Effects of grazing and drought on population dynamics of salt desert shrub species on the Desert Experimental Range, Utah

Abstract

We examined natality, mortality, and population turnover of dominant salt desert shrub species under different seasons (winter vs. spring) and intensities (light vs. heavy) of grazing during a drought period on the USDA Forest Service Desert Experimental Range in southwestern Utah. Species responses were more predictable from their life history and physiological traits than from past responses to grazing alone. Heavy or spring grazing increased mortality of Artemisia spinescens, a cool-season shrub susceptible to past grazing, and of Sporobolus cryptandrus and Atriplex confertifolia , a C4 grass and shrub, respectively, that had increased under this grazing regime in the past. Light or winter grazing during this period increased survival and natality of S. cryptandrus, and of Ceratoides lanata, a shrub that had decreased in density but increased in cover under past grazing. Population turnover rates were generally positive for A. spinescens, but were highly negative for A. confertifolia in all but the heavy spring grazing treatment. A. confertifolia had exhibited high mortality during past droughts. C. lanata exhibited little population change reflecting past trends. Generally positive rates of turnover for the two grasses, S. cryptandrus and O. hymenoides, paralleled past trends, except in the spring-heavy treatment which had highly negative turnover rates. In a comparison of grass vs. shrub dominated vegetation types, C. lanata had higher mortality in grass dominated plots; O. hymenoides had higher mortality in shrub dominated plots. Both S. cryptandrus and O. hymenoides exhibited low or negative turnover rates for grazed plots within the shrub dominated type. Overall, light to moderate grazing and the removal of livestock before active physiological growth of cool season species had the least negative effects on population dynamics during a two-year drought period. This grazing regime increased survival or natality of certain species.