Stand Structure and Allometry of Trees During Self-Thinning of Pure Stands Author(s): C. L. Mohler, P. L. Marks, D. G. Sprugel Source: Journal of Ecology, Vol. 66, No. 2 (Jul., 1978), pp. 599-614 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2259153 . Accessed: 04/03/2011 11:24 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=briteco. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology. http://www.jstor.org Journalof Ecology(1 978), 66, 599-614 STAND STRUCTURE AND ALLOMETRY OF TREES DURING SELF-THINNING OF PURE STANDS C. L. MOHLER,* P. L. MARKS* AND D. G. SPRUGELt Sectionof Ecology and Systematics,Cornell University, Ithaca, New York14853*, and Radiologicaland Environmental Research Division, Argonne National Laboratory, Argonne,Illinois 60439, U.S.A.t SUMMARY (1) Stand structureand mean weight-densityrelations of nearlypure, dense, even-aged, naturalstands of Prunuspensylvanica and Abiesbalsamea in thenorth-eastern U.S.A. were examinedand relatedto allometricgrowth. Values forthe exponents of allometric and mean weight-densityequations wereestimated by principalcomponents analysis of logarithm- icallytransformed data. (2) It is proposed thatsoon aftera stand of woody plantsbecomes established the size- frequencydistribution is a negativelyskewed, bell-shapedcurve; the distributionsub- sequentlybecomes positively skewed, and eventuallyapproaches normality after substantial thinning.Maximum positive skewness occurs at thetiine self-thinning begins. (3) The weight-frequencydistribution undergoes a parallelseries of stages: roughly normal at first,but quicklychanging to lognormal,with maximum skewness attained at the tiine thinningbegins. These curve-forinsare proposed only as approximationsto empirically observeddistributions. A consistenttendency toward bimodality is one commonly-observed departurefrom the idealized distributions. (4) The calculated exponentsof the inean weight-densityequations for Prunuspensyl- vanicaand Abies balsamea were - 1 46 and - 1 22, as comparedto theproposed value of - 15 (- 3/2power 'law' of self-thinning).In general,weight changes in plantparts during self-thinningdid not parallelthose for whole plants; in particular,the mean weight-density exponentfor foliage was approxiinately- 1 0. The exponentsof inean weight-density equationsfor total roots and totalshoots, however, approximately equalled theexponent for wholeplants. (5) It is concludedthat observed patterns of alloinetric growth are incompatiblewith inean weight-densityequations forwhole plants,unless a mutualadjustment between allometry and standstructure is assumed. INTRODUCTION Intraspecificcompetition causes thinningin plant populations throughdeath of the smallestindividuals. In single-speciesstands small initial differences in seed size, timneof germination,growth rate and otherfactors lead to small differencesin above-ground statureand rootinghabit of individualplants soon afterthey become established,and these differencesbecome amplifiedas the stand develops (Black 1957, 1958; Black & Wilkinson 1963). Extremeexpression of these differenceseventually results in very dominantand verysuppressed individuals in a stand,although the abilityto withstand suppressionis knownto varywidely among species. 0022-0477/78/0700-0599$02.00(D 1978 BlackwellScientific Publications 599 600 Standstructure of treesduring thinning It is of interestto know thegenerality of theeffects of the thinningprocess in plants, especiallysince much of theexperimental work has been done on annual crop plantsand weeds.Do radishplants and pinetrees respond to thinningin thesame manner?It is clear fromthe literature(Yoda et al. 1963; White & Harper 1970) that both herb and tree species,when growing in single-speciesstands that are thinning,exhibit a linearrelation- shipbetween the logarithm of wholeplant weight and thelogarithm of density: log w=C'+klog p or w=Cpk (1) wherew is the inean weightof individualplants thathave survivedthinning, p is their density,C' is a species-specificconstant, and C is the antilogof C'. Yoda et al. (1963) proposedas a generalfinding that the exponent of (1) shouldbe - 3/2for plants that are thinningin purestands (the - 3/2power 'law'). White& Harper(1970) postulatedfurther thatthe exponent might also be - 3/2for plant parts(leaves, roots,etc.), whichwould implythat weights of individualparts show thesame relationshipto plantdensity as the weightsof entireplants. In thispaper we considerthe weightsand dimensionsof plant parts in relationto one another(allomnetry), and in relationto density,for naturally thinningpopulations of two treespecies, pin cherry(Prunus pensylvanica L.) and balsam fir(Abies balsamea (L.) Mill.). We also considerthe mechanisms of thinningin relationto developmentof stand structurefor thesespecies. For comparison,allomnetric relation- shipsare developedfor several additional species using data in theliterature. Unlikemany tree species, both pin cherryand balsam firare well suitedto examining the thinningprocess in naturallyoccurring stands. Pin cherryis a small,fast-growing, successionaltree that lives forabout 30 yearsin northernNew England (U.S.A.); it is commonafter cutting or othermajor disturbancethroughout the Northern Hardwood and Boreal forestsof easternNorth America. It typicallyforms truly single-age stands because of itsburied seed habitand germinationrequirements (Marks 1974).The buried seed habit means thatstand age reflectsboth timnesince disturbanceand age of all pin cherrystems in the stand. Wheredormant seeds are abundantin the soil, pure,dense standsof pincherry are foundsoon afterdisturbance, and thinningbegins in thefirst 2 or 3 years. Samplingplant weightin plots of varyingdensity in such pure, dense stands shouldyield results comparable with those from experiments. Balsam firis a stronglyshade-tolerant conifer with a lifespan of 60-80 yearsin the mountainsof thenorth-eastern U.S.A., whereit oftenforms nearly pure stands(> 99% fir)immediately below the timberlineat an altitudeof 1100-1300 in. Many of these high-altitudefir forests are characterizedby 'wave-regeneration,'a peculiar type of naturaldisturbance in whichtree mortality is concentratedin long bands (waves) which movethrough the forest in thedirection of theprevailing wind at 1-3 in peryear (Sprugel 1976). When a wave passes throughan area of firforest the overstorey trees die off,and are replacedby a dense,even-aged stand of fir seedlings. A largetract of wave-regenerated firforest thus contains numerous naturally regenerated stands of nearlypure fir varying widelyin densityand in thedegree to whichthinning has occurred. METHODS Data collection Prunuspensylvanica Four-, six-,and fourteen-year-oldpin cherrystands, generally equivalent in aspect, C. L. MOHLER, P. L. MARKS AND D. G. SPRUGEL 601 slope and drainage,were selected for sampling in New Hampshire,U.S.A. (longitude71? 30' W, latitude440 00' N). The standswere small (0 25-05 ha in size), thusavoiding the patchydispersion of pin cherrycommon in largerstands (5-50 ha), and all standswere denselystocked withpin cherry.Mortality was already much in evidenceeven in the 4-year-oldstand. Density was measuredin randomlyplaced plots varyingin site and numberfrom fifteen1 x 1 in plots in the 4-year-oldstand to twenty2 x 3 in plots in the 14-year-old stand. Plant weightwas estimatedusing dimensionanalysis (Whittaker& Woodwell 1968).In each standa setof sample trees (about twenty-five)was chosento span therange of trunkdiameters observed in theplots, and thesesample trees were excavated carefully by hand to includeroots (roots smallerthan c. 0 5 mm in diameterwere not included). Trunkwood plus barkand rootsof all sampletrees were oven-dried and weighedwithout sub-sampling.Branches were sub-sampled by age-class, and leaves,current twigs (1 -year- old branchesplus 1-year-oldtwigs on older branches),and older partsof each sample branchwere weighed (dry), so thateach of thesetissues could be regressedon branch basal diaineterfor each branchage-class. After estimating dry weights of branchparts on all sampletree branches, another set of regressionswas calculatedto relatethe weight of each tissue(leaves, current twigs, older branch wood and bark,trunk wood and bark,and roots) to trunkdiameter, and theseequations wereused to calculatethe weight of each organon each pin cherryplant in each plot. Abiesbalsamea Plantweight was estimatedby dimension analysis procedures