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The Role of Symbiotic Nitrogen Fixation on Nitrogen Availability, Competition, and Plant Invasion into the Sagebrush Steppe
AdvisorChambers, Jeanne C.
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In the semi-arid sagebrush steppe of the Northeastern Sierra Nevada, resources are both spatially and temporally variable, arguably making resource availability a primary factor determining invasion success. N fixing plant species, primarily native legumes, are often relatively abundant in sagebrush steppe and can contribute to ecosystem nitrogen budgets. Lupinus argenteus (Pursh), a native legume abundant in high elevation areas of western North America, is one of the most common native legumes in sagebrush steppe. L. argenteus responds positively to disturbance and prior studies indicate that it can increase the availability of soil nitrogen. Thus contribution of nitrogen by L. argenteus can potentially have a large effect on maintaining native species diversity and productivity of sagebrush ecosystems. However, if a non-native seed source is present, increased nitrogen associated with L. argenteus can create conditions favorable for invasion by non-native species. This study examined the role of L. argenteus on resource availability in the sagebrush steppe and the implications for invasion with four interrelated studies. Results indicate that L. argenteus can modify available soil N and increase productivity in sagebrush ecosystems both through rhizodeposition and litter decomposition. Further, modification of the local resource pool by L. argenteus can alter competitive outcomes among native and non-native species and can increase plant establishment and growth of both native and non-native species. However, higher establishment and growth rates give the non-native a greater advantage. The ability of L. argenteus to increase N availability can serve to promote resilience of native ecosystems, but also may create an avenue for invasion.