Increasing fire severity changes the assembly and coupling between plant taxonomic and functional diversity in a dry conifer forest understory

Disturbance regimes are changing globally with uncertain consequences for the assembly and functioning of ecosystems. In western North America, wildfires are increasing in frequency and severity relative to historic fire regimes. How plant community assembly and function will respond as disturbance intensifies is poorly understood. Ecosystem functions rely on the functional traits of species within a community. Trait diversity represented at the community level is expected to increase as taxonomic diversity (TD) increases. Altered disturbance regimes may influence trait and species diversity and their relationship during recovery. We measured TD and functional diversity (FD) across a burn severity gradient in a ponderosa forest understory for five years following a mixed severity wildfire. To measure the effects of fire severity on community assembly, we quantified taxonomic and functional beta diversity and measured functional redundancy, which is a measure of the coupling of taxonomic and functional alpha diversity, over time and across the severity gradient. Using a null model to measure the deviation of beta diversity from random assembly, we found that high‐severity fire changed the trajectory of community assembly relative to unburned and low‐severity fire. We also found that both low‐ and high‐severity fire decoupled TD and FD relative to unburned communities, and that high‐severity fire destabilized temporal patterns of functional redundancy. These alterations to the assembly and coupling of TD and FD were associated with altered taxonomic and functional composition compared to unburned and low‐severity fire. Our results demonstrate that as disturbance regimes become more severe, processes of community assembly may change and, therefore, community composition and ecosystem function are unlikely to rapidly return to pre‐disturbance conditions.