Forest disturbance accelerates thermophilization of understory plant communities

1. Climate change is likely to shift plant communities towards species from warmer regions, a processtermed ‘thermophilization’. In forests, canopy disturbances such as fire may hasten this processby increasing temperature and moisture stress in the understory, yet little is known about the mechanismsthat 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-scalenatural 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 disturbanceseverity should increase community dominance by species with southern-xeric biogeographicaffinities. We also analysed how climatic productivity mediates the effect of disturbance severity,and quantified the functional trait response to disturbance, to investigate potential mechanismsbehind thermophilization.3. The proportion of north-temperate flora decreased, while the proportion of southern-xeric floraincreased, with greater disturbance severity and less canopy closure. Disturbance caused a greaterreduction of north-temperate flora in productive (wetter) forests, while functional trait analyses suggestedthat species colonizing after severe disturbance may be adapted to increased water stress. Forestswith intermediate disturbance severity, where abundances of northern and southern species weremost equitable, had the highest stand-scale understory diversity.4. Synthesis: Canopy disturbance is likely to accelerate plant community shifts towards species fromwarmer regions, via its effects on understory microclimate at small scales. Understory diversity canbe enhanced by intermediate disturbance regimes that promote the coexistence of species with differentbiogeographic affinities.