Effects of drought on shrub survival and longevity in the northern Sonoran Desert

Abstract

Permanent vegetation plots in the northern Sonoran Desert, USA, provided an opportunity to assess the effects of recent drought on desert shrubs and to examine survival in relation to rainfall variability during the past 76 years. Survival and maximum longevity of six species were determined for eight intercensus periods between 1928 and 2004. Average annual survival was Ambrosia deltoidea, 0.9167 ± 0.0415; Encelia farinosa, 0.7952 ± 0.0926; Janusia gracilis, 0.9334 ± 0.0247; Krameria grayi, 0.9702 ± 0.0270; Larrea tridentata, 0.9861 ± 0.0174; and Lycium berlandieri, 0.9910 ± 0.0077. The longest-lived species were Larrea, Lycium, and Krameria, with average maximum life spans of 330, 211, and 184 years. Janusia, Ambrosia, and Encelia were much shorter lived, with average maximum longevity of 53, 40, and 16 years. Winter rain equalled or exceeded 90% of the long-term average accumulation except during 1948 to 1959 (65% of average) and from 2001 to 2003 (49% of average). Summer rain did not drop below 90% of the average accumulation in any period. The 1950s drought caused modest declines in survival of Ambrosia, Encelia, Janusia, Krameria, and Lycium. The effects of the recent drought were much more pronounced, resulting in sharp declines in survival and maximum longevity of Ambrosia, Encelia, Krameria, and Larrea, and modest declines for Lycium. Despite heightened mortality during the recent severe drought, 72% of the deaths observed between 1928 and 2004 occurred during periods of average or better-than-average rain, providing support for the idea that demography of shrubs in arid regions is influenced by continuous as well as episodic processes.