Biogeographical And Evolutionary Processes Influencing The Assembly Of Deciduous Forest Plant Communities

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Ecological theories of community assembly and structuring are often predicated on the overriding importance of small-scale processes operating over short periods of time. However, it is becoming increasingly clear that longer-term, larger-scale processes, such as migration and the diversification of evolutionary lineages, are also important factors influencing the distributions of individual species and the diversity of communities. In this investigation, I first examined the geographic distribution of an ant-dispersed forest plant, Jeffersonia diphylla, to assess whether the population structure of its geographic range in eastern North America exhibited patterns consistent with a distribution in equilibrium with the environment, or whether the species’ limited dispersal ability on local scales might lead to a non-equilibrial distribution at large geographic scales. Population size and performance did not decline toward the northern range edge and seed sowing within and beyond the species’ northeastern range edge demonstrated potentially suitable habitat up to 300 km outside its range. As such, the range of J. diphylla may not be in equilibrium with the environment and its restricted distribution in the Northeast may trace to limited post-glacial migration. These findings highlight the potential for limited migratory responses of plant species to climate change, raising the possibility that human intervention or ‘assisted colonization’ may be necessary to aid some species in tracking modern climate change. Finally, I investigated a prominent plant species richness gradient in the forests of the northeastern U.S. to determine whether long- term evolutionary processes, including phylogenetic niche conservatism, may contribute to high species richness in communities on fertile, calcium-rich soils. Analyses of the phylogenetic ‘depth’ of communities along this gradient demonstrated that species-rich communities on calcium-rich soils included a disproportionate representation of Basal Angiosperm and Lower Eudicot angiosperm taxa. Parsimonybased reconstructions of the ancestral calcium niche of lineages present in the study also suggested a key role for fertile soils in the early diversification of angiosperms in Temperate Deciduous Forests. These patterns suggest that calcium-rich soils may be an ecological ‘zone of origin’ for angiosperms and highlight the potential for longterm evolutionary processes to influence species diversity in contemporary communities.

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