Forest disturbance accelerates thermophilization of understory plant communities

1. Climate change is likely to shift plant communities towards species from warmer regions, a process termed “thermophilization.” In forests, canopy disturbances such as fire may hasten this process by increasing temperature and moisture stress in the understory, yet little is known about the mechanisms that might drive such shifts, or the consequences of these processes for plant diversity. 2. We sampled understory vegetation across a gradient of disturbance severity from a large-scale natural experiment created by the factorial combination of forest thinning and wildfire in California. Using information on evolutionary history and functional traits, we tested the hypothesis that disturbance severity should increase community dominance by species with southern-xeric biogeographic affinities. We also analyzed how climatic productivity mediates the effect of disturbance severity, and quantified the functional trait response to disturbance, to investigate potential mechanisms behind thermophilization. 3. The proportion of north-temperate flora decreased, while the proportion of southern-xeric flora increased, with greater disturbance severity and less canopy closure. Disturbance caused a greater reduction of north-temperate flora in productive (wetter) forests, while functional trait analyses suggested that species colonizing after severe disturbance may be adapted to increased water stress. Forests with intermediate disturbance severity, where abundances of northern and southern species were most equitable, had the highest stand-scale understory diversity. 4. Synthesis: Canopy disturbance is likely to accelerate plant community shifts towards species from warmer regions, via its effects on understory microclimate at small scales. Understory diversity can be enhanced by intermediate disturbance regimes that promote the coexistence of species with different biogeographic affinities.