Phylogenetic depth of species turnover in time and space in Minnesota oak savanna communities

Background/Question/Methods —Measures of community dissimilarity and turnover are often based on species identity without considering species relatedness or functional similarity. In this study, we ask how communities change over time and at what phylogenetic depth turnover occurs in response to disturbance and in relation to critical environmental gradients. We test the hypothesis that closely related species are functionally redundant and can replace each other after disturbance (such as a severe drought) achieving constant functional composition and diversity. We also predict that turnover in species composition across steep environmental gradients will occur deep in the phylogeny while turnover in species composition in similar environments will occur near the tips of the phylogeny. We analyzed changes in species composition of understory Minnesota oak savanna communities in a long-term experiment at Cedar Creek Ecosystem Science Reserve, in which communities were subjected to differing frequencies of prescribed burns since 1964. Permanent 50 x 75 m (0.375 ha) study plots were established, each containing 24 1 x 0.5 m quadrats. Surveys of species abundance were conducted in 1984, 1990, 1995, 2000, and 2005. We also measured suites of life history and functional traits of species in these plots and analyzed shifts in functional and phylogenetic composition of communities in relation to light availability, N mineralization, and fire frequency.

Results/Conclusions —We found that the depth of species phylogenetic turnover is proportional to the steepness of critical environmental gradients and that communities with similar environments and successional history attain comparable functional composition despite differences in species composition. We attribute these results to functional redundancy of close relatives. The study highlights that environmental sorting operates on functional traits of organisms rather than on species identity. This has important consequences for understanding community assembly, species turnover across environmental gradients and large-scale patterns of biodiversity.