Deterministic tropical tree community turnover: evidence from patterns of functional beta diversity along an elevational gradient

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1 Downloded from rspb.roylsocietypublishing.org on Februry 8, Proc. R. Soc. B () 78, doi:.98/rspb..69 Published online September Deterministic tropicl tree community turnover: evidence from ptterns of functionl bet diversity long n elevtionl grdient Nthn G. Swenson, *, Pedro Angld-Cordero nd John A. Brone Deprtment of Plnt Biology, Michign Stte University, Est Lnsing, MI 88, USA Institute for Ecosystem Studies, University of Puerto Rico, Rio Piedrs, Puerto Rico 78 Deprtment of Biology, Columbus Stte University, Columbus, GA 97, USA Explining the mechnisms tht produce the enormous diversity within nd between tropicl tree communities is pressing chllenge for plnt community ecologists. Mechnistic hypotheses rnge from niche-bsed deterministic to dispersl-bsed stochstic models. Strong tests of these hypotheses require detiled informtion regrding the functionl strtegies of species. A few tropicl studies to dte hve exmined trit dispersion within individul forest plots using species trit mens in order to sk whether coexisting species tend to be more or less functionlly similr thn expected given null model. The present work tkes n lterntive pproch by: (i) explicitly incorporting popultion-level trit vribility; nd (ii) quntifying the functionl bet diversity in series of 5 tropicl forest plots rryed long n elevtionl grdient. The results show strong pttern of decy in community functionl similrity with elevtion. These observed ptterns of functionl bet diversity re shown to be highly nonrndom nd support deterministic model of tropicl tree community ssembly nd turnover. Keywords: community ecology; functionl trits; lef re; Puerto Rico; specific lef re; wood density. INTRODUCTION Tropicl communities re renowned for their biodiversity. Not only is there n elevted level of species diversity within communities (i.e. lph diversity) with respect to less diverse temperte communities, but species turnover between tropicl communities (i.e. bet diversity) is lso elevted. Perhps, nowhere re ptterns of tropicl diversity more often documented thn in tree communities (e.g. [ 5]), yet substntil debte still exists on the extent to which these ptterns re the result of deterministic or stochstic processes (e.g. [6 8]). Understnding the degree to which ptterns of diversity re the result of deterministic or stochstic processes is prticulrly criticl for those trying to conserve nd predict the fte of tropicl biodiversity. The mjority of reserch into ptterns of tropicl tree diversity nd turnover hs focused on one type of biodiversity: species diversity. The functionl component of biodiversity in these communities hs been less well quntified (but see [8 ]). This is unfortunte becuse documenting the functionl strtegies of species in communities my provide refined or novel mechnistic insights into their structure nd diversity []. One principle concern for community ecologist is tht trditionl nlyses of species bet diversity re unble to determine whether or not the communities re functionlly similr or dissimilr. In figure, we * Author for correspondence (swensonn@msu.edu). Electronic supplementry mteril is vilble t 98/rspb..69 or vi present one exmple of this where species my be dispersl limited cusing complete species turnover between two communities, but the environment lso plys key role in determining the community ssembly, thereby generting functionlly nlogous communities (figure ). This would result in decoupling of species nd functionl bet diversity where species bet diversity is high nd functionl bet diversity is low. We would expect this result to be prticulrly importnt when compring ssemblges between regions tht shre similr environments. Despite the potentil insights one cn gin from nlysing the functionl similrity of communities, the concept of functionl bet diversity is rther new nd it still remins to be deeply explored (but see [,]). To dte, the mjority of the work on this topic hs regrded the development of functionl bet diversity metrics similr to those designed to nlyse species bet diversity (e.g. []) or phylogenetic bet diversity [5,6]. To our knowledge, there hve been no nlyses of functionl bet diversity of communities rryed long brod environmentl grdients nd no nlyses of diverse ecosystems such s tropicl rinforests. Thus, tests using such dtsets nd multiple metrics re needed. When compring communities long n environmentl grdient it is probble tht species bet diversity nd functionl bet diversity will be, on verge, positively relted. In such instnces, the question becomes whether the functionl turnover between communities is higher or lower thn tht expected given the species turnover. For exmple, higher thn expected functionl turnover between communities given the species turnover could occur if the species in the two communities re more Received 5 June Accepted August 877 This journl is q The Royl Society

2 Downloded from rspb.roylsocietypublishing.org on Februry 8, 878 N. G. Swenson et l. Tropicl functionl bet diversity community B trit pool community A b high species turnover higher thn expected functionl turnover high species turnover rndom functionl turnover high species turnover lower thn expected functionl turnover z z y community C t m b c d... m n o... x y z community D b Figure. A crtoon depicting the species nd functionl turnover between set of four hypotheticl communities. The shpes indicte species identity nd the letters indicte the functionl strtegy of the species. The trit pool represents ll of the functionl strtegies tht could potentilly colonize community. Community A shres no species in common with ny of the other communities nd it therefore hs the sme level of species turnover from A to B, A to C nd A to D. Community A nd community B shre no functionl strtegies. Further, the strtegies present in community B re significntly more dissimilr from those in community A thn expected if one were to rndomly pull three strtegies from trit pool. This would be expected if n underlying environmentl grdient determines species turnover nd community ssembly. Community A nd community C shre no functionl strtegies, but the functionl turnover is indistinguishble from rndom pull of three functionl strtegies from the trit pool. This would be expected under stochstic model of species turnover nd community ssembly. Community A nd community D re functionlly nlogous nd there is less functionl turnover thn expected given rndom pull of three strtegies from the trit pool. This would be expected where dispersl is limited but community ssembly is deterministic with respect to the environment. The figure illustrtes tht three different ecologicl processes cn be determined by exmining the functionl nd species turnover between communities simultneously, while nlyses of species turnover lone could provide erroneous ecologicl inferences. functionlly divergent thn expected given the species pool. This would indicte tht the species turnover long the grdient is not simply owing to dispersl limittion nd tht the underlying environmentl grdient plys key role in influencing species turnover nd community ssembly. Thus given the importnce of quntifying the functionl strtegies of species within nd mong communities, ecologists re incresingly promoting functionl trit-bsed pproch to community ecology []. Plnt community ecologists re incresingly integrting functionl ecology into their reserch progrmmes by quntifying functionl trits on the individuls nd species in their study systems (e.g. [7 ]). Functionl trits re morphologicl nd physiologicl trits tht re indictive of the ecologicl strtegies of species. An dvntge of functionl trits is tht they llow for rpid nd stndrdized inventory of severl mjor xes of plnt functionl differentition cross ll species in diverse communities [ ]. Indeed this dvntge hs led to the pioneering work into the reltive influence of deterministic nd stochstic processes on ptterns of species coexistence in tropicl tree communities [8]. Despite the promise of functionl trit-bsed prdigm in community ecology there re severl outstnding chllenges. First, the ppropriteness of the functionl trits used in community ecology investigtions is routinely questioned nd determining which re the right trits to use is importnt. In recent yers, there hs been gret del of focus on severl plnt functionl trits tht hve been proposed to be importnt indictors of plnt ecologicl strtegies [ ]. These soft functionl trits re proxies of ctul physiologicl processes nd they re reltively esy to mesure compred with hrd functionl trits tht re often more closely linked to the physiologicl processes of interest, but much more difficult to quntify cross diverse communities [5]. While the debte of which re the best trits or the right trits for community ecology investigtions will continue, n initil step is to determine when nd where those often used soft functionl trits cn provide novel insights into the mechnisms determining community ssembly nd diversity. A second chllenge hs been tht of intrspecific functionl trit vribility [,6]. Mny functionl trit investigtions of communities hve ssigned specieslevel trit mens to ll individuls or popultions (e.g. [8,]). In some cses, these men vlues re clculted from few individuls from the study system, nd in others these vlues come from globl dtbses where Proc. R. Soc. B ()

3 Downloded from rspb.roylsocietypublishing.org on Februry 8, Tropicl functionl bet diversity N. G. Swenson et l. 879 the mesured individuls my be from very different geogrphicl region (e.g. [9,9]). Given tht the trits of interest re known to vry with respect to the locl-scle environment, ssigning species-level mens my introduce severl bises nd reduce the power of functionl trit-bsed community ecology. Perhps nowhere will this be more importnt thn when exmining the functionl similrity of communities (i.e. functionl bet diversity) rryed long substntil environmentl grdient. In prticulr, lrge chnges in popultion-level trit vlues owing to physiologicl responses to n environmentl could promote further decoupling of species nd functionl bet diversity, whereby functionl bet diversity would increse t fster pce thn species bet diversity. A result such s this would provide further evidence tht deterministic fctors ply lrge role in determining the functionl composition nd ssembly of communities. Thus, when fesible, quntifying trends in popultion-level trit mens long environmentl grdients re criticl next step in functionl trit-bsed community ecology. The present study provides, to our knowledge, the first nlysis of functionl bet diversity long substntil environmentl grdient. Specificlly, here we quntify severl importnt functionl trits representing plnt ecologicl strtegies in series of 5 permnent forest inventory plots rryed long 7 m elevtionl grdient in Puerto Rico. Given the potentil for substntil intrspecific vrition in trit vlues for species with lrge elevtionl rnges, ll nlyses in the present work use popultion-level trit mens in lieu of species-level trit mens. The trit dt were used to quntify the functionl bet diversity of the forest plots long the grdient in order to ddress the fundmentl question of whether tropicl tree community turnover is stochstic or deterministic nd to sk the following sub-questions: (i) is there distnce decy in functionl similrity between the forest plots similr to the distnce decy in species similrity? nd (ii) is the functionl turnover between forest plots fster or slower thn expected given the observed ptterns of species turnover? We propose tht deterministic species turnover nd directionl species functionl responses to the environmentl grdient should led to higher thn expected functionl turnover given the species turnover. If the functionl turnover is rndom with respect to species turnover or lower thn expected, this would indicte lrger role for stochstic processes influencing the species turnover nd community ssembly in the study system.. MATERIAL AND METHODS () Study re nd forest inventory plots The present study took plce in the Luquillo Experimentl Forest (LEF) locted in estern Puerto Rico (889.6 N, W). The LEF is pproximtely h in re with elevtions rnging from to 75 m bove se level (.s.l.) [7]. At m.s.l. the temperture verges.58c during the wrmest month of September nd nnul rinfll totls mm on verge. At 75 m.s.l. the temperture verges.8c during September nd the nnul rinfll totls 6 mm on verge. The elevtionl grdient spns from premontne rinforest to cloud forest. Permnent forest inventory plots were estblished from to long the Sondor River wtershed within the LEF [8]. The forest plots were rryed every 5 m in elevtion from to m.s.l. The sites selected for ech forest plot hd little to no previous humn disturbnce nd did not include rvines [8]. Ech forest plot is. h in re (5 m) with ll free-stnding woody stems cm in dimeter t brest height ( cm bove the ground) mpped, tgged nd identified. As the flor of the LEF is well-known, over 99 per cent of the stems could be identified to species nd voucher specimens were deposited in the El Verde Field Sttion Herbrium. There re totl of 656 stems nd species found in the 5 forest plots. In ech plot, we clculted the species lph nd bet diversity. The species bet diversity ws clculted using Bry Curtis Distnce in the R pckge VEGAN. (b) Functionl trits nd dendrogrm A totl of six trits were chosen for this study to quntittively represent severl xes of plnt functionl strtegy []. Specific lef re (), lef nitrogen content (%N) nd lef phosphorus content (%P) were used to represent the well-documented lef economics spectrum [9]. Woodspecific grvity ws used to represent wood economics spectrum []. Lef re ws used to represent trde-off between re deployed for light cpture nd lef tempertures [], nd mximum height ws used to represent the dult light niche of the species []. Ech trit ws mesured on multiple individuls in ech forest inventory plot following the protocols in Cornelissen et l. []. When possible, 5 individuls were smpled for lef nd height trit mesurements nd individuls were used for wood-specific grvity mesurements. A recent power nlysis of lef functionl trits in tropicl forest community hs suggested tht similr smpling intensity is necessry for relible estimtions of popultion men trit vlues [6]. Smpling ws fewer thn 5 nd individuls respectively, only when tht mny individuls could not be locted t the given elevtion. For ech trit, distnce mtrix ws constructed to represent the trit similrity between ll popultions using popultion-level men trit vlues for ech species. This distnce mtrix nd UPGMA clustering were used to construct dendrogrm for ech trit []. This dendrogrm represented the trit similrity between popultions. Two of the functionl dissimilrity metrics shown below, nd, cn lso be implemented without using trit dendrogrms, wheres the other, F sor, cnnot. Thus, for consistency we hve chosen to use the dendrogrms for ech metric. Further studies re needed to ddress the degree to which using dendrogrms versus rw trit distnces influence functionl bet diversity metrics. As this study ws lso interested in composite mesure of functionl similrity between species, we used principle coordintes nlysis (PCA). To perform the PCA, we first logged the trit dt nd converted it to stndrd norml devites to identify the mjor xes of multivrite functionl similrity. The first three PCA xes explined over 96 per cent of the vrition (see the electronic supplementry mteril, tble S) nd the Eucliden distnce between popultions in this three-dimensionl spce nd UPGMA clustering were used to construct composite functionl dendrogrm representing the overll functionl similrity between popultions. Proc. R. Soc. B ()

4 Downloded from rspb.roylsocietypublishing.org on Februry 8, 88 N. G. Swenson et l. Tropicl functionl bet diversity (c) Functionl bet diversity The present study implemented three metrics of functionl bet diversity. The concept nd mesurement of functionl bet diversity re still in their infncy nd further work will be needed to compre nd contrst the reltive benefits of vrious metrics. Here, we hve chosen to use three potentil metrics of functionl similrity between communities s strting point without mking ny prticulr judgement with respect to their reltive merits. The first metric used we term Functionl Sorensen s Index, F sor. This metric is presence bsence weighted nd defined s: BL k k F sor ¼ ðbl k þ BL k Þ= ; where BL k k ; is the totl dendrogrm brnch length common to ll species in communities k nd k, nd BL k nd BL k re the totl dendrogrm brnch length common to the species within communities k nd k, respectively. This metric is functionl trit nlogue of the phylogenetic metric, PhyloSor, introduced by Brynt et l. [5]. It is lso nlogous to trditionl Sorensen s Index. Thus, the F sor metric provides n overll indictor of the shred function between two communities. The next two metrics we employed use bundnce informtion. First, we employed n bundnce weighted nerest functionl neighbour dissimilrity,, between two communities, k nd k. This metric ws originlly published by Ricott & Burrscno [] nd is defined s: P nk i¼ ¼ f i min d ik þ P n k j¼ f j min d jk ; for species i = species j where n k is the number of species in community k, f i the reltive bundnce of species i in community k, nd min d ik the dendrogrm brnch lengths between species i in community k nd its nerest functionl neighbour in community k tht is not the sme species. Thus if two communities shre functionlly nlogous or nerly nlogous species, the verge nerest neighbour distnces will be much lower thn two communities tht hve very dissimilr community trit distributions. The third metric ws n bundnce weighted pirwise dissimilrity,, between communities k nd k modified from the metric described bove. This metric is defined s: P nk i¼ ¼ f id ik þ P n k j¼ f jd jk ; for species i = species j where d ik is the men pirwise distnce on the dendrogrm between ll species in community k nd ll species in community k excluding conspecific species. The is somewht different thn most bet diversity metrics becuse it compres ll pirwise distnces between species in the two communities. Thus we my not expect it to correlte with ptterns of species bet diversity s well s the previous two metrics. (d) Null model nd stndrdized effect sizes As discussed bove, it is resonble to expect species nd functionl bet diversity to be correlted. While we were interested in documenting ptterns of functionl bet diversity in the study system, we were lso interested in determining whether the functionl bet diversity ws higher or lower thn expected given the species bet diversity. In order to ccomplish this we performed null modelling nlyses. A null distribution of functionl bet diversity vlues ws generted for ech trit by rndomizing the nmes of the popultions cross the tips of the trit dendrogrms 9999 times. During ech itertion, the functionl bet diversity ws clculted cross the entire elevtionl grdient. These vlues were used to generte the null distribution. Therefore, the rndomiztion procedure only rndomized the functionl similrity of popultions while mintining the observed species occupncy rtes, community species richness levels, community bundnce distributions nd species bet diversity. Further, s the null model mintins the observed distributionl ptterns of species nd their popultions, the observed dispersl limittion of species is conserved in the null model. A stndrdized effect size (SES; [6]) ws clculted for functionl bet diversity using the men nd stndrd devition of the null distribution s follows: SES ¼ X obs X null s:d:ðx null Þ ; where X obs is the observed dissimilrity vlue (i.e. ) between two communities, X null the men of the null distribution nd s.d.(x null ) the stndrd devition of the null distribution. Vlues greter thn.96 indicte higher thn expected functionl dissimilrity between the communities nd vlues below.96 indicte lower thn expected functionl dissimilrity between the two communities. Non-rndom SES vlues could be generted by functionlly non-rndom replcement of species long the grdient nd/or popultion-level trit mens shifting directionlly long the grdient.. RESULTS () Species lph nd bet diversity There ws generl decline in species lph diversity with incresing elevtion (see the electronic supplementry mteril, figure S) lthough the pttern my hve become unimodl if lower elevtion forests could hve been censused. The species lph diversity in the plots rnged from 8 species t 95 m.s.l. to mximum of 5 species t m.s.l. The species bet diversity ws lso clculted nd the results show generl increse in community dissimilrity with incresing elevtion using Bry Curtis Distnce (see the electronic supplementry mteril, figures S nd S). (b) Functionl bet diversity nd stndrdized effect sizes The functionl bet diversity of the forest plots ws determined using three different metrics. In the min text, we only present the functionl dissimilrity between the plot t the lowest elevtion nd the other forest plots long the grdient, but ll pirwise comprisons between plots re presented in the electronic supplementry mteril. The results show generl decy in functionl similrity with elevtion cross ll trits using both the presence bsence weighted F sor metric nd the bundnce weighted nerest neighbour metric, (figure nd electronic supplementry mteril, figure S). The results were not s consistent using the bundnce weighted pirwise metric,. Specificlly, dissimilrity Proc. R. Soc. B ()

5 Downloded from rspb.roylsocietypublishing.org on Februry 8, Tropicl functionl bet diversity N. G. Swenson et l. 88 () height (b) height (c)..6 F sor... height F sor.8 lef re. lef re lef re F sor.5 lef N. lef N.9 lef N.7 F sor.5.5 lef P. lef P.5. lef P F sor F sor.. WSG. WSG WSG F sor.. PCA PCA PCA Figure. The bet functionl diversity long the elevtionl grdient in the Luquillo Experimentl Forest presented s the dissimilrity between the plot t the lowest elevtion ( m.s.l.) nd the other forest plots spced every 5 m in elevtion. () Clcultes bet functionl diversity using the presence bsence weighted F sor metric, (b,c) clculte bet functionl diversity using the bundnce weighted nerest neighbour metric, nd the pirwise metric,, respectively. incresed with elevtion for mximum height, lef re, lef nitrogen content, lef phosphorus content nd PCA, but the wood-specific grvity results were unimodl (figure nd electronic supplementry mteril, figure S). In n dditionl nlysis we compred the trit dissimilrity vlues between plots with the Bry Curtis Distnce between plots. The Bry Curtis Distnce ws positively correlted with the F sor nd trit dissimilrity metrics (see the electronic supplementry mteril, figure S). The metric on the other hnd ws generlly not correlted with the Bry Curtis Distnce (electronic supplementry mteril, figure S). Given the strong reltionship between some of the functionl bet diversity metrics nd species bet diversity nd our desire to know whether the functionl bet diversity ws higher or lower thn tht expected given the species bet diversity, we conducted null model nlyses. The SESs for the F sor, nd, metrics were generlly positive cross ll trits nd consistent cross elevtion with effect sizes often greter thn.96 (figure ). The constncy in the positive vlues greter thn.96 is indictive of higher thn expected functionl trit dissimilrity or turnover with elevtion. A notble exception to this generl pttern is the results for the metric where there ws lower thn expected turnover initilly long the grdient followed by lrger thn expected turnover when compring the lowest plot to the high elevtion plots (figure ).. DISCUSSION The present study sked whether the turnover of tropicl tree communities long n elevtionl grdient is deterministic or stochstic with respect to species function. To dte, tropicl tree community turnover hs often been nlysed using lists nd bundnces of species tht re treted s functionlly equivlent or binned into brod ctegoriclly defined functionl groupings. Recently, Proc. R. Soc. B ()

6 Downloded from rspb.roylsocietypublishing.org on Februry 8, 88 N. G. Swenson et l. Tropicl functionl bet diversity SES F sor SES F sor SES F sor SES F sor SES height height height SES lef re lef re lef re SES lef N lef N lef N SES lef P lef P lef P SES SES SES SES SES F sor SES F sor SES SES WSG WSG WSG SES SES SES F sor PCA SES PCA SES PCA Figure. The bet functionl dispersion long the elevtionl grdient in the Luquilllo Experimentl Forest. The dt re presented using the stndrdized effect size (SES). SES vlues bove.96 (the upper dshed grey line) indicte significntly higher thn expected bet functionl diversity, or dissimilrity, given the observed bet species diversity. SES vlues below.96 (the lower dshed grey line) indicte significntly lower thn the expected bet functionl diversity, or dissimilrity, given the observed bet species diversity. species men functionl trit vlues hve been used in two tropicl forest plots to quntify whether the functionl lph diversity in forest subplots is higher or lower thn expected given the species richness [8,]. To dte, this work hs not ddressed the turnover of tree species in these diverse systems nd it hs not explicitly delt with the potentilly high level of intrspecific vrition in functionl trit vlues. In this study, to our knowledge, we hve presented the first nlysis of functionl bet diversity in tropicl tree communities, nd we hve provided the first functionl trit nlysis of tropicl tree communities tht explicitly incorportes intrspecific trit vrition. The functionl bet diversity results showed generl non-rndom compositionl turnover of function long the elevtionl grdient, suggesting lrge role for deterministic processes in controlling community turnover long the grdient. Below, we discuss these results nd their implictions in more detil. The primry gol of the present study ws to sk whether the turnover of tropicl tree communities is reltively deterministic or stochstic with respect to species function. To ddress this we quntified the functionl bet diversity or dissimilrity between the forest plot t the lowest elevtion to the other forest plots t higher elevtions using three different metrics (figure nd electronic supplementry mteril, figure S). In generl, the functionl bet diversity incresed with elevtion using ll three metrics (figure nd electronic Proc. R. Soc. B ()

7 Downloded from rspb.roylsocietypublishing.org on Februry 8, Tropicl functionl bet diversity N. G. Swenson et l. 88 supplementry mteril, figure S). In other words, there ws cler distnce decy in functionl similrity from the lowest to the highest forest plot. Thus, the functionl composition of the tree communities long the elevtionl grdient chnges directionlly for ll of the trits studied. Further nlyses reveled tht two of the metrics used were strongly correlted with species bet diversity, thereby mking it difficult to determine whether functionl turnover long the grdient ws ny different from tht expected given the underlying species turnover. Consequently, it is tenuous to reject stochstic model of community turnover nd ssembly from functionl distnce decy results lone nd null model nlyses re necessry. The results from the null model nlyses for ll of the metrics were generlly non-rndom with SESs often greter thn.96 (figure ). Thus, the functionl composition of the communities long the elevtionl grdient turns over fster thn expected given the species compositionl turnover nd chnges in species bundnce. The fster thn expected turnover in function between communities could occur for two resons. First, those species tht replce ech other from one community to the next re functionlly more divergent thn expected given the species pool. For exmple, sy tht there ws complete turnover in the species composition when trnsitioning from to 9 m elevtion. If the species t 9 m re non-rndom subset of the species pool tht is functionlly more distnt thn expected t rndom from those found t m, then this would indicte tht the species turnover is not simply stochstic. Second, if there re substntil directionl trends in popultion-level trit mens long the environmentl grdient, the functionl turnover would be fster thn tht expected given the species turnover. While there is high level of species turnover long the grdient studied (see the electronic supplementry mteril, figures S nd S), nowhere is there complete turnover in species composition between forest plots, nd few species spn the entire elevtionl grdient. We therefore suggest tht both of the bove processes probbly generted the fster thn expected functionl turnover reported in this study. Further, both functionlly non-rndom replcement of species long grdient nd directionl trends in popultion-level trit vlues long grdient re evidence for deterministic species turnover nd community ssembly long the grdient. The primry gol of the current study ws to document ptterns of functionl turnover in tree communities long substntil environmentl grdient in the tropics. To chieve this gol, we implemented three metrics of functionl bet diversity. In conducting the work we found tht the first two metrics, F sor nd, were strongly relted to Bry Curtis Distnce while the third metric,, ws not (see the electronic supplementry mteril, figure S). Thus, the first two metrics my be considered more closely ligned with wht is trditionlly considered to be bet diversity while the third metric my just serve s n lterntive mesure of similrity tht cnnot be esily fitted into diversity-prtitioning frmework. As the topic of functionl bet diversity becomes more brodly studied, detiled investigtions into when nd why these prticulr metrics nd other metrics re preferred will be needed. In summry, here we hve provided, to our knowledge, the first functionl bet diversity nlyses of series of tropicl tree communities rryed long substntil ecologicl grdient, to ddress whether the compositionl turnover between these communities is reltively stochstic or deterministic. We lso present, to our knowledge, the first nlysis of functionl bet diversity tht explicitly incorportes intrspecific vrition in trits long brod grdient. The results presented re lrgely non-rndom nd the turnover of the functionl composition of communities is generlly fster thn tht expected long the grdient. The results re lso consistent cross ll of the trits studied nd ech of the metrics used, which indictes tht the compositionl chnge long the grdient involves severl xes of plnt functionl differentition nd is not just shift in ny one trit or xis. These results suggest the predominte role of deterministic fctors promoting the compositionl turnover of the tropicl tree communities long the grdient nd lesser role for stochsticity. 5. CONCLUSIONS AND FUTURE DIRECTIONS Here, we hve presented n initil test of whether or not the compositionl similrity of tropicl tree communities is non-rndom with respect to species function. The results show tht tropicl tree community similrity in our study system is indeed functionlly non-rndom nd supports deterministic model of tropicl tree community ssembly nd turnover. Future work should im to repet similr pproch cross severl grdients within nd outside the tropics to determine whether or not similr distnce decy reltionships occur in other systems. The work hs lso successfully integrted intrspecific functionl trit vrition into the nlyses. Accounting for this vrition will become incresingly importnt s the scle of functionl trit investigtions into community structure brodens nd even more so s functionl bet diversity becomes incresingly mesured cross brod ecologicl grdients. Additionl studies will be needed to quntify to wht degree intrspecific vribility will bis functionl bet diversity nlyses tht use specieslevel men trit vlues nd how this bis my be enhnced or mitigted in systems with more or less vrible species. N.G.S. is supported by Michign Stte University. An NSF grnt (DEB-858) to the Institute for Tropicl Ecosystem Studies, University of Puerto Rico nd to the Interntionl Institute of Tropicl Forestry, US Forest Service (US Deprtment of Agriculture), s prt of the Long-Term Ecologicl Reserch Progrmme funded the originl instlltions of the forest inventory plots. N.G.S. would like to thnk Ctherine Hulshof for her ssistnce in the field. N.G.S. would like to thnk the NCEAS Working Group on Bet Diversity (NCEAS Project 7) for inspirtion nd converstions regrding this topic. We would like to thnk Jess Zimmermn nd Nick Brokw for their logisticl support nd encourgement. REFERENCES Gentry, A. H. 98 Ptterns of neotropicl plnt species diversity. In Evolutionry biology, vol. 5 (eds M. K. Hecht, B. Wllce & E. T. Prnce), pp. 8. New York, NY: Plenum Press. Gentry, A. H. 988 Chnges in community diversity nd floristic composition on environmentl nd geogrphicl Proc. R. Soc. B ()

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