Patterns of growth and soil-water utilization in some exotic annuals and native perennial bunchgrasses of California

In western California, exotic cool-season annuals appear to have widely replaced native perennial bunchgrasses as the herbaceous community dominants in grasslands, oak savannas, and oak woodlands. We argue that because these two herbaceous plant types possess very different life histories, this invasion may have correspondingly altered seasonal patterns of soil-water availability. To begin to assess this hypothesis, in this study we compared exotic cool-season annuals and native perennial bunchgrasses in terms of growth, biomass allocation, rooting distribution, root morphology, and soil-water utilization. Exotic cool-season annuals completed their life cycle early in the dry season through rapid growth apparently made possible by a high proportional allocation to shoots in combination with the efficient production of roots of high specific root length. Further, annuals tended to concentrate root growth and soil-water utilization in the upper soil profile. In contrast, native perennial bunchgrasses allocated a high proportion of their biomass to the production of a deep root system, which allowed them to continue soil-water utilization well into the dry season and contribute to the formation of a very dry soil profile. Taken together, these contrasting patterns suggest that the invasion of exotic cool-season annuals might have produced a corresponding increase in the amount of water present at depth in the soil profile during the dry season. (C) 1996 Annals of Botany Company