Phytolith evidence for the extent and nature of prehistoric Californian grasslands

Abstract

Californian grasslands have been radically transformed by cultivation and exotic species invasions that began with European settlement in 1769. Because native species contribute only a small percentage of total vegetation cover at most grassland sites in California today, there has been a long-standing controversy regarding the extent and composition of prehistoric grasslands. We used phytolith analysis of soil samples from 153 spatially diverse grassland sites currently cultivated or dominated by exotic annual grasses to provide the first direct evidence that, except for areas near the coast, grass was generally a minor component of the vegetation. Most of the arid, non-wetland Central Valley and surrounding foothills probably had very low grass cover. Combining our data with historical accounts of vegetation encountered by early explorers in California and relict analysis of native species in existing grassland patches, it appears that while the extent of prehistoric grassland was probably similar to the current extent of exotic annual grassland, most areas were likely dominated by a highly diverse assemblage of herbaceous species, composed largely of annual forbs adapted to exploit local environmental heterogeneity. Phytolith analysis should be routinely performed in California to guide grassland restoration efforts. The technique can also be used to reconstruct prehistoric vegetation of highly altered grassland ecosystems worldwide.