability toobtain regeneration of the desired species, andto have that regeneration develop into mer-chantable size classes. Conversely, failures withuneven-aged silviculture are typically associ-ated with an inability to obtain desired regen-eration (Guldin 1996; Guldin and Baker 1998).There is good reason to expect that the detailsof regeneration establishment and develop-ment under an uneven-aged system in longleafpine stands will be difficult, if the experienceassociated with the development of the shel-terwood method in longleaf pine (Croker andBoyer 1975) is any indication. There is anec-dotal evidence to suggest that uneven-aged

silvicultural prescriptions can be successful inlongleaf pine stands (Farrar and Boyer 1991;Farrar 1996; Moser et al. 2002). There is alsoconsiderable debate about the implications ofhabitat quality for red-cockaded woodpeckersin uneven-aged longleaf pine stands (e.g., En-gstrom et al. 1996; Rudolph and Conner 1996).In view of the current situation, the oppor-tunity to develop and refine the applicationof uneven-aged silviculture in longleaf pine istimely.

This review of the selection method and ofthe principles that underlie its application forlongleaf pine is based on another southernpine species in which experience has been suc-cessful over a long period of time—specifically,naturally regenerated stands of loblolly pinewith a minor and varying proportion of short-leaf pine in the upper West Gulf CoastalPlain in southern Arkansas. Following thatoverview, thoughts about the application andmodification of the method to longleaf pinewill be discussed in detail.

Definitions and Concepts

The goal of any silvicultural system is to ad-vantageously utilize the resources in a givenstand for social benefit through the emulationof natural processes of succession and distur-bance. Helms (1998) defines a silvicultural sys-tem as a planned series of treatments for tend-ing, harvesting, and reestablishing a stand, anda regeneration method as a cutting procedureby which a new age class is created withinthe stand. Smith (1986) also makes this dis-tinction, using the term “reproduction cuttingmethod” instead of “regeneration method.”These terms, “regeneration method” and “sil-vicultural system,” are commonly misappliedin two ways. The first is that they are oftenmistakenly used interchangeably. The formerrefers to the short period of time during whicha new age cohort of regeneration of the desiredspecies is obtained, and the latter refers to theentire program of treatments for the life of thestand. The second is that they are often mis-takenly applied to a forest rather than to theindividual stand, which is their intended scope

7. Uneven-Aged Silviculture 219

(Smith 1986; Helms 1998). The confusion isin part because the treatment prescriptions inthe silvicultural system are closely related tostand structure at any given point in time, andstand structure is established primarily usingthe reproduction cutting method at the pointof stand or cohort establishment.

The choice of regeneration method deter-mines the scale of disturbance that foresterscan imitate (Brockway et al. this volume).Even-aged regeneration methods such asclearcutting, the seed tree method, or the shel-terwood method are designed to emulate vary-ing intensities of stand-replacing disturbanceevents, resulting in a new cohort of regen-eration across the entire stand. Unevenagedregeneration methods such as the selectionmethod are designed to emulate a small-scalewithin-stand disturbance event, resulting in anew age class only in that subset of the standwhere the practice was imposed. In eithereven-aged or uneven-aged methods, the mainindication of success in the execution of a re-generation method is whether a new standof trees is successfully obtained to replace thetrees that were removed during the harvest.Thus, the reproduction cutting method is theprimary element of the silvicultural system inthat the actions that comprise the silvicultural

Sta

nd b

iom

ass

Rotation length r

Reg

ener

atio

n cu

tting

Site

pre

para

tion

Rel

ease

trea

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tmen

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(e.g

., th

inni

ng)

Reg

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atio

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tting

in n

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otat

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FIGURE 1. Chronosequential ap-plication of individual practicesof a silvicultural system in even-aged stand. During the rotationage r, treatments are appliedacross the entire stand to meetsilvicultural objectives that arerelated to tree age.

system depend upon the origin of the regener-ation, its age distribution, and its spatial distri-bution over the stand.

The choice of regeneration method also hasan inordinate influence on the overall courseof silvicultural treatments and the way thosetreatments are imposed in the stand (Fig. 1).In an even-aged stand, the sequence of sil-vicultural practices depends upon stand age.The new stand is obtained using a regenera-tion method that results in a new cohort of re-generation across the entire stand. Subsequenttreatments such as site preparation in advanceof the new cohort, release of that cohort, andintermediate treatments such as thinning alsooccur across the entire stand. Each treatment isimposed in a manner that is correlated with theage of the new cohort of regeneration. Eventu-ally, when the stand reaches maturity, a newreproduction cutting method is implementedat the rotation age r , which gives rise to a sub-sequent stand managed with a subsequent sil-vicultural system.

Conversely, in an uneven-aged stand, thereis no rotation age r . Instead, treatments arebased on a cutting cycle c , the basic inter-val of stand entry, which varies from 5 to 20years. Cutting cycle harvests are imposed ineach stand every c years. However, the trees


Sta

nd

bio

ma

ss

C1 C2 C3

Cutting cycles C1 − C3

Regeneration cutting

Site preparation

Release treatments

Intermediate treatments(e.g., thinning)

FIGURE 2. Concurrent applica-tion of individual practices of anuneven-aged silvicultural systemduring a cutting cycle harvest ina balanced uneven-aged stand.Treatments are applied to sub-units of the stand depending onconditions within each subunit.Each cutting cycle harvest willsupport similar treatments.

removed during a cutting cycle harvest aretaken for different reasons—regeneration cut-ting, release, or intermediate treatment such asthinning (Fig. 2). As a result, the different sil-vicultural practices that occur chronosequen-tially in an even-aged stand and that cover itsentire area are conducted at the same time inan uneven-aged stand, but only over a portionof the stand area. This complicates the prac-tical implementation of the method. Uneven-aged systems are often thought to be less inten-sive than even-aged systems, and that may betrue in relation to, say, the degree of site expo-sure during regeneration cutting or the capitaloutlay required to establish a new stand. Butuneven-aged systems require more attentionon the part of the forester, and can be moreinefficient to conduct in some ways becausethe scale of operation is at the scale of subunitswithin a stand rather than across the entirestand.

The Selection Method—AnOverview

Uneven-aged silviculture is implemented us-ing a reproduction cutting method called the

selection method, used to regenerate andmaintain a multiaged structure by removingtrees either singly, or in small groups or strips(Helms 1998). By definition, an unevenagedstand has at least three age classes (Smith 1986;Helms 1998). In practice, age is not measured;stands are managed by controlling either thevolume that is harvested, the diameter distri-bution in reference to a target distribution, orthe area within the stand that is cut.

In an uneven-aged stand, growing space issubdivided among trees of all size classes, fromregeneration through the largest overstorytrees. A starting point to determine the appro-priate basal area to maintain in an uneven-aged stand is to apply the basal area found ina mature even-aged stand of the same speciesthat is marginally fully stocked or slightly un-derstocked. A stand in that condition will havea slight amount of growing space available inthe understory for the regeneration establish-ment but will be marginal for regeneration de-velopment. If that basal area is translated to anuneven-aged stand, the heterogeneous condi-tions that typify an uneven-aged stand will re-sult in fully stocked clusters of overstory treesin some areas, and other areas that are suf-ficiently understocked such that regenerationdevelopment can occur. This concept gives rise


to two generally accepted methods by whichthe selection method is implemented—one us-ing very small openings and the other usinglarger openings.

Single-Tree Selection MethodSingletree selection imitates the smallest scaleof disturbance, such as when a single treefalls or dies while standing in the woods.Possible causes of such a disturbance are in-sects, lightning, disease, windthrow, or someother agent. In unmanaged stands, this pro-cess results in gap-phase dynamics indicativeof late-successional conditions (White 1979;Runkle 1982; Pickett and White 1985). Whena large tree in an uneven-aged stand is re-moved, the growing space used by that treeis made available to adjacent trees, to smallertrees in the midstory, and to regeneration inthe understory. In the smallest gaps, the gapmay close before the regeneration can accedeinto the main canopy, and the regenerationmay then persist without further growth ormay even succumb to suppression. The occur-rence of multiple gaps (where a nearby treesuccumbs and creates a second nearby open-ing, either concurrently with or soon after thefirst), or expansion of existing gaps (wheregap-bordering trees fall) can tip this ecologicalbalance in favor of regeneration survival anddevelopment.

In the single-tree selection method, indi-vidual trees of all size classes are removedmore or less uniformly across the stand (Smith1986; Helms 1998). This is typically conductedby first identifying the trees that are to re-main in the stand. The trees to remove thenbecome obvious because their removal pro-motes the continued growth and develop-ment of the trees that are to remain. Thediameter of the tree being removed is di-rectly related both to the silvicultural objec-tive for its removal, and to the retention ofstocking levels by size class deemed desir-able across the residual diameter distribution.Small-diameter trees, such as pulpwood orsmall sawlogs, are removed according to clas-sical thinning rationale—to free an immatureneighboring tree, presumably one with better

form, condition, or other attribute, from thecompetition provided by the tree being re-moved. Removal of a large-diameter tree, suchas one at or larger than the maximum diame-ter desired for retention, is intended to createa canopy opening within which regenerationis to become established and to develop.

Group Selection MethodIn the group selection method, trees are re-moved and new age classes are established insmall groups (Smith 1986; Helms 1998). Groupselection imitates a small-scale natural distur-bance that kills a small group or cluster of treeswithin a stand; examples include mortality oftrees from a blowup of a surface fire or an infes-tation of pine beetles. In theory, the nature ofthe disturbance should promote a suitable andreceptive seedbed for seedling establishment,and the size of the gap that is created by thedisturbance is large enough to promote regen-eration development. The seed source for thatnew cohort can be advance growth of regener-ation in place prior to creation of the opening,seed from trees that border the gap or stored inthe soil beneath the gap, or as seedfall dissemi-nated from the trees being removed within thegap at the time of their removal. The size of thegap affects the species composition of that newcohort. Larger gaps favor species of greater in-tolerance to shade, while smaller gaps favorshade-tolerant species.

The group selection method is applied orsuggested when there is a desire to use anunevenaged silvicultural system in a stand,yet still regenerate shade-intolerant species;hence the interest in the method relative tothe southern pines. The maximum size forgroup openings depends on one’s interpre-tation of ecological literature as modified byprevailing forest management guidelines. Itis generally agreed that the upper ecologi-cal size limit for tolerant species within a cir-cular group selection opening is one whoseradius equals the height of the surroundingtrees in the stand (Helms 1998). In trees witha height of 25 m, the ecological upper limitwould be on the order of 0.2 ha. Larger open-ings would then be suggested for intolerant


species such as longleaf pine. The suggestedmaximum group opening size on national for-est lands within the Southern Region of theUSDA Forest Service is 0.8 ha, which should al-low for acceptable regeneration establishmentand development of any of the southern pines.In practice, some trees are typically removedfrom the stand matrix between the groups aswell, so as to promote stand health and accept-able basal area levels between group openings,and to prepare trees as seed producers in sub-sequent group openings.

The decision to locate a group openingwithin the stand is usually done to alleviate un-derstocking, rectify excessive stem density, ortake advantage of existing regeneration. If partof the stand is understocked, creating an open-ing gives the forester an opportunity to regen-erate that group, and thus to restore that partof the stand back to full stocking. At the otherextreme, if there is a place within the standwhere the trees are all in a surplus size class onthe marking tally, creating an opening harveststhose trees and helps the forester more quicklyachieve the desired residual stand density. Fi-nally, if a species is difficult to regenerate nat-urally, one should not fail to create openingswithin the stand where advance regenerationis found.

Group selection has a number of administra-tive advantages over single-tree selection thatcontribute to its popularity. Most group open-ings serve as points of concentration for loggingoperations in the immediate area; logs are fre-quently decked in the openings, and haul roadstypically run from one group to another. As aresult, group openings are often heavily scar-ified. This is an advantage in promoting pineregeneration, which requires exposed mineralsoil for optimal seed germination and estab-lishment. Moreover, the group opening is theonly part of the stand in which regenerationis expected. Site preparation or release treat-ments can thus be restricted to the groups,which is an advantage in that less area mustbe treated and the specifications for treatmentcan be made clearer than when a compara-ble treatment is prescribed in a single-tree se-lection stand. The advent of geographic posi-tioning system technology adds to the ease of

contracting treatments, since the precise loca-tion of each group can be specified.

Selective CuttingThe term “selective cutting” is often usedto describe harvesting activity that resemblesuneven-aged regeneration cutting in that treesremain on the site. But a strict definition ofthe term is “select some trees to cut and cutthem”; it has no commonly accepted pro-fessional meaning except a derogatory one.The term does not refer to, and is not syn-onymous with, the practice of uneven-agedharvests using the selection method. All toooften, stands harvested using selective cuttingare high-graded—harvest is uncontrolled, thebest trees are cut, and the poorest remain.Perhaps the most important silvicultural dis-tinction between the selection method and se-lective cutting is that under the latter, no provi-sion is made for establishment or developmentof desired species of regeneration. This is a trapinto which improperly applied harvests underthe selection method can fall.

Regulation ofUneven-Aged Stands

Regardless of whether single-tree selection orgroup selection is implemented, the methodsfor ensuring the regulation of growth and har-vest are the same. Two methods of regula-tion have historically been associated with theselection method—regulation of the sawtim-ber volume in the stand through harvest ofgrowth, and regulation of the structure of thestand through conformance with a target di-ameter distribution. With group selection, athird method enters the picture, in which regu-lation is based on proportion of harvested areaduring each cutting cycle.

These methods have varying degrees towhich they conform to the origins of uneven-aged silviculture (Guldin 1996). The selectionmethod, the most recently developed of theregeneration methods historically, traces itsorigins to the Dauerwald in Germany, which


evolved as a counterpoint to area-based regu-lation methods. Under the Dauerwald, allow-able cut was based on stand volume growth; avolume equal to roughly 15 times the annualgrowth was retained on the site (Troup 1928,1952). Other European work at the turn ofthe century used a negative exponential alge-braic relationship among the number of treesby diameter class to quantify the reverse J-shaped curve that approximates an uneven-aged stand (deLiocourt 1898), and that ap-proach was further developed in North Amer-ica in modern approaches to forest manage-ment (Meyer et al. 1961). Little historic evi-dence exists to support an area-based regula-tion method in the evolution of uneven-agedsilvicultural systems. On the other hand, Smith(1986) points out that regulation methodshould be independent of silvicultural system;under such logic, any of the following systemsmight legitimately be applied to regulation ofuneven-aged stands.

Regulation of VolumeThe most successful application of the selec-tion method in southern pines has been atthe Crossett Experimental Forest (EF) in Ash-ley County, AR, in the mixed loblolly–shortleafpine forest type of the upper West Gulf CoastalPlain. The Crossett EF, managed by the South-ern Research Station of the USDA Forest Ser-vice, was established in 1934 and is still ac-tive. It supports several long-term researchstudies and demonstrations, notably the Goodand Poor Farm Forestry Forty DemonstrationStands and the Methods of Cutting Study.

The uneven-aged stands of loblolly–shortleaf pine at the Crossett EF were regulatedusing the volume control-guiding diameterlimit (VCGDL) method (Reynolds 1959, 1969;Baker et al. 1996; Guldin 1996, 2002). In thismethod, sawlog volume and volume growthare used to calculate harvests, and trees aremarked based on whether their individualgrowth rates are sufficient to allow them tomaintain acceptable sawlog volume growth.

Implementation of the VCGDL method hasfour broad steps. First, a current inventory ofthe stand is taken. That inventory is used to

prepare a stand and stock table that quantifiesthe stem density and sawlog volume beforeharvest by diameter class per unit area. Thisrequires application of an appropriate localsawlog volume table, so that the sawlog vol-ume by diameter class can be included in thetable.

Second, the compound growth rate of thestand must be determined. This is usually doneby averaging the growth rate for a number oftrees of varying size in the stand, using 10-year radial increment measured from incre-ment cores, and calculated per tree using theformula

G = exp

[(V0/Vp

)n

]− 1

where

G = growth rate (%)Vp = previous tree volumeV0 = present tree volumen = growth interval (years)

Alternatively, experience shows that in man-aged stands, one can use an appropriate com-pound growth percentage for sawlog volumeincrement. Values between 6% and 8% aretypical in unmanaged and managed loblolly–shortleaf pine stands, respectively, in southArkansas. An appropriate range for longleafpine would be based on local experience.

Either approach then requires the forester todetermine an after-cut volume for the plannedharvest. The after-cut volume is the cumula-tive volume to which the current stand mustbe reduced. That level is set by predicting thefuture cutting cycle length, by selecting the fu-ture stand volume sought at that time, andthen by calculating the volume to which thecurrent stand must be reduced so that it willgrow to the intended future volume at the ap-propriate rate of growth.

Third, the allowable cut is calculated as thedifference between the before-cut volume andthe planned after-cut volume. This leads di-rectly to the calculation of the guiding diam-eter limit (GDL), which is that diameter classthat meets the allowable cut if all trees in largerdiameter classes are cut. Usually, part of the


GDL class must also be cut to exactly matchthe allowable cut.

Finally, the field crews are given the GDLclass and the percentage of the GDL class to cut.Markers are instructed to retain trees above theGDL if they are growing acceptably, and then tomark an equivalent volume to that retained indiameter classes smaller than the GDL. Mark-ing crews can only do this efficiently by memo-rizing the appropriate local volume table. Thecrews then must keep a running tally, eithermentally or on a notepad, of cumulative vol-ume retained above the GDL and that removedbelow the GDL. At the end of the marking, thevolume marked below the GDL should balancethat retained above the GDL (Fig. 3).

The VCGDL method has a number of ad-vantages. It requires the field crew to examinesawlog component of the stand from the per-spective of trees that should be retained. As aresult, crews can balance whether to cut andleave trees across a range of diameter classes. It

also requires that large high-value trees abovethe GDL have a compelling record of growthto be retained.

However, there are several limitations ofthe VCGDL method. The approach does notprovide any evidence to the forester aboutthe growth of trees below the sawtimber sizeclass. Foresters must judge in some other waywhether regeneration is being established, andwhether sub-sawtimber size classes are devel-oping at an acceptable rate. That judgment istypically based on experience rather than ob-jective standards, and that can be a limitation.Second, the method requires a high degree ofexperience on the part of the field crews whoare marking the stand, especially in regard toestimation of the volume of trees above andbelow the GDL that are being retained andmarked, respectively, such that the cumula-tive volume tally balances when the marking iscompleted. Finally, because decisions are madein the field about retaining trees above the GDL

0

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FIGURE 3. The volume control-guiding diameter limit (VCGDL) regulation approach conceptually appliedin the 1978 before-cut inventory from the Good Farm Forestry Forty demonstration stand, an uneven-agedloblolly–shortleaf pine stand on the Crossett Experimental Forest in southern Arkansas. Before-cut, GDLtarget, and after-cut diameter distributions are drawn as curves rather than histograms. The GDL targetreflects the allowable cut in sawtimber cubic volume based on 6% growth rate. The after-cut diameterdistribution illustrates one possible outcome resulting from retaining trees above the guiding diameterlimit, and removing trees below the limit such that the volume of the stand is retained at the guiding level.


and removing trees below the GDL, the useof computer models to predict stand develop-ment in advance of harvest is difficult.

The VCGDL regulation approach evolvedas a means to regulate single-tree selectionstands. However, modifying the approach toregulate a stand being managed using groupselection is relatively straightforward. All treeswithin the group would be marked, and addedto the marking tally. Trees smaller than theGDL within a group would have their volumeadded to the below-GDL cumulative volumetally. That would lead to retaining an equiva-lent volume of trees at or above the GDL in thematrix between groups.

Regulation of Stand StructureThe regulation of stand structure is based onthe notion that the diameter distribution of abalanced uneven-aged stand has an ideal the-oretical relationship, which can be comparedwith the actual stand structure for generatingan after-cut residual stand (and indirectly, amarking tally) that carries the existing standcloser to the theoretical ideal. Several ap-proaches can be developed to quantify thisideal theoretical stand, such as use of standdensity index (Long 1998) or leaf area in-dex (O’Hara 1996). But the most common insouthern pines is based on the assumption ofa constant ratio q in the number of trees inadjacent size classes, according to the simpleformula

q = tnt(n+i)

where

q = q ratiotn = number of trees per unit area in the nth

diameter classt(n+i) = number of trees per unit area in the

next larger class of class width i

Thus, q is dependent on diameter class width,and the use of a given q ratio must include ref-erence to the class width i. If the maximumdiameter class D of trees to retain in the standis known, one can use q to construct a neg-ative exponential relationship that can be fit

to any desired residual basal area B per unitarea in the stand. Specification of B, D , and qthus constitutes a unique solution of diameterdistribution. This approach, called the BDq ap-proach, is used to generate the target balanceddiameter distribution against which the exist-ing stand structure is compared. The methodwas developed by Leak (1964), and its prac-tical implementation was described in detailby Marquis (1978). Modifications for uneven-aged stands of loblolly–shortleaf pines in theWest Gulf region were described in Baker etal. (1996), and for southern pines generally byFarrar (1996).

Simply stated, selection of the target BDqparameters allows the forester to calculate aunique hypothetical target diameter distribu-tion. This is typically prepared on a unit area(per-hectare) basis. The diameter distributionof the before-cut stand, prepared from a pre-harvest inventory, is then compared to thattarget. In an ideal case, the before-cut standwill contain a surplus of trees in every diame-ter class compared with the target; that sur-plus then becomes the marking tally. How-ever, far more common is the situation inwhich some diameter classes in the prehar-vest stand will contain a surplus of trees com-pared to the target, and others will contain adeficit of trees. Here, the basal area of thosedeficits must be calculated, and that basal areadeficit must be accounted for by retaining moretrees than called for in those diameter classesthat have a surplus relative to the target stand(Fig. 4). Ultimately, deficits in a given diameterclass will be corrected through ingrowth fromsmaller diameter classes over time.

When the final tally of trees to cut is deter-mined for each diameter class, the proportionof trees to cut by diameter class is calculated.That information—number of trees to cut, andpercentage, by diameter class—is given to thefield crews, who use that information as theymark the stand. Field crews will find it easierto base their marking on the proportion ratherthan the absolute number, since it is easier tothink about removing a set percentage of agiven diameter class rather than an absolutenumber of trees in a given diameter class perunit area. That also allows the reinforcement


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BDq target

After cut

FIGURE 4. Regulation of stand structure using the basal area–diameter–q ratio (BDq) method conceptuallyapplied in the 1978 before-cut inventory from the Good Farm Forestry Forty demonstration stand, anuneven-aged loblolly–shortleaf pine stand on the Crossett Experimental Forest in southern Arkansas.Before-cut, BDq target, and after-cut diameter distributions are drawn as curves rather than histograms.The BDq target reflects a B = 14 m2 ha−1, D = 57.5 cm, and q (5-cm classes) of 1.44. The after-cut diameterdistribution illustrates the compensation by basal area according to the q ratio; the basal area in deficitdiameter classes is retained in surplus diameter classes such that the target basal area is retained.

to be given that the poorest percentage of treesin the diameter class should be marked for har-vest, and the best trees retained. The numberof size classes the field crews must work withcan be reduced if broader product classes areused. For instance, a fivefold product classifica-tion that includes small pulpwood, large pulp-wood, small sawtimber, medium sawtimber,and large sawtimber would be very convenientfor field crews to apply.

If some method other than the BDq ap-proach is used to generate the target structure,the process for implementation still is most ef-ficient if conducted as described above. Sup-pose a target diameter distribution is generatedusing a power function, for example, ratherthan the negative-exponential BDq approach.Once the target diameter distribution is ob-tained, there is still need to compare the ex-isting stand to that target, generate a mark-ing guide, and to compensate for diameterclass deficits between the preharvest stand andthe target, so as not to overcut the preharvest

stand, and finally to determine if the projectedharvest is operable.

Structural regulation has the advantage ofobjectivity. When generating a target diam-eter distribution, target diameter class datacan be calculated for submerchantable diam-eter classes as well, which can provide guid-ance about whether cutting-cycle harvests areproviding acceptable regeneration establish-ment and development through the submer-chantable component and acceptable recruit-ment into the merchantable component of thestand. This depends on the assumption that themathematical relationship used to characterizethe stand structure is biologically meaningfulat the smallest size classes, and this may notbe the case for the negative exponential rela-tionship upon which q is based (Baker et al.1996).

The main disadvantage is that the processfor calculating the marking tally is cumber-some, especially in cases in which deficit di-ameter classes are adjusted according to the


mathematical relationship used in the initialtarget calculation. Spreadsheet programs areavailable to assist this calculation (Baker et al.1996). A second disadvantage is that the ap-propriate B, D, and q parameters for applica-tion to longleaf pine have yet to be identifiedthrough research or practice.

Area-Based RegulationRegulation of stands managed by group selec-tion has been advocated using the area-basedregulation concept borrowed from even-agedforest management (e.g., McConnell 2002).Under this simple device, the initial decisionis to establish a rotation age, r , for the trees inthe uneven-aged stand, essentially the age atwhich trees for the species under managementare typically harvested in a comparable even-aged context. The area a of the stand is thendivided by the rotation age r , and the quotientrepresents the proportion of the stand area ato be cut annually. That is converted to a, anarea to be cut in a given cutting cycle harvestby multiplying the annual percentage of areato cut times the length of the cutting cycle, ac-cording to the simple formula

Ac = (a/r)c

where

Ac = area to be cut in a cutting-cycle harvesta = stand arear = rotation age andc = planned length of the cutting cycle (years)

The problem in using area-based regulationwith group selection is more theoretical thanpractical. It is difficult to distinguish be-tween group selection and patch clearcutting,the small-opening even-aged variant of theclearcutting method that is also regulated usingthis approach. The best way to draw a distinc-tion between the area-based regulation of thegroup selection method versus patch clearcut-ting is through applications that increase thewithin-stand heterogeneity of structure. Ex-amples include varying the area cut in any onecutting cycle, varying group size and shape, orplacing openings in a pattern that is not ge-ometric or predictable. Use of group opening

sizes less than two tree heights in radius, suchthat the entire group opening is under the eco-logical influence of the gap-bordering trees,would also provide ecological distinctions withpatch clearcutting.

Adaptive Experience inSouthern PinesThe long-term studies and demonstrationsat the Crossett EF in south Arkansas pro-vide keys to the successful implementation ofthe method in mixed loblolly–shortleaf pinestands. That background is the best source ofexperience with the method in the South andserves as a point of departure for consideringthe application to other forest types.

RegenerationAt the stand level, the first indicator of long-term forest sustainability is whether adequateregeneration is obtained when a regenerationcutting is made in a stand. This is especially truewith the selection method, which requires adelicate balance between the stocking and de-velopment of the merchantable component ofthe residual stand versus the stocking and de-velopment of seedlings and saplings. It appliesalso in situations where conversion from even-aged condition to uneven-aged condition isimposed. It is critical to obtain regeneration af-ter a cutting cycle harvest in any conversion,transition, or initial steps in implementationof either the single-tree or group selectionmethod.

Abundant seed crops are an excellent at-tribute on which to rely when prescribing areproduction cutting method that depends onnatural regeneration. Long-term data on seed-fall in loblolly–shortleaf pine stands on theCrossett EF show that, on average, natural re-generation is adequate or better four years infive, and rarely do seed failures occur in twoconsecutive years (Cain and Shelton 2001).This prolific seedfall is one of the reasons un-derlying the successful application of eithereven-aged or uneven-aged reproduction cut-ting methods that rely on natural regeneration


in this forest type and region (Baker and Mur-phy 1982; Zeide and Sharer 2000; Cain andShelton 2001).

Conversely, irregular seed crops can reducethe probability of success in obtaining naturalregeneration under the selection method. Thisis important both for initial period of conver-sion to uneven-aged structure, and in mainte-nance of that structure. Experience at the Cros-sett EF suggests that failure to secure a newage class following a given cutting cycle harvestwas not in itself an impediment to maintain-ing desirable uneven-aged structure, but miss-ing two age classes in consecutive cutting cycleharvests is to be avoided (Reynolds 1969). If agiven cutting cycle harvest fails to secure a newage cohort of regeneration, supplemental sitepreparation efforts should be conducted at thenext cutting cycle harvest to ensure that regen-eration is obtained (Guldin and Baker 1998).

Rehabilitation of UnderstockedConditionsThe stands on the Crossett EF originated as cu-tover understocked stands, and were managedin a manner by which stocking was built overtime. The stands had been harvested by theCrossett Lumber Company in 1915 to a 38-cmstump limit, roughly equivalent to a 30-cm di-ameter limit. No management occurred on thearea until it was leased to the Forest Servicein 1934 (Guldin 2002). These stands were notfully stocked, homogeneous, and even-aged;rather, the stands showed considerable within-stand heterogeneity at the start. The researchand demonstration work that began at thatpoint successfully restored understocked andmarginally stocked stands back to full stockingthrough harvest of a portion of growth. Twoelements of this work were especially impor-tant.

The first was the reaction of these pines toremoval of competition. In the upper West GulfCoastal Plain, both loblolly and shortleaf pinerespond to release at advanced age. Data fromstudies at the Crossett EF (Baker et al. 1996)suggest that pine stems in the 10- to 15-cmdiameter class will respond to release if their

FIGURE 5. A loblolly pine sapling on the CrossettEF that meets the minimal size criteria for responseto release—a 20% live crown ratio, and diameteroutside bark at the base of the live crown of 5 cm. Asimilar rule of thumb for response to release wouldbe helpful to have in applying the selection methodto longleaf pine. (James M. Guldin)

live crown is greater than 20% and if the stemdiameter at the base of the live crown exceeds5 cm (Fig. 5).

The second was the use of herbicides to con-trol hardwoods that were competing with thepines. Effective hardwood control was criticalboth as site preparation for the establishmentand development of new seedlings and alsoas a release treatment and liberation cutting(Smith 1986) to free established pine saplingsand small merchantable stems. Thus, the abil-ity to use herbicides effectively to control hard-woods competing with pines, and to then havethe pines respond quickly to the growing spacemade available, lies at the root of success in


using the selection method to manage loblolly–shortleaf pine stands in the West Gulf re-gion.

Developmental DynamicsUneven-aged stands exist in a delicate balancebetween understocking and growth. Bakeret al. (1996) describe this balance in the con-text of shade management. That balance iscontrolled by three factors: the distribution ofbasal area retained in the residual stand imme-diately after a cutting cycle harvest, the lengthof the cutting cycle, and the operability of thefuture cutting cycle harvest.

After a cutting cycle harvest, the residualoverstory trees grow and some degree of in-growth into the overstory also occurs. As a re-sult, stocking levels increase in the overstoryover time. But this increased stocking serves toincreasingly inhibit the development of regen-eration. Subsequent cutting cycle harvests willbe needed to reduce overstory stocking suffi-ciently to allow the continued development ofthe initial regeneration cohort, and to obtaina new cohort. On the other hand, if a givencutting-cycle harvest removes too much basalarea, the stand will not grow rapidly enoughto allow an operable cutting cycle harvest inthe subsequent cutting cycle.

One simple metric for the upper limit ofacceptable basal area to carry in an uneven-aged stand is to quantify the residual basalarea in a classic low thinning at which acciden-tal regeneration just begins to be suppressed.This point approximates the highest acceptablebefore-cut basal area in uneven-aged stands.For example, pine regeneration can becomeestablished in even-aged Coastal Plain loblollystands thinned to 16 m2 ha−1, but will cease tomake acceptable height growth if basal areaexceeds 17–18 m2 ha−1. Because overstorytree distribution in uneven-aged stands is moreheterogeneous, these basal area levels repre-sent an upper limit to the acceptable basalarea range for successful uneven-aged pre-scriptions.

The lower limit is defined by maintaining ac-ceptable overstory growth over the expectedduration of the cutting cycle. In uneven-aged

loblolly–shortleaf pine stands at the CrossettEF, the target residual basal area after a cuttingcycle harvest is roughly 14 m2 ha−1, and thestands grow approximately 0.55–0.7 m2 ha−1

in basal area annually. After 5 years, the standswill have grown in basal area to about 17 m2

ha−1, and the subsequent cutting cycle harvestcan then be imposed.

Because basal area can also be related tostand volume, operational feasibility of har-vests can be tested. In south Arkansas, loblolly–shortleaf pine stands support after-cut volumesof roughly 26–27 m3 ha−1 of sawlog volume,and the stands grow on the order of 2.2–2.4m3 ha−1 of sawlog volume annually. After5 years, the stands will support roughly 37–40 m3 ha−1 of sawlog volume. Operable har-vests in the vicinity are about 9 m3 ha−1 ofsawlog volume. Thus, sawlog volume growthof the stand can be cut on roughly a 5-yearinterval with operational harvests, which alsomaintains regeneration development. Metricssuch as these are needed for longleaf pinestands.

Another clue about the appropriate upperlimit of basal area is whether acceptable ratesof height growth can be maintained in the re-generation component. In loblolly and short-leaf pine stands, height growth of regenera-tion is a useful indication of maintaining theability to recover full growth potential. Mini-mum acceptable annual height growth in thesespecies is 0.15 m. If seedlings or saplings lessthan 1.3 m in height are not growing at thisrate, they will probably not survive.

Finally, the Crossett EF experience sug-gests a final visual clue for determinationof acceptable balance between overstory andunderstory—the presence of foliage of the de-sired species at all levels of the canopy pro-file in the stand. If regeneration is success-fully established and making acceptable heightgrowth, and if repeated cutting-cycle har-vests are successful in obtaining regeneration,seedlings and saplings will be visible in thestand. The longer the period of successful silvi-culture under the selection method, the moreprominently will foliage of the desired speciesbe found at all levels of the canopy profile(Fig. 6).


FIGURE 6. A view of conditions in an uneven-aged stand of loblolly–shortleaf pine within the PoorFarm Forestry Forty Demonstration Area on theCrossett EF immediately following the cutting cy-cle harvest in the spring of 2003. Note the presenceof foliage of the desired pines at all heights in thecanopy profile, a simple visual clue that denotes sus-tainable regeneration cutting over time in uneven-aged stands. (James M. Guldin)

Marking RulesDuring his tenure at Crossett, CEF foundingscientist Russ Reynolds explicitly refused toidentify his volume-control method as “single-tree” or “group” selection; he called it “se-lection.” Occasionally large openings wouldoccur, occasionally small ones would suffice.Reynolds’s key decision was whether the treebeing examined while marking was of accept-able form, size, and quality to retain. Reynoldscaptured this concept in the simple phrase, “cutthe worst and leave the best” (Reynolds 1959,1969). Attention to this simple marking ruleensured that stem quality was gradually im-proved over time. As practiced on the Crossett

EF, the selection method has a reputation asone that produces sawtimber of high quality(Guldin and Fitzpatrick 1991); the long-termapplication of a marking rule such as this con-tributes to that reputation.

This rule raises distinctions between regula-tion by volume under the VCGDL and regula-tion by structure under the BDq method. It iseasier to leave the best trees under the volumecontrol regulation method versus the BDq, be-cause the marking tally in the BDq is specificto a given diameter class whereas the markingtally of the volume control method cuts acrossdiameter classes. Under VCGDL, a residual treeis judged to be part of the population of “best”trees regardless of diameter class. Conversely,in the BDq method, a tree that is retained isjudged relative to other trees in that diameterclass only, and the proportion to cut changesfrom one diameter class to another.

For example, suppose that a BDq markingtally requires removal of 1 in 10 trees in the 45-cm class, but half of the trees in the 30-cm class.Field crews will invariably come across a treein the 20th percentile of quality in the 45-cmclass immediately adjacent to a smaller tree ofbetter absolute form and with better develop-mental potential in the 40th-50th percentileof quality in the 30-cm class, and will com-plain about marking the better tree and leavingthe poorer one. The answer for the field crewsin that event is to use common sense, and tomark the poorer tree. Carrying that logic to itsconclusion leads to a critical point relative tothe BDq method. Of the B, D, and q variables,residual basal area is most important to retain,followed by maximum diameter; q is least im-portant. Some thought has been given to mod-ifications of the BDq method as a BD method(Baker et al. 1996); this would result in essen-tially a basal area control method implementedin a manner similar to regulation by volume,but in which the basis for compensation amongtrees being retained is by equivalence of basalarea rather than volume.

Reynolds’s marking rule also raises distinc-tions between group selection and single-treeselection. A rule that guides the forester to “cutthe worst and leave the best” can be morestrictly followed in single-tree selection than


in group selection. In most instances, the lo-calized area of the stand within which a groupopening is planned will contain trees that un-der an individualistic evaluation would qual-ify for retention. That might allow one to fur-ther refine the logic for placement of groupopenings—locate the opening in those partsof the stand where a disproportionate num-ber of the trees within the planned group areof poorer condition than those in other partsof the stand. Such manipulation in the loca-tion of group openings is possible if the groupselection method is being implemented usingregulation by volume or structure. However,improvement in residual stand condition as aresult of this marking rule is by definition un-likely to occur under area regulation of groupselection, especially if imposed using strict ge-ometric patterns.

The Selection Method inLongleaf Pine

Interest in implementing the selection methodin the longleaf pine forest type is driven bya number of considerations. Foremost amongthem is to develop habitat conditions in lon-gleaf stands that favor the species that inhabitthese stands, such as bobwhite quail (Moseret al. 2002) and the red-cockaded woodpecker(McConnell 2002). To a certain extent, argu-ments about habitat condition that can be de-veloped in uneven-aged stands of longleaf pineare premature without a careful examinationof what a sustainable application of the se-lection method would look like in longleafpine, using the subjective metrics developedfrom our understanding of the method in theloblolly–shortleaf pine forest type.

The state-of-the-art treatise on the selectionmethod in longleaf pine (Farrar 1996) is a pri-mary source for managers to consider as the se-lection method is operationally applied in lon-gleaf pine. Equally important in application tothe selection system in longleaf pine is researchon longleaf pine autecology that culminatedthree decades ago on the Escambia EF nearBrewton, AL (Croker and Boyer 1975), where

FIGURE 7. A view of the shelterwood method in ap-plication to longleaf pine in 1982 on the EscambiaExperimental Forest, Brewton, AL. The residualbasal area in the overstory was 7 m2 ha−1, andseedlings have emerged from the grass stage severalyears following the seed cut. (James M. Guldin)

detailed studies of the reproductive biologyand silvics of longleaf pine were fundamen-tal to the development of the even-aged shel-terwood method (Fig. 7). A subjective inter-pretation of these sources suggests that a suc-cessful prescription for the selection methodin longleaf pine will require attention to re-generation establishment, the pattern of im-plementation, the approach to regulation, anddevelopmental dynamics. Among the largestchallenges will be the integration of prescribedfire as a standard element of the method.

RegenerationThe application of natural regeneration in aselection method for longleaf pine will be


difficult. Seed production is much less reliablein longleaf pine, where adequate seed cropsonly occur between 10 and 20% of the time(Wahlenberg 1946), than in loblolly pine. Thedegree of silvicultural attention required tomake a successful prescription involving nat-ural regeneration will be greater for longleafpine than for loblolly pine, if for no other rea-son than the greater infrequency of adequateseed crops. This can be especially problematicin mixed-species southern pine stands that in-clude longleaf pine as part of the mix, becausethe other pines will be more prolific seed pro-ducers.

Careful attention to silvicultural detail isneeded to ensure practical success with natu-ral regeneration in longleaf pine. For example,the key to the development of the shelter-wood method in longleaf pine was the de-tailed work by Croker (1973). He reported thatover a 7-year period, cone production in a lon-gleaf pine stand reached an optimum whenthe stand basal area of longleaf pine was 6.88m2 ha−1 and declined as basal area decreasedor increased from that level. Greater overstorybasal area resulted in less seedfall and reducednumbers of seedlings. A uniform residual over-story of 10.33 m2 ha−1 resulted in virtuallyno surviving saplings over time (Boyer 1993).Fieldcraft such as that described in the devel-opment of the shelterwood method (Crokerand Boyer 1975) would improve natural seed-fall in any selection method applied in lon-gleaf pine stands. Because cone productionis a highly inherited trait genetically (Croker1964), marking crews should include an eval-uation of past cone production as a decisionelement in whether to retain a tree during cut-ting cycle harvests (Fig. 8).

On national forest lands in the South, an-other practical approach for management oflongleaf pine using the selection method isto plan for natural regeneration, but to useplanting as a fallback position to prevent ex-cessive delays in reforestation. There will betwo opportunities for successfully obtainingnatural regeneration prior to planting. Thefirst chance is that associated with the ini-tial harvest. Foresters with the USDA ForestService generally allow a logger a multiyear

FIGURE 8. Longleaf pine seed after seedfall in fall1982, a good seed year in a managed even-aged lon-gleaf pine stand in central Louisiana. A prescribedfire had been used that year to prepare the seedbedfor the anticipated seed crop. (James M. Guldin)

window within which to complete a cuttingcycle harvest. Hopefully, the period when thestand is actually cut would occur in conjunc-tion with an adequate seed year. However,the administration of sales on National Forestlands precludes the ability to guarantee har-vest in conjunction with seed crops and recep-tive seedbeds. Logging contractors are typicallygiven several years to harvest a timber sale,and cultural work to improve seedbed condi-tion must be programmed a year in advance.

The second chance is to catch the first goodseed crop after the sale closes. Site preparationshould be conducted to prepare a receptiveseedbed when that seed crop occurs. This, too,is constrained by administrative procedures.Site preparation on National Forest lands isfunded using proceeds from timber sales under


the Knutsen–Vandenberg Act of 1933, whichlimits expenditures to a 5-year period follow-ing sale closure. If longleaf pine produces onegood seed crop in 5 years, the chances arethat site preparation can be timed to that ex-pected seedfall. However, site preparation dol-lars must be requested in the fiscal year in ad-vance of that in which they would be spent—and the ability to predict a good seed year islimited to 6 months in advance of seedfall (Cro-ker and Boyer 1975).

Practically, then, a forester with the USDAForest Service has 4 years after the cutting cy-cle harvest is concluded to obtain natural re-generation; the fifth year must be devoted tospending the available funds to prepare the siteand plant seedlings. Standards should be de-veloped that provide guidance to silviculturistsabout when natural regeneration difficultiesare likely to be profound (such as poor seedproducers left on the site, an absence of ad-vance growth, or the lack of adjacent standsthat could contribute seed to a cutover areafrom adjacent mature trees). That informa-tion could help foresters identify stands thatmight be good candidates to plant initially un-der residual overstories, rather than to tacklethe chain of efforts to synchronize site prepa-ration to seedfall.

In the past decade, a focus of regenerationresearch in longleaf pine has been to exam-ine longleaf seedling establishment, survival,and growth in openings of various size andcondition. Results are generally of the opinionthat a clumped residual overstory condition,suggestive of the group selection approach,promotes early seedling development whencompared with homogeneous overstory con-ditions. Seedlings initiate height growth pri-marily in the center of gaps, but height growthis reduced along the borders of gaps or adja-cent to residual trees. This border effect is onthe order of 12–20 m. The major reasons forthis seedling growth pattern are related to in-creased light intensity in gaps (Grace and Platt1995; Palik et al. 1997; McGuire et al. 2001;Battaglia et al. 2002; Gagnon et al. 2003) anddecreased levels of intraspecific competitionfor soil resources in gaps (Brockway and Out-calt 1998).

More than any of the other southern pines,longleaf pine will benefit from some applica-tions that involve planting as the source ofregeneration. However, research experiencewith that is limited. Probably a minimum num-ber of seedlings is rationally feasible to plant.If natural regeneration is inadequate, andplanting is needed, one should plant enoughseedlings to ensure a fully stocked stand ofplanted seedlings. The number to plant willvary depending on seedling stock and thelevel of understory competition. For example,1000–1200 seedlings per hectare may be suf-ficient using containerized planting stock anda preplant herbicide treatment in a group se-lection opening. Without these refinements,more seedlings will be needed.

Planting should be done in association withan effective site preparation prescription thatpromotes survival and height growth of theplanted seedlings. Genetic improvement ofplanting stock has produced seedlings thatmake rapid early height growth under openconditions with intensive site preparation. Al-though families selected for rapid growth in theopen would probably be successful if plantedbeneath a residual overstory or within a groupopening that is under the ecological influenceof the overstory trees that surround the group,there are opportunities to explore the best fam-ilies to plant in conditions that are subject topartial shade or neighbor influence. However,there is little basis for this at present.

Pattern of ImplementationBecause the presence of overstory trees affectslongleaf pine seedling establishment and de-velopment, most recent research has concen-trated on the possibilities associated with useof the group selection method in longleaf pine.Farrar (1996, 1998) suggests that a “modifiedgroup selection” approach be used in whichgroups are not removed until regeneration isestablished beneath the group. He further sug-gests that this be integrated with cyclic pre-scribed burning (Fig. 9). Either the VCGDL orthe BDq regulation approaches would be fea-sible (Farrar and Boyer 1991). Under the BDq,Farrar’s suggested target structure for longleaf


FIGURE 9. Longleaf pine seedlings and saplings of natural origin in a group selection opening on theEscambia Experimental Forest in Brewton, AL, during the 1982 growing season. (James M. Guldin)

pine includes a residual basal area target Bof 14 m2 ha−1, maximum retained diameterD of 50 cm, and a q ratio for 5-cm diameterclasses of 1.44, and a suggested cutting cyclelength of 10 years (Farrar 1996). His uneven-aged marking guidelines contain a descriptionof how to implement either method for lon-gleaf pine. The burning program is requiredto keep competing hardwoods in check and tokeep seedbeds prepared for any seedfall thatmight occur. When seedlings become estab-lished at acceptable densities within an area(local distributions equivalent to 1800–2000trees ha−1), cutting cycle harvest to remove theoverstory trees will allow seedlings to initiateheight growth. Subsequent cutting cycle har-vests can be used to expand existing groups orto establish new groups, and as a free thinningin the matrix of the stand between the groupopenings.

Given the success in regenerating longleafpine naturally using the shelterwood method,another possibility, or perhaps a variant ofFarrar’s modified group approach, is to adaptthe shelterwood prescription for longleaf pine(Croker and Boyer 1975) in the context of agroup selection regeneration method, where

the groups are treated as small shelterwoodopenings. Smith (1986) noted that althoughgroup selection is usually imposed by remov-ing all trees within the group, groups could cer-tainly be created that retained overstory treeswithin them as silviculturally appropriate. Thismethod would resemble a group selection withreserves (cf. Helms 1998) except that the re-serves are explicitly retained for the silvicul-tural purpose of obtaining natural regenera-tion.

A shelterwood-based group selection ap-proach in longleaf pine would use groupswithin which longleaf pine seed trees are re-tained at shelterwood (6–7 m2 ha−1) residualbasal area levels. During one cutting cycle har-vest, groups would be marked to resemble theseed cut of a shelterwood residual basal area(Smith 1986), using the same decision vari-ables for seed tree retention as described inCroker and Boyer (1975). During the inter-vening cutting cycle, prescribed fires would beimposed as an element of the prescription, soas to prepare the site for seedfall, and Farrar(1998) offers suggestions to accomplish that.The seed trees in the group openings wouldeventually maximize their ability to produce


cones, seedfall would be optimized 3 to 5 yearsafter the cutting cycle harvest, and at the nextadequate seed year seedlings would becomeestablished in the shelterwood group opening.The residual seed trees within the group wouldthen be removed in the subsequent cuttingcycle harvest to release established seedlings.If the removal cut of the shelterwood resid-uals within the group opening is not timely,seedling survival would be compromised.

Alternatively, if groups are initially createdwithout residual trees, an area adjacent to thegroup opening could be retained at shelter-wood basal areas such that group expansionwould occur in the subsequent cutting cy-cle harvest, resulting in an expanded groupcoalescence. The same suggestions regardingmarking, use of prescribed fire, seedling devel-opment, overstory removal, and so on wouldapply in this modification of the practice.

It might be that stands larger than 15 hacould be managed this way, and also that themethod would be amenable to group open-ings larger than the 0.81-ha maximum forgroup selection suggested by the USDA For-est Service. Research would be needed to de-termine an effective range of group openingsize such that longleaf seedlings can initiateheight growth in a short period of time. It islikely that the size would be larger rather thansmaller. This approach could be regulated us-ing any of the usual regulation methods thatapply to uneven-aged stands.

Approach to Stand RegulationMore than in other southern pines, managersin longleaf pine are looking for ways to re-tain larger trees, in some cases much larger,above the diameter limits that have been estab-lished in other uneven-aged experience. Thiswould be done to meet resource attributes,values, or needs for associated species withinthe longleaf forest ecosystem, such as legacytrees or for nest construction by species suchas the red-cockaded woodpecker. There is littletheory available in the uneven-aged literatureto account for the influence of large trees re-tained above the maximum diameter or in ad-dition to the desired target residual stand. But

even if only a few large trees are retained, theirpresence can adversely affect the developmentof the stand, because retaining them preventsgrowing space from being used by trees ofother sizes. The regulation method and mark-ing guides must account for any trees that areretained for special purposes, simply becauselarge trees usurp considerable growing spacethat if unaccounted for could disrupt stand de-velopment.

For example, the BDq method calls for har-vest of the worst trees and retention of the bestin diameter classes at or below the maximumretained diameter D, but calls for all trees abovethe D to be cut. There are no active researchstudies on the Crossett EF that test whethertrees larger than D can be retained. As a startingpoint, the basal area of retained trees should beincluded in the calculations of stand structure,simply because they create a very large influ-ence in basal area calculations. An 80-cm treehas a basal area of 0.5 m2, or roughly 4% ofthe residual basal area target of 13 m2 ha−1 af-ter a typical cutting-cycle harvest. If three treesabove D per hectare were retained for specialpurposes and the stand below D was man-aged for the after-cut residual basal area tar-get of 13 m2 ha−1, the actual basal area in thestand would be 14.5 m2 ha−1, more than 10%higher than the target. Over time that wouldadversely affect stand development. Similarly,under volume control, the volume and the vol-ume growth of those big trees must be aver-aged into the calculation used to determine theallowable cut and the guiding diameter limit.Trees above the limit can be retained at the dis-cretion of the marker, provided that an equiva-lent volume is then marked below the guidingdiameter limit.

Growth and YieldEmpirical data on growth and yield of uneven-aged stands of longleaf pine are difficult tofind because uneven-aged stands managed fora sufficient length of time are relatively rare(Kush et al. this volume). One of the few pa-pers to cite data on growth and yield directlyunder the selection system is that of Farrarand Boyer (1991), who describe the growth of


two uneven-aged stands on the Escambia EFover a 10-year period in comparison to that ofa demonstration stand being managed usingthe shelterwood method. In all three stands,sawtimber volume growth was comparable atroughly 2 m3 ha−1, or roughly two-thirds therate expected in the study. At these rates ofvolume growth, cutting cycles of 10 years orlonger will be required to generate operableharvests.

Farrar and Boyer (1991) also speculated thatover time, volume yield from selection standswill likely be less than that produced from aforest of large even-aged stands, because thezone of competition between large and smalltrees is minimized with large blocks found ineven-aged stands (Farrar and Boyer 1991). Theuse of group selection, which would also haveless area within the stand in this zone of com-petition relative to single-tree selection, mightpartly compensate for that hypothesized vol-ume shortfall.

In a study of longleaf pine regenerationdevelopment under varying residual over-story basal area levels, Boyer (1993) reportedthat even a few residual longleaf pine par-ent trees resulted in substantial growth re-ductions of the new age cohort. The growthof two-aged stands in this study was lessthan half the growth reported in the natu-rally regenerated even-aged stands releasedfrom overstory competition when young. Thisprovides another estimate of the total mer-chantable volume growth that might be pro-duced in uneven-aged stands—less than halfthat found in comparable released even-agedstands.

Given the shortage of long-term data ongrowth and yield from uneven-aged stands inthe literature, among the first priorities for re-search is to better quantify the growth andyield that one can expect from application ofthe selection system in the longleaf pine for-est type. At a minimum, one should expectreduced rates of volume growth, especiallyin total merchantable cubic volume, in theuneven-aged stands. This point has been ob-served elsewhere in application of the methodin southern pines (Guldin and Baker 1998).

Developmental Dynamics

If uneven-aged silviculture is applied in lon-gleaf pine stands similar to that in other for-est types where the method has been success-ful, a number of attributes will be apparent.Longleaf pine trees in the sawtimber size classwill constitute roughly two-thirds of the resid-ual basal area, but only 25% of the number ofstems 10 cm and larger in the stand (Farrar etal. 1984; Baker et al. 1996). Cutting cycle har-vests will require two visits by field crews tothe stand—one to obtain the before-cut standinventory upon which regulation calculationsare based and to determine operability of theproposed cutting cycle harvest, and a secondto actually mark the stand using the mark-ing guidelines that the regulation calculationsproduce. Marking will follow a pattern of cut-ting the worst longleaf pines, and leaving thebest, during every cutting cycle. Some formof competition control that meets the multi-ple silvicultural objectives of site preparation,release, and liberation cutting will be requiredon the order of every other cutting cycle. Inlongleaf pine stands, that will probably takethe form of prescribed fire, with perhaps oc-casional herbicide use or mechanical felling ofcompeting hardwood species. Regeneration oflongleaf pine must be monitored after eachcutting cycle harvest. Some regeneration willbe expected to become established and to ini-tiate height growth after every cutting cycleharvest, and especially after the first cuttingcycle harvests in stands recovering from un-derstocked conditions or being converted fromeven-aged fully stocked conditions. After sev-eral decades of implementation using a 10-year cutting cycle, visual examination of standswill reveal foliage of longleaf pine present at alllevels of the canopy profile from the groundup through the main canopy. It will requirethree or more 10-year cutting cycle harveststo approach an uneven-aged structure, andlonger to develop a well-balanced stand struc-ture. Fewer cutting cycles will be needed if theinitial stand condition is understocked and ifmultiple age cohorts are already present; morewill be needed if the stand under management


FIGURE 10. A within-stand view of a new regeneration cohort in a longleaf pine stand managed as a quailplantation in southern Georgia. (James M. Guldin)

was initially a well-stocked even-aged stand atmidrotation or later.

This description is at odds with several ex-isting and notable approaches to managementof longleaf pine—that proposed for manage-ment of the red-cockaded woodpecker, andthat reflected in the Stoddard–Neel approachfor management of uneven-aged longleaf pinestands for quail habitat. Open understoryconditions are sought for both red-cockadedwoodpeckers and for bobwhite quail. An un-derstory full of trees in the submerchantableand smallest merchantable size classes aresought in an uneven-aged stand. The ques-tion is whether uneven-aged practices canbe, or have been, modified so as to simulta-neously maintain an open understory condi-tion, while concurrently maintaining the de-velopment of regeneration cohorts in sufficientnumber and distribution within the uneven-aged stand (Fig. 10).

Consider the Stoddard–Neel approach ofuneven-aged silviculture, an excellent descrip-tion of which was recently published by Moseret al. (2002) (see also boxes A and B). Inthis approach, single-tree selection is used to

maintain low residual basal area at 15 m2 ha−1

or less; open midstory conditions are main-tained, removals are made from below, andregeneration occurs in patches. The purpose ofthis variant of the single-tree selection methodis to provide habitat for northern bobwhitequail. Graphs of the diameter distributions forseveral quail plantations (Moser at al. 2002)differ from the reverse J-shaped curve typicallyassociated with uneven-aged silviculture at thestand level, specifically in a lack of trees inthe smallest diameter classes. For example, no1-cm diameter class less than 10 cm in any ofthe plantations managed using the Stoddard–Neel approach has more than 5% of the to-tal number of pines per hectare. Conversely,the 1995 preharvest inventory on the GoodFarm Forestry Forty demonstration stand onthe Crossett EF showed 4450 trees ha−1 inthe submerchantable diameter classes (1.5–8.9centimeters inclusive), corresponding to 92%of the total trees per hectare in the demon-stration stand (Guldin unpublished data). Thisraises the question of whether regenerationis being recruited at a sufficient rate underthe Stoddard–Neel approach to ensure the


long-term sustainability of not only uneven-aged stand structure, but also the habitatvalues for which those stands are justifiablyprized.

Similarly, debate is currently active in theliterature about the habitat values providedby uneven-aged stands for the red-cockadedwoodpecker (Engstrom et al. 1996; Rudolphand Conner 1996; Hedrick et al. 1998). This de-bate has its roots within the application of theStoddard–Neel selection system as well, since itis largely upon that experience that habitat de-scriptions of uneven-aged longleaf pine standsare based. But the debate embraces other sit-uations in which desired habitat for the red-cockaded woodpecker is inconsistent with in-formation available from the literature aboutuneven-aged stand dynamics. It is not really aquestion of the conditions that are appropriatefor the red-cockaded woodpecker, which arefairly well defined (Conner and Locke 1982;Hooper 1988; Hooper et al. 1991; Conner etal. 1994; Ross et al. 1997). Rather, the ques-tion is one in which a regeneration methodthat provides desired residual stand conditionsis sustainable, according to the first rule of sus-tainability at the stand level—that an imposedregeneration method must result in the suc-cessful establishment and development of re-generation.

Summary

The successful practice of uneven–aged silvi-culture in mixed loblolly–shortleaf pine standsof the upper West Gulf Coastal Plain hasbeen characterized by a number of attributes(Guldin and Baker 1998). Key factors are at-tention to stand-level regulation, use of appro-priate residual basal area levels that approx-imate those found in slightly understockedmature even-aged stands, establishment anddevelopment of regeneration, and attentionto a marking rule that cuts the poorest treesand leaves the best across a range of diameterclasses.

Longleaf pine shares some silvical attributeswith loblolly and shortleaf pines, but not all.

The favorable elements will be useful in devel-opment or refinement of the selection methodin longleaf pine. First, dominant or codomi-nant longleaf pines respond to release, thoughsuppressed trees do not (Boyer 1990). Thus,cutting cycle harvests in which codominant orbetter longleaf pines are released from com-petition of others will stimulate a growth re-sponse in the residual stand, which promotescontinued stand development. This attributehas been a feature of the Stoddard–Neel vari-ant of the selection method in longleaf pineas well (Moser et al. 2002). Second, it can besuccessfully managed using the shelterwoodmethod (Croker and Boyer 1975), which hasbeen observed in other forest types where theselection method has been applied (Guldin1996).

Where longleaf differs most prominentlyfrom other southern pines is in the periodicityof seed crops and the difficulty in securing nat-ural regeneration. This will require new inter-pretations of existing knowledge, and the de-velopment of new knowledge, to ensure thatlongleaf pine seedlings can become establishedand can develop properly following regen-eration cutting under the selection method.With respect to natural regeneration, refine-ments of existing knowledge from the appli-cation of the shelterwood method might bepromising as a variation under modificationsof the group selection method (Farrar 1996).Conversely, should natural regeneration tech-niques fail or result in unacceptable delays inregeneration establishment or development,technology should be developed for applica-tion of planting as an alternative or a preferredmethod of obtaining establishment of regen-eration at acceptable levels. More than anyother southern pine, or for that matter anyother species in which the selection methodhas been used, planting seedlings for reforesta-tion of uneven-aged stands will have a promi-nent place in the successful application of theselection method for longleaf pine.

The biggest question that remains unre-solved is the level at which regeneration de-velopment can be considered acceptable inthe selection method. The Stoddard–Neel se-lection method points in one direction about


this, and experience from the selection methodas practiced in mixed loblolly-shortleaf pines,and in other forest types, points in another.The Stoddard–Neel approach differs from mostother instances of successful application ofuneven-aged silviculture due to the smallernumber of stems in the submerchantable class.That difference leads to allied relationships inhabitat condition that promotes open midstoryconditions in one case, and midstory condi-tions occluded by development of seedlingsand sapling in the submerchantable diame-ter classes in other conditions. Ultimately, thequestion relates to the degree to which de-viations from the reverse J-shaped structurecan be considered to be sustainable at thestand level. This is the most prominent re-search gap in our understanding of the regen-eration dynamics of uneven-aged longleaf pinestands, and one that is critical in order to ulti-mately evaluate what constitutes sustainabil-ity of uneven-aged structure in longleaf pinestands in the long term.

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