Facilitation of an invasive grass by native shrubs

Much of biological invasion theory is built upon models of negative ecological interactions, but the importance of positive interactions in specific invasions is gaining attention. This research examines the role of facilitation of the invasive annual grass, Bromus tectorum, by native shrubs in the western Great Basin Desert. Plots were established under the canopies of Artemisia tridentata and Purshia tridentata, and in intershrub spaces near Mammoth Lakes, CA (elevation 2150 m). Multivariate analysis of microclimatic and edaphic variables showed that intershrub plots were potentially less suitable habitat. The survival, growth, and fecundity of over 2000 individuals of B. tectorum were recorded during the 2005 and 2006 growing seasons (April-July), and estimates of germination and establishment were obtained from experimental seedbanks adjacent to each plot. Modeled isolated populations of B. tectorum under shrub canopies exhibited higher growth rates than intershrub populations in both years despite substantial variation between years. Across both years, seed production per unit ground area was nearly five-fold greater in shrub understory microhabitats than in intershrub spaces. When dispersal between microhabitats was included in the model, growth rates were more similar across microhabitats and corresponded well with observed 2005-2006 values (mean λ = 2.1). Community sampling at several sites spanning 125 km and 500 m in elevation detected a predominance of B. tectorum in shrub microhabitats and lower species diversity compared to intershrub spaces. Overall, this research indicates that facilitation by native shrubs can be significant at the population level for high elevation populations of B. tectorum.