A major disturbance like wildfire can re-shape the ecological characteristics of a system, but these effects are not typically homogenous across a landscape. Within a single wildfire perimeter, understory plant communities regenerate in post-fire sites across a spectrum of disturbance severity classes. For land management agencies working in wildfire-prone ecosystems, the recovery of biodiversity in a site is often a priority. In temperate forest ecosystems, understory plant species contribute significantly to overall community diversity, thus understanding the disturbance-related factors that shape understory community composition is imperative. However, there remains a gap in our understanding of post-fire diversity in mixed-conifer, unmanaged sites in the Sierra Nevada. We conducted complete understory plant censuses of plots across a spectrum of fire severity classes in eight wildfire perimeters of varying age to examine patterns in understory diversity. Our objectives were to determine how understory plant richness and cover varied across fire severity classes, whether environmental variables related to wildfire severity could be used to predict plant richness and cover, and how these results compared across wildfires of varying ages.
Our results are largely consistent with the intermediate disturbance hypothesis, since moderate fire severity classes were often associated with higher values of diversity while understory diversity indexes in low and high severity classes were significantly lower. We also found understory plant compositions were similar within fire severity classes across wildfires, but varied greatly depending on the time since wildfire. These results we attribute to variations in environmental characteristics brought about by wildfire severity type that may impose lasting effects through successional time. High and low severity classes have a strong initial filtering effect on understory species composition, while the heterogeneity of environmental variables like overstory cover in moderate severity classes allows for a diverse assemblage of species. This information should allow land managers in wildfire-prone systems to more confidently commit resources towards the goal of diversity while the effects of global change and legacies of fire suppression make high severity wildfire more frequent.