Causes and consequences of exotic species invasion in the ponderosa pine forests of Colorado's Front Range

Biological invasions that alter the structure and composition of ecosystems can change nutrient dynamics, storage, and loss. Understanding the causes and consequences of biological invasions is therefore of paramount concern to ecologists and managers.; I conducted three experiments to elucidate factors that promote invasions and their biogeochemical consequences in Colorado's ponderosa pine ecosystem. Two experiments investigated how interactions among environmental factors and disturbances influence invasibility. These experiments explored invasion by (1) all exotic plant species and (2) by Bromus tectorum . My final experiment examined the effect of B. tectorum invasion on pools of labile carbon and nitrogen.; I found that exotic species are successful in Colorado ponderosa pine ecosystems. The most important variable for predicting native and exotic species richness was increasing site accessibility (distance along roads from population centers), which increased richness. Accessibility may increase richness by maintaining high levels of propagule pressure and/or physical disturbances.; Successful invasion of B. tectorum was limited by nitrogen and water. B. tectorum biomass was a saturating function of water and nitrogen. Even though an experimental plant mortality treatment increased nitrogen availability, plant mortality had a negative impact on invasion. This result may be sensitive to drought conditions that persisted during this experiment.; Successful B. tectorum invasion had significant biogeochemical consequences. Averaged across the growing season, all soil pools of nitrogen and carbon were larger beneath B. tectorum than perennial grasses. Incorporating seasonal variation was crucial for detecting this difference because it was not statistically significant at each sampling period within the growing season. Throughout the growing season, carbon and nitrogen pools beneath B. tectorum became progressively larger than those beneath native grasses, probably because B. tectorum senesces earlier than native grasses.; My research suggests that in this ecosystem (1) spatial and temporal variation in environmental conditions, resource availability, and disturbances interact to regulate community invasibility, and (2) the consequences of invasion have significant consequences for soil carbon and nitrogen pools. Furthermore, if invasion by B. tectorum results in persistent increases in N availability, B. tectorum invasion could create a positive feedback loop between N availability and further B. tectorum or exotic invasion.