Phylogenetic repulsion in the assembly of Floridian oak communities J Cavender Bares1 2 3 D D Ackerly4 D A Baum5 and F A Bazzaz2 Running head Phylogenetic repulsion of Floridian oaks 1 Corresponding author Smithsonian Environmental Research Center 647 Contees Wharf Road Edgewater MD 21037 2 Harvard University Department of Organismic and Evolutionary Biology Cambridge MA 02138 3 Contact info March 1 Aug 31 2003 Centre d Ecologie Fonctionelle et Evolutive CNRS 1919 route de Mende 34293 Montpellier cedex 05 Tel 33 0 467613289 Fax 33 0 467412138 Cavenderj si edu 4 Department of Biological Sciences Stanford University Stanford CA 94305 5 Department of Botany University of Wisconsin Madison WI 53706 Keywords Plant community structure null models phylogenetic repulsion independent contrasts niche overlap Quercus conserved and convergent traits soil moisture 2 Abstract Plant species that occur together in communities and experience similar environmental conditions are likely to share phenotypic traits due to the process of environmental filtering At the same time species that are too similar are unlikely to cooccur due to competitive exclusion Communities that are dominated by multiple species of a single genus are challenging to explain because they appear to present an exception to the principle of competitive exclusion We examined the phylogenetic structure of oak dominated forest communities in north central Florida in relation to ecological and physiological traits of the seventeen species of oaks that occur in this region Oak species co occurring within communities are more distantly related than expected by chance and the most closely related species occur in the same communities less often than expected providing evidence for phylogenetic repulsion Within individual clades species tend to show spatial overdispersion among plots and greater niche partitioning across a soil moisture gradient than expected However cumulative distributions of species in the two major clades i e red oaks and white live oaks representing the deepest phylogenetic node show greater niche overlap than expected Hence communities are more likely to include members of both the red oak and the white live oak clades than expected by chance and the distributions of the two clades across a soil moisture gradient are significantly more similar than expected This pattern of phylogenetic repulsion within clades can be explained because traits important for habitat specialization show high evolutionary convergence At the same time we hypothesize that certain conserved traits permit coexistence of distantly related congeners 3 Introduction A critical challenge in community ecology is to understand how the ecophysiological attributes of species influence community assembly and how these attributes and hence community composition change over evolutionary time scales There are two central processes involved in the assembly of communities 1 the ecological filtering of species that can exist within a community based on the abiotic environment Weiher and Keddy 1995 Weiher et al 1998 and 2 limitations to longterm coexistence including competitive exclusion Gause 1934 Elton 1946 MacArthur and Levins 1967 Chesson 1991 Leibold 1998 These two processes make opposite predictions about the phenotypic and phylogenetic similarity and dissimilarity of cooccurring species Tofts and Silvertown 2000 Webb 2000 If closely related species share similar physiological limitations and exhibit evolutionary niche conservatism the former process ecological filtering will tend to cause closely related species to co occur this pattern can be termed phylogenetic attraction or underdispersion In accord with this prediction several previous studies have shown that related species are more likely to cooccur than expected by chance Tofts and Silvertown 2000 Webb 2000 On the other hand competitive exclusion should limit the coexistence of closely related species that are phenotypically similar leading to the opposite pattern of phylogenetic repulsion with co occurring species being phylogenetically overdispersed This latter pattern has been hypothesized but has yet to be demonstrated using phylogenetic analysis Communities that are dominated by multiple species of a single genus are challenging to explain because they appear to present an exception to the principle of competitive exclusion Most studies of this problem have focused on niche and character differentiation of sympatric congeneric species Wilson and Lee 1994 Price et al 2000 in an effort to find ecological and evolutionary mechanisms that may prevent competitive exclusion Armbruster 1985 Schluter and McPhail 1992 Schluter 1996 Radtkey et al 1997 Adams and Rohlf 2000 It is also important to look beyond the genus designation and consider phylogenetic relationships at higher resolution Phylogenetic repulsion or attraction may occur at any level of phylogenetic resolution Silvertown et al 2001 4 reflecting patterns of trait evolution and the relative importance of ecological filtering and competitive exclusion Webb et al 2002 Oak dominated forests provide an important model system to address these questions in plant communities In North Central Florida seventeen species of oaks cooccur The high diversity of congeneric species can be explained in part by habitat differentiation at the landscape level with species occurring in three major community types hammock sandhill and scrub differentiated by contrasting fire regimes and or soil moisture and soil fertility regimes Cavender Bares et al submitted However at small spatial scales up to six species of oaks can still occur within a single 0 10 ha plot In this study we investigated the phylogenetic structure of co occurring oak species in these communities and examined the phenotypic traits and habitat features likely to give rise to this structure We estimated the phylogeny of the co occurring oaks using molecular data and we used environmental ecophysiological and morphological measurements in the field and greenhouse reported in Cavender Bares and Holbrook 2001 Cavender Bares et al submitted to identify key functional traits and habitat features We set out to test the hypothesis that co occurring congeneric species would show patterns of either phylogenetic repulsion due to factors limiting coexistence of phenotypically similar species or phylogenetic attraction due to niche conservatism or both patterns at different phylogenetic scales In
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