Environmental and biotic influences on the abundance and distribution of an introduced grass species : implications for management in the Okanagan Valley, British Columbia

Introduced plant species exert major influences on the structure and function of ecosystems, and are often implicated in biodiversity declines. The Eurasian annual cheatgrass, Bromus tectorum Linnaeus, has spread extensively in western North America since its introduction over 150 years ago; it extirpates native species, appears to have increased fire cycle periodicities, and provides cattle with inadequate nutrition. Because cheatgrass abundance recently increased in pastures of grassland in the Okanagan Valley of British Columbia, Canada, assessing environmental and biotic factors that influence its abundance is important from a management perspective. In an observational study at five heterogeneous sites, I isolated a number of highly significant correlations; cheatgrass abundance was positively correlated with proximity to focal ponderosa pine (Pinus ponderosa C. Lawson) trees, but negatively correlated with other plant diversity. Also, soil pH and soil moisture were significantly lower in proximity to trees than at distances further away, suggesting soil chemistry could have affected cheatgrass abundance. Because other analyses indicated that cheatgrass abundance differed in relation to the identity of the other species present, I conducted community-wide and species-specific co-occurrence analyses; I asked whether invaded communities featured different assembly patterns and isolated the species that had the strongest co-occurrence patterns with cheatgrass. I found that communities lacking cheatgrass were more diverse in terms of grass species and appeared to be structured non-randomly. Invaded communities, however, displayed patterns indicative of ‘disassembly’ as co-occurrence relationships did not differ from null predictions. Five grass species grew relatively more frequently if cheatgrass was present; these were bluebunch wheatgrass (Pseudoroegnaria spicata, Pursh. A. Love), Kentucky bluegrass (Poa pratensis L.), western needlegrass (Stipa occidentalis Thurb.), sand dropseed (Sporobolus cryptandrus Torr. A. Gray) and needle-and-thread-grass (Stipa comata Trin & Rupr. Barkworth). These results suggest that selective herbicide use in proximity to pine trees could be effective in controlling cheatgrass in these grasslands. I recommend manipulative experiments to assess the potential of this technique, as well as seeding experiments designed to characterize the most effective natural competitors against cheatgrass.Science, Faculty ofZoology, Department ofGraduat