Species associations structured by environment, space, and land use history promote beta-diversity in a temperate forest (Powdermill Nature Reserve, Rector, PA)

Background/Question/Methods

Patterns of diversity and composition in forests are controlled by a combination of environmental factors, historical events, and stochastic/neutral mechanisms. Each of these processes has been linked to forest community assembly, but their combined contribution to alpha and beta-diversity in forests has not been fully explored. We used variance partitioning to analyze data on ~40,000 individual trees of 49 species, collected within 4,365 plots spread across a ~900 ha forest reserve in southwestern Pennsylvania to ask 1) To what extent is site-to-site variation in species richness and community composition of a temperate forest explained by environment, space, and land use history? and 2) How do individual species within the community correlate with these factors? Measured environmental variables included topography, soil type, and distance to nearest stream. Land-use history was quantified using aerial imagery available from 1939–2006, and included land-use type (i.e. mined, cultivated, developed, or none), as well as stand age and intensity of land-use. Additionally, eigenvectors obtained through principal coordinates of neighbor matrices were used to identify the spatial signature of potentially important, yet unmeasured, environmental variables, or stochastic processes like dispersal limitation. Habitat associations were then assessed using multivariate regression tree and indicator species analysis.

Results/Conclusions

Environmental variables, space, and land use history explained about half of the variation in both species richness and community composition. Environmental variables explained 28% of the variation for species richness and 29% for composition. Land-use history explained 9% and 4%, respectively. Spatial eigenvectors explained by far the most variation in both cases (richness = 45%; composition = 47%), and also the most variation that was uncorrelated with that explained by environment or land-use history (richness = 20%; composition = 22%). In contrast, much of the variation explained by environment and land-use history was also explained by space. Individual species revealed variable responses to each of these sets of predictor variables. Thirty-four of the 49 species sampled (69%), were positively associated with habitats defined using environmental variables, spatial eigenvectors, or land-use history categories. Several species were associated with stream habitats, and others were strictly delimited across opposing north and south-facing slopes. Several species were associated with areas that experienced human land use, including several late-successional shade-tolerant species. These results indicate that deterministic factors, including environmental and land use history impacts, are important drivers of community response in a temperate forest.