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

Abstract

Biological invasions that succeed in altering the structure and composition of ecosystems can change the dynamics, storage, and efflux of carbon and nutrients within and from ecosystems. Understanding the causes and consequences of biological invasions is therefore of paramount concern to ecologists and managers. I conducted three experiments intended to elucidate both the factors that cause or promote exotic species invasion and the biogeochemical consequences of these invasions. Two of my experiments investigate the effects of and interactions among environmental and disturbance factors in influencing the invasibility of ponderosa pine communities along Colorado's Front Range by exotic plant species. My first experiment explored the invasibility of this ecosystem by all identified exotic plant species, and the second experiment investigated invasion by Bromus tectorum specifically, as an example of a highly invasive and potentially damaging species. My final experiment investigated some of the consequences of B. tectorum invasion on pools of available and microbial carbon (C) and nitrogen (N), while attempting to control for variation in daily and seasonal time and water availability. I found that exotic species are relatively successive along Colorado's Front Range. The main drivers of exotic and native species richness are largely similar, but the factors that influence the success (percent cover) of exotic and native species do so in different directions or with varying strengths. The most important variable for predicting both native and exotic species richness was accessibility of the site from large population centers. Native and exotic richness increased with increasing human accessibility, which was estimated as shortest distance along roads from major population centers. The importance of accessibility to species richness could be due to the impact of historical and/or current high levels of physical disturbances related to land use and increased propagule pressure. Successful invasion of B. tectorum was limited by N and water. Biomass at the end of the experiment was a saturating function of water and N. Even though plant mortality increased N availability, plant mortality had a generally negative impact on invasion success. This result may be sensitive to the drought conditions that persisted during this experiment. In this ponderosa pine ecosystem, successful B. tectorum invasion seemed to have significant biogeochemical consequences. Averaged across the growing season, all labile and microbial pools of N and C were higher beneath B. tectorum than beneath perennial grass communities. Incorporating seasonal variation was crucial for detecting this pattern because this difference was not large or statistically significant at each sampling period within the growing season. As the growing season progressed, pools of C and N beneath B. tectorum became increasingly larger than those beneath native grasses. This trend is probably due to the comparatively early senescence of B. tectorum versus native C3 and C4 grasses. My research suggests that (1) spatial and temporal variation in environmental conditions, resource availability, and disturbances may interact to regulate the invasibility of a community and (2) the consequences of a successful invasion may have significant consequences for soil and microbial pools of C and N. Furthermore, if invasion by B. tectorum results in persistent increases in N availability, my results suggest that B. tectorum invasion could create a positive feedback loop between N availability and further B. tectorum or exotic plant invasion. Research that addresses the interactions between climatic variables, resources, disturbance types, and the causes and consequences of successful species establishment will hopefully increase our ability to understand, predict, and manage these complex events.