Professor University of Minnesota; Institute for Global Change Biology, University of Michigan, Minnesota, United States
Abstract: The number of plant species that an area can sustain is related to a host of environmental factors. Species richness increases with area of a habitat up to a plateau that, similarly, varies from one habitat to another. Experimentally it has been demonstrated that increasing levels of nitrogen deposition typically reduce local diversity. There is as yet no consensus regarding the impacts of elevated CO2 on local diversity. The response of these two perturbations together has been studied extremely rarely and evidence is unclear regarding whether CO2 and nitrogen together increase or reduce species loss relative to the individual effects of the treatments. However, the influence of environmental perturbations may vary across spatial scales, and most experimental analyses are conducted at a single scale. Here, we use a set of high spatial resolution measurements taken in the BioCON experiment in Cedar Creek, Minnesota to calculate species-area curves for grassland species across three spatial orders of magnitude in the year 2019 in plots that were originally seeded with 16 different species in 1997. These scales represent potential neighborhoods at which nearest neighbors or more distant neighbors might interact.
We find that both CO2 and the combination of CO2 and nitrogen deposition consistently decrease diversity, measured as richness and through a set of three common diversity indices, across all spatial scales. Nitrogen alone, however, appears to have a spatially dependent influence – decreasing diversity at finer scales, including those at which species counts are often taken in these experimental settings. Interestingly, though, we found no statistically significant effect of N addition on species richness at the largest spatial scale studied, the full plot. However, this spatially dependent effect is sensitive to the metric of diversity used, and only appears for richness. Other diversity metrics, which discount the influence of rare species relative to richness, show a consistent decline in diversity across all spatial scales and for all treatments. This suggests that nitrogen deposition first suppresses diversity at the finest neighborhood scale, but small scale differences in the plot level environment result in a lower likelihood that the last member of a species will be lost due to nitrogen deposition alone, even over a 20-year time period.