Dynamics of shrub die-off in a salt desert plant community.

Abstract

Mortality of shadscale (Atriplex confertifolia [Torr. & Frem.] Wats.) was severe in Great Basin valley bottoms between 1983 and 1988. Puddle Valley, Utah, just west of the Great Salt Lake, typifies areas of extensive shrub loss in which density decreased from over 12,000 ha-1 to less than 200 ha-1. We analyzed vegetation along a radial transect established in the bottom of Puddle Valley in 1987. Mortality was greatest at the lowest elevations where shrubs were initially most dense. These sites occurred where soil moisture, fine-textured soils, and bulk density were greatest of all sites evaluated. Soil was most saline at the margins of the valley bottom. Higher densities of live shadscale occurred where slopes are greater, soil is more droughty, and soil moisture was lower during the 3 years of data collection. The die-off "front" continued about 5 km to the west of the valley center in 1989. Refugia of live shadscale populations were found where soil salinities were higher. Population dynamics of annuals, including summer-cypress (Kochia scoparia [L.] Schrader), cheatgrass (Bromus tectorum L.), and halogeton (Halogeton glomeratus [Bieb.] C.A. Mey.) were highly variable between 1987 and 1989.