Pine barrens in central Wisconsin still phylogentically clustered after 54 years of fire suppression
Fire suppression has greatly changed the species diversity and composition of plant communities. Its effects on phylogenetic community structure, however, are largely unknown. To study these effects, we compared 1958 survey data to 2012 resurvey data from the same 30 pine barrens sites in central Wisconsin not burned since the 1930s. We built a phylogeny for 177 taxa found at these sites using sequence data from two plastid ‘barcode’ loci (rbcL and matK). We also measured several key functional traits for most of species. We then applied phylogenetic generalized linear mixed models (PGLMMs) to identify phylogenetic patterns as well as underlying mechanisms involving environmental factors and species traits of these communities in both 1958 and 2012. Specifically, we tested (1) whether communities show phylogenetic attraction (i.e., closely related species are more likely to co-occur) or phylogenetic repulsion; (2) whether phylogenetic structure can be explained by environmental variables, especially canopy cover that increased between 1958 and 2012; and (3) whether functional traits can explain species abundance patterns better than phylogeny.
These pine barrens communities showed very strong phylogenetic attraction in both 1958 and 2012, according to either null models or PGLMMs. Species responded individually to environmental variables and these different responses were independent of phylogeny. Therefore, the observed phylogenetic attraction cannot be explained by the environmental variables we measured. Similarly, sites selected different species based on their functional traits. However, including the effects of traits only explained a fraction of the phylogenetic attraction, even though most functional traits in our study showed strong phylogenetic signal. Our results suggest that environmental filtering was a dominant driver of these communities in both 1958 and 2012. The specific filters, however, have changed over time, favoring different sets of species. Furthermore, while in principle phylogenetic structure should be explained by functional traits, in practice the large number of functional traits we measured were not sufficient to explain phylogenetic community structure. These results argue for the value of phylogenetic community analyses even when abundant trait data are available.