Effects of Warming, and Nitrogen on the Colonization and Establishment of a Novel Species

Aspects of global change - specifically, elevated soil nitrogen deposition and warming - are resulting in significant changes in native plant communities. These effects can be direct through changes in productivity, or indirect through the spread of novel species, as organisms respond to changing climates by shifting their range. Although ecologists have used a variety of approaches to understand how much and through what mechanisms elevated soil nitrogen and warming may influence plant species, a predictive level of understanding is still lacking. Identifying vital traits that might be related to species' success in colonizing new habitat could be useful for making predictions about how species might respond to elevated soil nitrogen and warming. Further, a mechanistic understanding of how nitrogen deposition, increasing temperature and a novel species affect native plant communities is necessary for understanding and predicting effects of a changing environment. An initial study investigated the effects of fire and density on my chosen study species, the perennial composite Pityopsis aspera, using Life Table Response Experiments. The shape of the relationship between population growth rate (λ) and density was significantly modified by fire, Fire affected adult flowering stasis and survival and first-year survival, resulting in more positive contributions from these vital rates to λ at both the lowest and highest density values. These results demonstrate the first application of a regression-design life-table response experiment to elucidating the interactive effects of density and fire. They highlight the utility of this approach for both capturing the complex dynamics of populations and establishing a means of determining how vital rates might contribute to differences in demography across densities. I also studied the effects of elevated soil nitrogen and warming on the demography of Pityopsis aspera, a novel species introduced to an old-field in the Tall Timbers Research Station and Land Conservancy in North Florida. I conducted a factorial experiment, crossing nitrogen addition, warming, native plant reduction, and transplanting by P. aspera in different life cycle stages. My dissertation investigated two general topics. First, using a life table response experiment, I asked how increasing temperature, soil nitrogen and native plant reduction differentially affect survival, growth and reproduction of a colonizing species, P. aspera, in the field. Second, using structural equation modeling, I examined hypotheses related to the environmental factors responsible for mediating the effects of warming, elevated soil nitrogen, and the establishment of this novel species on a native plant community. The life table response experiment showed that the estimated population growth rate (λ) of the colonizer P. aspera was reduced by nitrogen addition, and increased by warming, warming + native reduction, and nitrogen + native reduction. Fecundity was important in modifying differences in λ between warming and nitrogen addition treatments. Growth of young individuals also contributed to differences in λ, but only when warming and nitrogen addition were combined with the native reduction treatment. The structural equation modeling demonstrated that, in the absence of colonization by a novel species, soil pH mediates the effect of nitrogen addition on the plant community. In the presence of colonization, however, the direct effect of nitrogen on the native plant community is removed entirely and soil moisture becomes important for mediating nitrogen effects. I did not find effects of warming on the native plant community in the absence of colonization by a novel species. In the presence of colonization, however, warming had negative effects on functional richness directly, while percent invasive species in 2011 and herbivory mediated an overall positive effect of warming. This work illustrates how relationships among effects of elevated soil nitrogen and temperature, and the introduction of a novel species can be complex and non-additive. Population level responses of a colonizer to nitrogen deposition and warming are mediated by different vital rates and may occur via indirect effects through established species. Future research should identify the identity and the magnitude to which biotic and abiotic variables are responsible for mediating relationships among elevated soil nitrogen, warming, and native plant communities.