Increased rainfall variability and N addition promote woody encroachment and extend phenology in a restored prairie
Climate model projections suggest that precipitation patterns will become more variable as the planet warms, increasing the frequency of large rainfall events and prolonged drought. Concurrently, chronic anthropogenic nitrogen deposition has become a leading factor of global change in many terrestrial ecosystems. These two global change factors have the potential to strongly alter plant community composition and ecosystem function. However, while nitrogen fertilization has been the focus of many decades of empirical field studies, the impacts of rainfall variability and its interaction with other global change factors on ecosystem structure and function are largely unknown. Using a factorial combination of partial rainout shelters and a slow-release nitrogen fertilizer, we examined how more variable rainfall patterns and nitrogen addition influenced the growth, phenology, and litter quality of two woody species, Quercus palustris and Lonicera maackii, in a restored prairie. After two years of experimental manipulation, seedlings of each species were planted into established communities. Stem height and diameter were monitored May through December, when seedlings senesced and phenology was recorded. Litter from each seedling was collected and analyzed for carbon and nitrogen content. Seedlings were harvested once they were fully senesced and their final dry weight was recorded.
Overall, L. maackii was more responsive to increased rainfall variability and nitrogen addition than Q. palustris. While Q. palustris had enhanced litter quality under our treatments and subsequently experienced an extended phenology, seedling growth remained largely unresponsive to either nitrogen addition or increased rainfall variability. In contrast, L. maackii seedlings were larger and retained their leaves longer in communities exposed to either increased rainfall variability or nitrogen addition, suggesting an increased propensity for invasion of L. maackii into grasslands in the future. However, performance of both woody species was largely mediated by the surrounding herbaceous community, which exerted strong control over seedling growth and mortality via competition for light. While increased rainfall variability and nitrogen addition stimulate L. maackii growth, they also strongly favor denser canopies through the expansion of clonal forbs. Thus, the benefits of these two global change factors to L. maackii growth may be tempered by increased mortality brought on by greater competition from clonal forbs, leading to fewer, but larger individuals persisting in grassland communities of the future.