N:P ratio explains plant species composition of grasslands independent from other environmental factors

Availability of water and nutrients impose important environmental constraints on species composition in terrestrial ecosystems. Species composition may be further constrained by saline and/or acid conditions, which can act as potential stressors for which many plant species have limited tolerance. Heinz Ellenberg estimated indicator values for these factors for c. 2800 European vascular plant species. He assigned ordinal values to species for each factor, indicating a species’ relative position along a tolerance/requirement gradient. These values were based on expert judgement but were later calibrated by other researchers using field measurements. The Ellenberg values are now accepted as a means to infer environmental conditions from plant species composition. The Ellenberg factor ‘nutrients’ is considered to be a general proxy for total nutrient availability. Total nutrient availability is often regarded as the most important determinant of species composition. However, many recent studies from a variety of ecosystems have shown that the relative amount of nutrients (stoichiometry) is also an important environmental factor. In this study we analysed to what extent the N:P(:K) ratio in aboveground biomass explains grassland species composition. We compared the explanatory strength of the N:P(:K) ratio with that of productivity and other environmental factors (the latter based on Ellenberg values). For this purpose we used a database containing more than 600 sites and c. 600 species. Our results indicate that N:P ratio was the second most important explanatory variable for species composition. This finding has important implications for current nature management strategies, which generally only aim to reduce nutrient availability per se. Future strategies could target specific nutrients to adjust soil nutrient ratios in a conservation area.