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The relative importance of spatial and environmental processes in structuring diversity on a forested landscape.
Gilbert, Benjamin*,1, Lechowicz, Martin1, 1 McGill University, Montreal, Quebec, Canada
ABSTRACT- Niche specialisation and neutral interactions among dispersal limited species are two competing hypotheses that attempt to explain plant species diversity and community membership. We tested these hypotheses at Mont St. Hilaire: a temperate, old growth forest near Montreal, Quebec that contains approximately half of the regional flora in an area of 10 km2. We used a sampling technique that decoupled spatial and environmental patterns on the landscape and incorporated diverse plant functional groups (trees, shrubs and herbs) and phylogenetic groups (Asteraceae, Cyperaceae, Poaceae, and seedless vascular plants). Constrained ordinations and distance decay tests were used in conjunction with path analyses to determine the relative importance of each contributing factor. Tree community composition was structured mainly along environmental gradients, but also showed evidence of dispersal limitation. The tree functional group was the only group that showed support for the neutral theory. The other groups were predominantly structured by environmental gradients, although a distance by environment interaction was sometimes present. Niche partitioning differed among groups, with trees and shrubs mainly structured by light availability, and understory plants mainly structured by water and pH or nitrate. Our results indicate that dispersal limitation in this old growth forest is either nested within or secondary to environmental gradients. Models that attempt to predict species richness and species turnover must incorporate a better understanding of fine scale environmental differences, and incorporate space by environment interactions.
Key words: niche, spatial patterns, beta diversity, neutrality