UC Davis

Forests in the North American Mediterranean Climate zone (NAMCZ) are increasingly defined by novel disturbance regimes and manifestations of climate change. Many regions in the NACMZ, including the yellow pine/mixed-conifer (YPMC) forests of the Sierra Nevada, California, that historically experienced low to moderate intensity fire regimes are seeing increased area burned at high intensity resulting in high post-fire severity as a result of interactions between high fuel loads and climate warming with a number of negative ecological effects. It is important to understand species responses to the ecological disturbance of wildfire in this context, especially from the perspective of the understory plant community where the bulk of regional plant diversity is found and vital ecosystem functions take place. We examined understory plant responses across gradients of post-wildfire severity, shrub cover and a suite of other post-fire environmental features in over 400 plots of YPMC forests of the Sierra Nevada. We found a generally unimodal ‘hump-shaped’ relationship between local (alpha) understory plant diversity and fire severity. High severity burning resulted in lower local diversity and there was some indication of homogenization of the flora at the regional scale. Fire severity class, postfire litter cover, and annual precipitation were the best predictors of understory species diversity. In the YPMC forests of the NAMCZ, shrub lifeforms tend to be promoted by high intensity fires, and the recent increase in the extent of high intensity burning is driving major changes in the spatial patterns of shrub-dominated vegetation. We further examined understory plant community response to shrub cover and demonstrated that understory plant species diversity and evenness decline as shrub cover increased while understory richness was less affected. We used structural equation modeling to better understand relationships among shrub cover, understory richness and diversity and other environmental variables. Our examinations of specific lifeforms and life history categories in the understory revealed that while many environmental factors influence richness, water availability, rock and shrub cover mostly determine diversity and evenness. Our research suggests that increases in post-fire severity in systems historically characterized by low to moderate intensity wildfire may lead to understory plant diversity losses. Anticipated increases in shrubfield area due to these changing wildfire regimes will also likely negatively impact native herbaceous plant species diversity. These findings collectively indicate that global patterns of increasing fire size and severity may have important implications for regional plant species biodiversity.