Shocks to the system: Community assembly of the oak savanna in a 40-year fire frequency experiment

Fire is a major force driving the evolution of plants and the structure and function of ecosystems globally. It thus likely operates as an important environmental filter that selects for species that have evolved to tolerate and depend on fire. Across a 40-year experimental fire gradient from frequently burned open savanna to unburned dense woodland in Minnesota, USA, we examined the relationships among community assembly, evolutionary history, and functional trait composition. Close relatives had similar abundance patterns across the fire gradient, providing evidence for phylogenetic conservatism in fire adaptation and highlighting the importance of shared ancestry in predicting species responses to fire. Phylogenetic beta diversity was greatest between the most extreme fire treatments across the gradient, indicating that species in the most contrasting fire regimes were most distantly related. Fire strongly influenced diversity, co-occurrence patterns, and leaf trait means and variances within communities. The most frequently burned communities had the highest species richness, exhibited the most resource-conservative leaf traits, and spanned the greatest number of phylogenetic lineages but harbored more close relatives within those lineages than other communities. In contrast, unburned communities had the lowest species diversity, the most acquisitive leaf traits, and the fewest phylogenetic lineages, but close relatives cooccurred less frequently. The largest difference in abundance between treatments occurred within the Rosales, Asteraceae, Vitaceae, and the Poaceae; woody Rosales were strongly selected for in unburned communities, while composites and grasses of the Poaceae were strongly selected for under frequent burning. A major climatic perturbation of consecutive hot, dry summers in the late 1980s prompted a significant shift in the functional and phylogenetic composition of communities. Greater than expected turnover in species composition occurred following the drought years, and then again during the subsequent five-year rebound period. Just after the drought year, turnover was greatest among recently diverged taxa, whereas during the rebound period turnover was greater among taxa that diverged deep in the phylogeny. The drought years also caused a short-term shift in functional traits, including declines in specific leaf area and leaf nitrogen content and an increase in leaf length. These results indicate that the phylogenetic and functional trait composition of communities are responsive both to fire gradients and to shocks to the system, such as climatic perturbation.