The type of nutrient limitation affects the plant species richness–productivity relationship: Evidence from dry grasslands across Eurasia

Abstract

The species richness–productivity relationship is one of the most debated patterns in ecology. Species coexistence theory suggests that it could be tightly linked to the type of nutrient limitation (no limitation, single-nutrient limitation, colimitation by several nutrients). Yet, the effects of nutrient limitation on the species richness–productivity relationship have been rarely studied at the regional and continental scales. Combining the predictions of the humped-back model and the niche dimension hypothesis, we hypothesized that an increase in plant species richness with the number of different limiting nutrients is detectable only at higher productivity levels, at which competition for nutrients is more intense. Therefore, we expected the shape of the diversity–productivity relationship to differ between sites colimited by nitrogen (N) and phosphorus (P), sites limited by a single nutrient (either N or P), and sites not limited by any of these nutrients. To test this hypothesis, we used species richness data collected in 10 m × 10 m plots at 694 temperate dry grassland sites across eight regions in northern Eurasia. Productivity ranged from 10 to ~500 g/m2 of above-ground standing biomass. The type of nutrient limitation was identified by critical nutrient ratios alone and their combination with critical nutrient concentrations measured in the plant tissue. Relationships were analysed using generalized linear and mixed-effect models. In line with our expectations, species richness of Eurasian temperate dry grasslands increased more steeply and peaked higher under higher productivity levels at N&P-colimited sites. When nutrient limitation was assessed by both ratios and concentrations, species richness at N&P-colimited sites continued to increase monotonically until the maximum productivity sampled in this study. In contrast, at sites with a single-nutrient limitation or no limitation, the peak in species richness was lower and occurred at a lower productivity of about 300–400 g/m2. Synthesis. We provide the first evidence that the species richness–productivity relationship may depend on the type of nutrient limitation as predicted by the species coexistence theory. To generalize these findings, the role of nutrient limitation needs to be tested in other ecosystems, including more productive plant communities.