1.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 (NL: no limitation, single‐nutrient limitation, co‐limitation by several nutrients). Yet effects of NL on the species richness–productivity relationship have been rarely studied at the regional and continental scales. 2.Combining the predictions of the humped‐back model and the higher niche dimensionality hypothesis, we hypothesised that plant species richness will increase with the number of different limiting nutrients but that it 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 co‐limited 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. 3.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·m‐2 of above‐ground standing biomass. The type of NL was identified by critical nutrient ratios alone and their combination with critical nutrient concentrations measured in the plant tissue. Relationships were analysed using generalised linear and mixed‐effect models. 4.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 co‐limited sites. When NL was assessed by both ratios and concentrations, species richness at N&P co‐limited 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·m‐2. 5.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 generalise our findings, the role of nutrient limitation needs to be tested in other ecosystems, including more productive plant communities. Shareable link: https://rdcu.be/baLBR
A summary of three case studies exploring environmental drivers of fine-scale species richness (one at regional and two at continental scale) that form of my PhD thesis.
Understanding environmental drivers of fine-scale species richness of vascular plants (henceforth, species richness) and their underlying mechanisms is important both for planning efficient conservation measures and for understanding the ecological and evolutionary processes that structure plant diversity in space and time. Dry grasslands have received much attention in this respect since their species richness can range from moderate to extremely high levels. Some factors that may considerably influence the variation in species richness in dry grasslands at local and regional scales are: (1) soil pH as a non-resource factor that has been shaping evolutionary history of regional floras and regional species pools; (2) productivity as an expression of water and nutrient availability; (3) the type of nutrient limitation (pure limitation by a single nutrient, co-limitation, or no nutrient limitation) as an expression of the unbalanced inter-specific competition under different relative nutrient supplies of nitrogen (N) and phosphorus (P); and (4) precipitation as another proxy of water availability. Most previous studies in this regard were conducted in Central and Western Europe, where dry grasslands reach their western distribution limits. In contrast, studies exploring ecological relationships of species richness in dry grasslands from other parts of Europe (e.g. Balkans) and from the core of the Eurasian dry grassland distribution range – the steppe and forest-steppe biomes in continental Asia – are relatively few. In addition, the relative effects and importance of soil pH and precipitation, on the one hand, and the influence of productivity and the type of nutrient limitation, on the other hand, have not yet been assessed for a single ecosystem at large study extents, i.e. at regional and continental scales. In this thesis, I attempt to answer the following questions: (1) What factors explain the richness of dry grasslands across Bulgaria (regional scale)? (2) What are the relative and interactive effects of soil pH and precipitation on species richness in dry grasslands? (3) Does nutrient limitation affect the diversity–productivity relationship in this habitat? Questions (2) and (3) were tested both within regions spanning large climatic and productivity gradients in northern Eurasia (regional scale) and across regions (continental scale). The results are summarized in the following papers: Paper I (Palpurina et al., 2015, Acta Oecologica): Using dry grasslands from Bulgaria as a study system, we revealed that climatic factors were the most important driver of species richness, separating the species-richer sites in sub-Mediterranean areas from the less species-rich sites in temperate-continental areas. Substrate properties and factors related to water availability were important only when the effect of climate was accounted for. We showed that the reason for this pattern is the increase in richness of annual species of sub-Mediterranean origin but not at the expense of temperate-continental species, which suggests that the species pool of dry grasslands in sub-Mediterranean areas is larger than that of dry grasslands in temperate-continental areas. Paper II (Palpurina et al., 2016, Global Ecology and Biogeography): We revealed a strong dependence of the richness–soil pH relationship on the precipitation gradient across dry grasslands in northern Eurasia. Soil pH had a small effect and most of this effect was shared with the effect of precipitation. We suggested that the species richness–soil pH relationship in dry grasslands over broad areas is substantially confounded by precipitation either indirectly, by shortening and shifting the pH gradient, or directly, by decreasing the negative effects of drought stress on richness. Paper III (Palpurina et al., manuscript): At the continental scale, the species richness–productivity relationship differed between types of nutrient limitation in dry grasslands. Species richness increased continually up to a productivity of ~500 g.m-2 in N&P co-limited grasslands. However, in purely N-, P-limited and unlimited by N and P sites, richness reached a peak at ~300 g.m-2. We hypothesized that the higher species richness of highly productive N&P co-limited grasslands is due to enhanced species coexistence under balanced nutrient supplies. Due to unbalanced prevalence of nutrient limitation within regions, no generalization could be made at the regional scale. To conclude, macroclimatic factors override the importance of site conditions when fine-scale species richness patterns are analyzed at regional and continental scales. Further, nutrient co-limitation might be another condition for maintaining high species richness in dry grasslands, especially in Europe.