Watercourse-Upland and Elevational Gradients in Spring Vegetation of a Mojave-Great Basin Desert Landscape

摘要

Springs in arid lands provide critical habitat for a variety of species and functions to humans, yet the ecology and management needs of springs to maintain these values are poorly understood. To examine plant communities along spring watercourse-upland gradients, we sampled 12 springs at low (desert) and high (forest) elevations on the Desert National Wildlife Refuge in the Mojave and Great Basin Deserts in southern Nevada. In contrast to the commonly reported positive relationship between native and exotic species richness in sampling studies, we did not find strong correlations (r~2 ＜ 0.05) between native and exotic richness at any distance from watercourses. Additionally, exotic species cover was lower nearest (0 and 2 m) watercourses than at uplands 20 m from watercourses, which also differs from the hypothesis that watercourses are more heavily invaded than uplands. Exotic species were more pervasive at low-elevation compared to high-elevation springs, but the proportion of total plant cover comprised by exotics was still small (0.03 - 0.06) at low-elevation springs. Species distributions and ordinations suggested that compositional watercourse-upland gradients were often readily detectable, but the composition of springs was individualistic. Some springs contained wetland species such as Juncus saximontanus, while other springs contained species of dry-site affinity. This study also illustrated challenges associated with estimating reference conditions for arid-land springs, as there are no known data prior to the development (i.e., modifying surface flow) of the springs and no known unmodified springs on this landscape.