Rainfall variability and nitrogen addition synergistically reduce plant diversity in a restored tallgrass prairie

Climate change is expected to bring fewer, larger rainfall events and prolonged droughts (i.e. increased rainfall variability). Concurrently, the burning of fossil fuels and reliance on nitrogen (N) fertilizers are expected to continue to increase N availability in many ecosystems. These changes in water and N availability have the potential to alter plant community composition and structure. We manipulated rainfall variability and N inputs in a restored tallgrass prairie over the course of two growing seasons. Greater rainfall variability led to wetter soils throughout the majority of both growing seasons and provided punctuated relief from a severe drought that occurred during the first three months of the experiment. Both rainfall variability- and fertilization-induced increases in resource availability favoured fast-growing, deeply rooted C₃ forbs, particularly the dominant Solidago canadensis, at the expense of species adapted to low resource conditions, particularly C₄ grasses and N-fixing forbs. This change in community composition decreased plant community diversity and evenness in plots receiving both supplemental N and more variable rainfall. Synthesis and applications. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable.