Change in California grasslands

Grasslands cover approximately 10 million acres in California. Yet intact, native grasslands are one of the state's most threatened ecosystems (Noss et al. 1995). The vast majority of California's native grasslands were converted to urban or agricultural uses or have become dominated by alien species, mainly annual grasses from the Mediterranean region. Moreover, the relatively few native-dominated grasslands that remain (even those in parks and reserves) appear to be disappearing rapidly, often without clear causes. Work proposed here, to integrate understanding of the distribution, ecology, status and trends of remnant native-dominated grasslands in California, would contribute to our understanding of the basic ecology of grasslands and improve the ability of conservationists and land managers to protect these rapidly disappearing natural communities. We will gather and analyze data relevant to understanding the following specific questions of California grasslands: What ecological factors control the distribution of the state's remnant native-dominated grasslands? What biotic and abiotic factors control the transitions between grasslands and other ecosystem types? At what rate are native-dominated grasslands continuing to be lost? How do the answers to these questions vary along edaphic and climatic gradients, and among different regions of the state? Many studies have been done on the ecology of grasslands in California (reviewed in Huenneke and Mooney 1989), and this previous work establishes that Californian grasslands differ ecologically in several crucial ways from those elsewhere in the United States and the world. First, the invasion and replacement of native perennial bunchgrasses by Mediterranean annual species has been uniquely thorough in California. In California, alien grasses appear to have outcompeted and replaced natives under a very wide range of conditions. Second, most California grasslands are not as strongly successional as the prairie and old-field systems of the midwestern and eastern US. The historical roles of fire and native ungulates in Californian grasslands are very poorly understood, and may be largely irrelevant now given the widespread, virtually complete conversion to exotic, annual grasslands that has occurred across California. Third, and following from the first two points, the restoration of native grasslands in California is not as simple as reimposing a "natural" regime of fire or grazing. Despite substantial efforts at restoration, Californian grassland ecologists have had very limited success in finding ways to recreate native-dominated grasslands that resist invasion. We propose to expand on this previous work by analyzing and synthesizing data from across the state, to determine how the structure, function, and dynamic trajectories of California grasslands vary along edaphic and climatic gradients. Ultimately, we hope to assess whether the composition of remnant grasslands has reached an approximate steady state, or alternatively, where and how fast continued changes are occurring. Understanding the factors determining variation and driving change in California grasslands at a larger scale is essential for conservation and restoration efforts within the state, and will provide valuable comparisons with similar work in other grassland communities.