Much of the coastal mountains and foothills of central and southern California are covered by a mosaic of grassland, coastal sage scrub, and evergreen sclerophyllous shrubs (chaparral). In many cases, the borders between adjacent plant communities are stable. The cause of this stability is unknown. The purpose of this study was to examine the water use patterns of representative grasses, herbs, and shrubs across a grassland/chaparral ecotone and determine the extent to which patterns of water use contribute to ecotone stability. Results support the hypothesis that grassland species deplete soil moisture in the upper soil horizon early in the drought, preventing the establishment of chaparral seedlings or bare zone herbs. Also, grassland plants are prevented from invading the chapparral because of low seed dispersability and high animal herbivory in these regions.