Livestock Use Has Mixed Effects on Slender Orcutt Grass in Northeastern California Vernal Pools

AbstractLand managers often face the dilemma of balancing livestock use with conservation of sensitive species and ecosystems. For example, most of the remaining vernal pools in California are grazed by livestock. Vernal pools are seasonal wetlands that support many rare and endemic species, such as slender Orcutt grass (Orcuttia tenuis Hitchc.). Although studies in other areas of California have demonstrated that livestock use may benefit some vernal pool specialist species, grazing has been considered a threat to slender Orcutt grass in northeastern California. We evaluated the effects of livestock use on slender Orcutt grass using a replicated, paired design across a range of environmental conditions and grazing management regimes. Frequency, density, cover, reproductive potential, and height of slender Orcutt grass was compared in plots where livestock had been excluded with plots where grazing occurred. We found that livestock do not directly graze slender Orcutt grass, so the effects of livestock use on this species are indirect. These indirect effects are complex, including both positive, neutral, and negative effects. Year had the largest effect on slender Orcutt grass, probably as a result of variation in annual precipitation patterns. Livestock use had no effect in some years; in other years slender Orcutt grass was twice as abundant in unfenced than in fenced plots. Litter cover was also lower in unfenced plots in these years, suggesting that livestock use may benefit slender Orcutt grass in some years by reducing litter accumulation. Conversely, livestock use negatively affected slender Orcutt grass in pastures where livestock hoofprint cover was high, including pastures that were grazed early in the season. By considering patterns of annual variation in environmental factors such as precipitation, site conditions, and season of grazing, land managers can balance the needs of sensitive vernal pool species with maintaining livestock utilization on public lands