Ecological Origins of California's Mediterranean Grasses

Abstract

Annual grasses from the Mediterranean Basin have become the dominant species in the extensive California grasslands, replacing native perennial and annual grasses. This study compared the ecology of Mediterranean annual grasses between California and their homeland to determine their preadaptation to conditions in California. In six regions of the Mediterranean Basin (southern France, western Spain, southeastern Spain, southern Tunisia, Crete, and Israel). annual grasses now common in California are mainly ruderal species in early successional grasslands or in open degraded maquis. In none of these regions do they form stable annual grassland communities as in California. In the Mediter- ranean Basin, climatic factors associated with highest abundances of annual grasses are high rainfall in winter or long, dry summers, which are both found in California. Ruderal life-history characteristics, combined with suitable climatic conditions, allowed the rapid spread of Mediterranean annual grasses during periods of overgrazing and drought in the last century, at the time of European settlement. Both alien and native perennial grasses have slower winter growth rates and lower reproductive outputs during their first year than annuals, and presumably they would have been reduced by the high densities of the introduced annuals.