17 Spatial and Temporal Variation in Species Composition in California Grasslands : The Interaction of Drought and Substratum

Water is the principal limiting resource for Californian Mediterranean-climate plants. The relationship between rainfall variation and the abundance and distribution of plant species was investigated at the Jasper Ridge Biological Preserve near Stanford University. Five years of rainfall data (1986-1990) were combined with a yearly census of plant species on a 100m transect to relate inter-annual variation in rainfall to changes in the vegetation on adjacent serpentine and greenstone grasslands. The four years of drought from 1986-1990 did not affect all species along the transect equally. Within the grasses, native bunchgrasses were less affected by four consecutive drought years than were the annual grasses. For non-native annual grass species, there was a significant interaction between substratum and drought on both species abundances and spatial dispersion of plants on serpentine soil. These species were more affected by drought on serpentine soil than was the common native annual grass, Vulpia microstachys. Dramatic changes in the distribution of non-native grasses in serpentine grassland were related to heterogeneity in the Mg/Ca quotient. Variation in responses of non-native grass species to drought in greenstone grassland was related to seed size; larger-seeded species were less affected by drought. It is suggested that drought plays an important role in structuring the plant community in serpentine soils at this site, and that the relationship between drought and serpentine vegetation requires further investigation.