Nitrogen deposition into natural systems is an increasingly pervasive environmental change that is associated with declines in species diversity worldwide. However, most studies of nitrogen deposition on plant diversity are conducted at a single sampling scale, which may present difficulty in translating experimental outcomes to broader contexts.
Approaches that quantify effects across a range of sampling scales can distinguish between multiple mechanisms of observed diversity loss, including changes to species relative abundance and spatial distribution of different patch types. Particularly in California annual grasslands, where vegetation and soil resources are highly heterogeneous, nitrogen-driven effects on species diversity may be mediated by spatial dynamics that are captured only at certain scales of observation.
To quantify scale-dependent diversity change, we established a series of 64m2 paired plots at three sites in winter 2016. At each of these sites, four sets of plot pairs were established. One plot within each pair was subjected to nitrogen enrichment through a slow-release urea fertilizer at a rate of 10g N m-2 yr-1. In the spring of 2017 and 2018, we characterized community response to fertilization using a series of 64 0.25m2 subplots spaced at 1m intervals.
Results/Conclusions
Consistent with nutrient enrichment studies in California grasslands, nitrogen fertilization increased biomass productivity and light limitation. Across scales, exotic annual grass cover increased. At the subplot scale (alpha diversity), this increase in exotic grass abundance was accompanied by a loss of community diversity. At the scale of the entire plot (gamma diversity), effects of nitrogen additions were more muted compared to the subplot scale, and were attributed primarily to decreases in community evenness rather than richness.
We further decomposed these diversity patterns into spatially explicit and randomized species accumulation curves to explore diversity change between the subplot and overall plot scales. Neither produced a consistent, linear trend; in spatial accumulation curves, proportional diversity loss in response to fertilization was most pronounced at intermediate sampling efforts. This effect of fertilization on random accumulation curves was weaker, suggesting that nitrogen enrichment interacts with underlying spatial heterogeneity to produce non-uniform shifts in community composition, in which biodiversity loss occurs in a spatially aggregated fashion.
Together, our findings emphasize that single scales of analysis can fail to detect changes in plant abundance and spatial distribution that are critical to translating outcomes of experimental studies to scales of interest to conservation and land management.