Development of size hierarchies prior to the onset of density-dependent mortality in irrigated and fertilized loblolly pine stands

Nine years of growth r~iicl stand developmcr~t were i~ivest~gatctl iri r\ 2 2 nutrient and water factorral experiment with four replications. The study was located on an rnfert~le, excessively dra~necl sandy site In Scotlancl County, North C:arolina, U.S.A. The hypothesis tested was that increased growth foll'o\vrng ~nrgation and fertilization would increase the ratc at which sizc hierarchies tlevelop. l'he hypothesis was investigated by comparing the coeficictat of variatlon (CV) of stem volume over trme and exanuning the stem ~ o l u m c relat~ve gl-owth rate (KGK) of trees of different riiitlal size in control and ~n-igated + krtllizcd stands. Even though tliere were no statist~cally s~gnificant differences In C'V among treatments, there was a tendency fbr rncrcascd CV over time In the control stands, whereas C'\' ~nitially increased, then decreased, arltf became constant In the irrigated + fertilized plots The lack of illcrease i r i (:V in the irrigated + fel-tilizcd plots was explained by unt~sually low variation 1x1 RGK across tree size classes and negative relation of RGR and size. Therefore. the hypothesis that increased growth resulted In a more rapid development of s ~ z c hierarchies was rejected. The hrgh RGR of small trees in the irrigated + ftrtil~zed treatment cot~ld not be explained by differences in vertical distrrbution of' needles compared with the control treatment. Small trees In the ~rrigated + fertilized plots were overtopped by neighboring trees to the same degree as small trees In the control plots. Resume : La croissance ct le deveIoppelnent d'un peuplerneitt ont titti ituditis au bout de 9 ant; ?I I'alde d'un d ispos~t~f Sactorrel cornportant deux quantlttis dc nutriments et deux intensites d'irr~gation avec quatre rtipltitlons. Le dispositif Cta~t situti sur un sol sablei~x non fertile ;tvec un drainage cxcess~f datis le corntd de Scotland en Caroline du Nord, aux ~ t a t s ~ n i s . L'objectif consistait i tester I'hypothcse selon laquclle une ~nc~lleurc croissance provoquee par I'lrrigation et la fert~lrsation entrainerait une aug~nentation du taux de hicrrtrchlsatio~i bask sur la di~nens~on. L'hypothcse a tit6 testkc en comparant le coefficient tle variation (CV) du voltnne de la tige dans le tcrnps et en exa~ninant I'accroissemcnt relattf (AR) du volume de la tige chez dcs arbres de dimension initirtle diifiirerite clans des peuplernents ttmoins et dans des peuplen~cnts n~igues et fert~lises. Bier1 que Ic ('V tie differe pas de f;ic;on statistiqtlement significative entre les 11-aitements, i l a tendance i augmenter avec le tenips ti;~ns les peuple~iients tPrnolns alors qu'il augmente au dtihut puis diminue et sc stabilisc dans les parcclics trrigui.cs et fertilisties. L'ahsence d'augmentatton du CV (fans les parcelles trriguties et fertilistics s'expliclue par la variat~on cxceptiorirtellcment farble tlu taux relat~f dc croissance entre les diffirentes classes cie dintension des arbl-cs et la rclatlon ntgative entrc I'AR et la d~rnensron. f'ar constiqt~ent, I'hypothCse seloli laquelle trne augnientation de cl-olssi~ncc entraine Ie dtivclopperncnt plus rapltlc d'une liidrarchisation tics diniensions est rejettic. L'AfI tilevti dcs petites tlgcs dans les p;trcelles trartkes nc pcut Ztrc cxpliquk par ies cfiiTCrciices dans la d~stl-ibution verticale dcs algurlles coriiparativement aux parcclles tcrnotris. I,es pct~ts arbres sont dommCs p:~r les arhres vorsins autant dans Ies parcelles tl~iiotns c l ~ ~ c cf;lns les p;~rcelles traittzs. Introduction etc (We~ner 1985) As the trees grow larger, they beg111 to Conifer stands often undergo a predictable developrnent of stand structure (Mohlcr et al. 1978; Knox et al. 1989). Soon aftcr planting. variation in seedling size and growth rate ts introduced because o f differences in genetics, heterogeneous planting spots, seedling quality. damage, planting quality, compete with each other, the variation In tree size that was introtiriced in the seedlrng stage is increased, and sizc hierarchies develop. Later, ;IS self-thinning begins, stand variation rrernains constant o r dccrcases as thc smallest 11-ees in the stands are removed front the population (Mohler et al. 1978; W e ~ n c r and 'Thomas 1080). C'ornpcttition between nclghbol-ing trees is a continuum beReceived 18 April 2001. Accepted 16 January 2002. tween oneand two-sided cornpetition (Weiner 1990). TwoPublished on the NRC Research Press Web site at sided competition means that competition is symmetric, and http:,/qfi-.n~-c.ca on 22 May 2002. trees ~nterfere with one another in proportion to their size. In I . . U. Nilsson.' Swctfish Uriiversity of Agricultural Sciences. contrast, one-sicfed competition o r resource depletion is Souther11 Swedrsh Forest Research Centre, Box 49. 230 53 asymmetric; large individuals are able to capt t~re a greater Alnal-p, Sweden. share o f a lirniting resollrce that) would be expected according T.J. Albaugh arid f-1.1,. Allen. Box 8008. North ('aroliti;~ to thcir size (Weiner 1900). Thus, in symmetric two-sided State Uliiversity. Raleigh, N<1 27695-8008. 11 S.A. co~nnetition. there should be n o cot-relation, or a negative 1 ' ( . ~nc \~ond ing author (e-mall Illban N~i\soiiicr e \ \ \lu \e) corrclnt~on, between re1;tttve glowti1 rate (RGR) dnd st7e Cnn J fo r . Kes. 32 9XO oh (2002) DO1 10 1 1 ?')IX02-02 I 'c. 2002 Nit(' ( 'anada Can. J . For. Res. Vol. 32, 2002 wtthtn ' 3 crowded poptllat~on ( \ V c ~ n c ~ 1900), u h ~ l e KGli \hould be p o s ~ t ~ v c l y correl,~ted to \ILe tn ~i\ymmctrlc. one\ ~ d e d compct~t~orl (Stoll et '11 1094) Size hierarchies are characterized by variation 111 trce slze. typically with milny small and few large trees nt~tl by large individuals rcpr-ese~iting ;I large proportion of st;tnd biomass (Wciner and Solbrig 1984). Several authors have ;tt-gt~ctf that size hierarchies result fiorn one-sicled cornpetition for light. because cornpetition for light often involves overtopping: I'irge ~nd~v~ciu, i l \ , te 'ible to \h,iccot~s plant\ 'ifter fert~ltr ' t t~on I ~effer \ and T ~ t u \ (I 989) found ~ncr en\cd w e ~neqtlallty for \hoot. toot, and total biomass aftcr fi-rtilization for white spruce. w h ~ l e fertilization only ;~ffected inequality of root t>iom;iss In lodgcpole pine. They coricluded that intert'crcnce from neighbors invoked a plastic I-esponsc in biom;tss allocation and that lodgcpole pitie was mol-e plastic than white spruce. Morris and Myerseotigh ( 1001) found that the onset of selfthinning was increased by improveti nutr~tion if time was used as a base but t l i ~ t t self-thinning was seduced by Increased nutrition IS standing biomiiss was usctl its a base ( i . ~ . , individuals in the fertilized poptllations coulti grou larger per ground ,ireW) and aftcr (1)ccen)ber 2000) tlie 9-year study period. lrrrgated + Year ( ontrol Ilrlgatcd Ferttl~zed fert~l~zed &lean; height ( n ~ ) 190 I 3 4 4 ~ 3 3 1 0 3 ?h(r 3 ? l a 2000 0 1 O c 10 2c I I Oh 13 20 Mean diameter (em) 1091 4 710 4 470 4 470 4 45