Determinants of perennial and annual grass distribution in Mediterranean-climate California

In California prairies, native perennial grasses have been almost entirely replaced by Eurasian winter-annual grasses. Landscape-scale descriptions of perennial versus annual grass distribution are lacking. To quantify perennial persistence in California grasslands, 50 randomly selected grassland meadows were sampled within three latitudinal belts of central California, from the Pacific coast inland to the Central Valley. Native perennial grasses were present at all but four of the sampling sites, although usually at low density. Proximity to the Pacific coast was positively correlated to perennial grass cover at all latitudes. Annual degree days above 18 A degrees C and increasing slope were significantly negatively correlated to perennial grass dominance, and average warm season precipitation was significantly positively correlated to perennial dominance. Cover of non-native annual grasses was significantly lower in prairies < 2 km from the coast than in all other sites. Non-native perennial grasses were occasionally important grassland community components at sites near the coast, but relatively uncommon inland. These results suggest that environmental conditions near the California coast tend to increase the success of perennial grasses, including exotic perennials. Intensity and duration of seasonal water stress likely play a central role in determining perennial grass distribution in California.