REGIONAL PATTERNS OF PLANT COMMUNITY RESPONSE TO CHANGES IN WATER: OWENS VALLEY, CALIFORNIA

Abstract

The conversion of large natural basins to managed watersheds for the purpose of providing water to urban centers has had a negative impact on semiarid ecosystems, worldwide. We view semiarid plant communities as being adapted to short, regular periods of drought; however, human induced changes in the water balance often remove these systems from the range of natural variability that has been historically established. This article explores vegetation changes over a 13-yr period for an entire water management area in eastern California. Using remotely sensed measurements of vegetation live cover, a recent vegetation map, field data and observations, precipitation records, and data on water table depth, we characterize the responses of xeric, phreatophytic, and exotic Great Basin plant communities. Despite the complexity of plant communities and land-use history, our technique was successful in identifying discrete modes of response. Differences in vegetation response were attributable to available groundwater resources (modified by water management activities), annual precipitation, and land cultivation history. Fifty-one percent of our study area, including phreatophytic and xeric communities, showed unchanging vegetation conditions and had experienced relatively minimal human disturbance. Nineteen percent of the area exhibited a linear decline in live cover during a drought when ground- water pumping lowered water tables. In portions of these areas, the decline in native phreatophytic cover was followed by an increase in exotic, nonphreatophytic species when the drought ended; in the remainder, cover was suppressed. Finally, vast regions that had been significantly disturbed showed live cover changes that were amplified with respect to precipitation, indicating the presence of exotic annuals. We view the increase in exotic species across the entire study area to be indicative of a fundamental shift in ecosystem function from one buffered from drought by stable ground water conditions to one sensitive to small changes in precipitation. The tools and techniques used here are applicable wherever large regions of land are being managed in an era of changing environmental conditions.