Contrasting effects of extreme precipitation and nitrogen addition on prairie community composition and invasion

Background/Question/Methods

Climate model projections suggest that anthropogenic climate change will result in more variable precipitation patterns, including increased frequency of large precipitation events and prolonged drought between events, independently of changes in total precipitation. Although the response of terrestrial ecosystems to changes in total rainfall amount has been widely studied, the response to an increase in precipitation variability, or “extremes”, is still relatively unknown. In this study, we used a multi-factor manipulation experiment to examine the response of a mixed-grass prairie community to a more extreme precipitation regime as well as nitrogen fertilization, which has increased as a result of increased agricultural intensity. For the precipitation treatment, we exposed experimental plots to a 50% reduction in ambient rainfall, and reapplied excluded precipitation every 30 days for the duration of the growing season. The nitrogen fertilization treatment consisted of an addition of 5 g/m² addition of slow-release, polymer coated urea to the experimental plots. Each manipulation was fully crossed, resulting in four plot types. Each plot type was replicated three times (12 plots total).

Results/Conclusions

The extreme precipitation treatment increased moisture availability during the early and middle portions of an unseasonably dry growing season, which increased total biomass, primarily due to an increase in the abundance of native forbs. In contrast, nitrogen addition had a larger positive impact on native grasses compared to native forbs, but also favored an invasive forb, Cirsium arvense. In the multi-factor experimental plots, we observed a larger increase in total biomass compared to single-factor plots as a result of an increase in native grasses, native forbs, and invasive forbs. Although the treatments did alter the composition of species within the plant community, neither treatment significantly altered plant species richness or diversity. Our results support the hypothesis that greater variability of future precipitation regimes and increased nitrogen deposition will significantly alter grassland soil water availability, plant community composition, and invasive plant species performance. However, these results suggest that different global change factors may have contrasting effects.