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Spatio-Temporal Heterogeneity and Habitat Invasibility in Sagebrush Steppe Ecosystems

Mazzola, M. B. 2008. Dissertation in Ecology, Evolution and Conservation Biology. University of Nevada, Reno

Abstract

Bromus tectorumL. (cheatgrass) is the most widespread invasive weed in sagebrush-steppe ecosystems. Invasion by B. tectorum produces large-scale changes ecosystem that negatively affect seedling establishment processes. Establishment of invasive and native species plays a key role in determining community invasibility and restoration potential. This study examined factors influencing seedling establishment and survival of B. tectorum and native species in ecosystems characterized by different degrees of B. tectorum invasion. Temporal and spatial patters of resource availability were examined in native and invaded shrublands. The replacement of native herbaceous species by B. tectorum affected resource availability and seedling establishment patterns. Within the native community, invasibility appeared to increase with increasing resources and was controlled by microsite conditions, with interspaces dominated by native grasses resulting in the highest resistance to invasion. Within the invaded community, spatial differences were less important and it appeared that B. tectorum facilitated its own establishment. In B. tectorum -dominated grasslands, short-term reduction of soil resources negatively impacted B. tectorum,but effects were temporary and were not effective in controlling B. tectorum. Perennial species establishment was affected by increasing B. tectorum density and appeared to be restricted by seed availability. Results indicate that invasibility and restoration potential are determined by dynamic interactions between temporal and spatial variations in resource supply, competition and seed availability of both B. tectorum and perennial species.