Consistent timelines, divergent end points: plant community change in multiple tallgrass nitrogen addition experiments.

Human activities have increased nitrogen (N) bioavailability broadly, resulting in increased plant productivity and decreased plant species richness globally. However, the determinants of the rates of change and the ultimate plant community composition resulting from N additions remain unclear. Here we synthesize data from six long-term N addition experiments in a tallgrass prairie ecosystem to (1) determine whether plant compositional change with N additions occurs gradually or is an abrupt response, (2) identify short-term (i.e., pulse) environmental drivers that may cause compositional changes, and (3) map the end points of plant communities following a decade or more of N addition. We find that most plant community change occurred gradually over the first several years of chronic N additions, with no evidence that other perturbations (i.e., precipitation, herbivory, fire) catalyzed the change. Rather, we think that the observed pattern reflects the long-lived nature of dominant plant species within this community. Additionally, we find that the final community composition differed across the six experiments we synthesized, seemingly driven by differences in the initial plant community composition. Despite different species coming to dominate the six experiments with N addition, these species shared functional identities as fast-growing, weedy species. The shifts in tallgrass plant community composition observed with increased N availability across the six experiments synthesized here may have consequences for the stability of ecosystem function in a world with altered N bioavailability. Long-term experiments are necessary to capture these responses to N addition, with short-term experiments likely underestimating the community responses to altered N.