Precipitation effects on plant composition in California rangelands

Background/Question/Methods

Rainfall is a key control on production and composition in California grasslands. While less studied, precipitation may also have a lagged effect, with previous year’s rainfall affecting composition the following year. Here, we ask how different functional groups respond to current and lagged effects of precipitation in order to forecast changes due to increased rainfall variability. We monitored grazed grasslands in three sites in northern California: Sunol Regional Wilderness, Pleasanton Ridge Regional Park and Vasco Caves Regional Preserve.  At each of the sites, species abundances were measured along four transects in six 500 m² circular plots. These measurements were repeated for seven years between 2006 and 2012. Using precipitation data collected at each site, we asked whether abundance patterns of major functional groups (based on growth form, life history, origin) were related to the current and previous year’s precipitation. We expected that grasses would increase in wetter years and forbs in drier years and that consecutive dry or wet years would strengthen their respective dominance.

Results/Conclusions

Over the past seven years, exotic grass and forb abundance is relatively stable. However, a closer analysis indicates that these functional groups are responsive to annual precipitation and lagged rainfall (previous year’s rainfall). Exotic grasses increase with higher precipitation, especially with lagged years, while in contrast, exotic annual forb abundance decreases. With more frequent drought years, it is likely that exotic annual forbs will become more dominant as exotic annual grasses decline.  Native perennial grasses appear stable, as lower annual precipitation did not significantly reduce abundance. Both native annual and perennial forb functional groups respond positively to precipitation, indicating that consecutive low rainfall years will coincide with a decrease in native species composition. Our results suggest that native diversity and forage production with likely decline in grazed grasslands with increased drought years in California.