Fire Effects on Plant Biodiversity Across Multiple Sites in California Vernal Pool Grasslands

Millions of dollars are spent annually to restore degraded grasslands throughout the world with little research conducted at scales relevant to guide restoration. Fire is commonly used as a management tool in grasslands, particularly in California grasslands where exotic plant cover often exceeds 85%. Seasonal wetlands (vernal pools) with high native biodiversity often occur within these invaded grasslands. Here I assessed the effects of fire on plant community composition in the heavily invaded uplands and less invaded vernal pools at four sites in the Central Valley of California, USA. Across all sites, fire reduced exotic grass cover but increased exotic forb cover, suggesting that fire cannot be used to control exotics generally. However, exotic forb cover increased only in the upland habitat outside the vernal pools while exotic annual grass cover was reduced throughout the habitat. Native plant species richness was 15% higher in burned versus unburned treatments across all sites one year but not two years post burn. At one site, I detected increased native plant biodiversity for at least three years post burn. Despite fire effects being ephemeral across all sites, these results provide compelling evidence for the use of fire to enhance vernal pool grassland plant communities at some sites. While the difficulty of implementation presents a major drawback, the ability to treat large areas and effectively control exotic grass species while providing a short boost in native biodiversity provide strong justification for maintaining fire as a management tool.