Perennial plant mortality in the Sonoran and Mojave deserts in response to severe, multi-year drought

Recent drought caused considerable shrub mortality in parts of the Sonoran and Mojave deserts (USA). The most severe impacts occurred in southern parts of the Mojave Desert and the adjacent Lower Colorado section of the Sonoran Desert in southernmost California and southwestern Arizona. In that region, mortality of the small, drought-deciduous shrubs Ambrosia deltoidea and Ambrosia dumosa was 100% or nearly so. The larger, drought-enduring evergreen Larrea tridentata fared much better, although nearly two-thirds of all L. tridentata plants succumbed at one location. Data on 21 additional perennial species showed that other species of small, drought-deciduous shrubs also sustained considerably higher mortality. The best predictor of plant responses was the Standard Precipitation Index (SPI) calculated for a 60-month time frame ending in March 2003, indicating the importance of the cumulative effect of successive drought years. Deficits of cool-season precipitation were more extreme than warm-season deficits and were probably the greatest contributor to plant mortality. Soil conditions, including differences in parent materials and texture, also influenced plant responses. Episodes of drought-induced, perennial plant mortality represent extensive, region-wide ecological disturbances and may be one of the most important processes affecting plant populations and community composition in deserts.