High-severity (stand-replacing) wildfires have become larger and more frequent in parts of the western United States in recent decades, causing substantial changes to forest landscapes. In many western forest types, recent large, high-severity fires are outside of the historical range of variation for wildfires. Long-term effects of severe fires on biodiversity remain poorly understood, in part because many previous post-fire plant diversity studies have focused on low-severity prescribed fires. Here, we analyze changes in herbaceous plant communities after the 2007 Angora Fire in El Dorado County, California. We sampled plant communities across a full fire severity gradient six times during the first nine years after the fire.
Plant species richness has increased gradually in the nine years since the fire in most burn severity classes, with the greatest increases in areas that burned at moderate severity. Nine years after the fire, species richness shows a distinct hump-shaped response to fire severity, with greater richness at moderate-severity plots than in high-severity, low-severity, or unburned plots. Beta diversity (species turnover among plots) and gamma diversity (landscape-level diversity) are also highest in areas of moderate severity fire. These results suggest that high-severity fire can have a homogenizing effect on the plant community at both local and landscape scales due to strong filtering of fire-sensitive species. Plant communities in forest types that were historically characterized by frequent, low-severity fire (Fire Regime I), but now experience high-severity fires, may be poorly adapted to high-severity fire. Pre-fire management to reduce burn severity may promote post-fire plant diversity in forests that are adapted to low-severity fire (Fire Regime I). Conversely, it could decrease diversity in forests that are adapted to high-severity fire (Fire Regime IV and V).