Functional diversity of species-rich managed grasslands in response to fertility, defoliation and temperature

Predicting plant functional composition of managed grasslands in response to stress (nutrients and temperature) and disturbance (grazing and cutting) is still a challenge. We aim to examine two issues. (i) Is the leaf dry matter content (LDMC) of Poaceae species (g) weighted by their abundance (LDMCgw) an appropriate indicator of grassland response to stress and disturbance? Comparison was done for stress and disturbance was assessed through proxy variables describing abiotic conditions (altitude) and management (fertilization practices and three defoliation regimes) or through measured indicators (temperature, N availability and canopy height at the time of defoliation). (ii) Does the degree of disturbance that maximises functional diversity increase as productivity increases? Using the distribution of the specific LDMCg values in the analyzed plant communities as a measure of functional diversity (FD), we explore its response to the gradients of stress and disturbance. For 136 vegetation datasets (meadows and pastures) recorded, the N-Ellenberg indicator value (N-Ell) was calculated to assess nutrient availability, and canopy height was measured. We show that LDMCgw was significantly negatively correlated with temperature, N fertilizer rate or N-Ell and canopy height (greater for meadows than for pastures). There was a parabolic relationship between FD and LDMCgw only if the temperature, the N-Ell and the defoliation regimes are considered together. This suggests the following framework for explaining the response of FD to stress and disturbance by a single plant trait: low stress (high temperature and nutrients) and high plant competition for light resulting from cutting favour plants with an exploitative strategy; high stress (low temperature and nutrients) and low plant competition for light resulting from grazing favour plants with a conservative strategy; intermediate stress favours the coexistence of both plant strategies for cutting as well for grazing. This framework can be used to define combinations of management practices and environmental factors, allowing a variety of management objectives to be achieved.