Complex plant community responses to modifications of disturbance and nutrient availability in productive permanent grasslands

Question: Do the effects of grazing and fertilization on taxonomic and functional diversity follow the dynamic equilibrium model? Location: Long-term permanent upland grasslands, Massif-central, France. Methods: Over 8yr, we monitored vegetation changes in 24 experimental plots in two productive grasslands after modification of grazing intensity (including abandonment) or fertilization regime (including cessation of fertilization). Botanical surveys and plant trait measurements (LDMC, SLA and reproductive plant height) were performed within each plot. We analysed taxonomic responses using PRC and functional responses from observed temporal changes of CWM traits and Rao index. Results: Grazing abandonment led to significant reductions in species richness and functional convergence toward taller plants with higher LDMC in both grasslands. Intermediate and high grazing pressure did not differ but showed strong variations in taxonomic diversity between years. Changes in functional vegetation structure were mostly driven by species turnover and weakly influenced by intraspecific trait variability. Community responses to modified fertilization regime were more complex, with strong differences between the two mown grasslands. Species richness did not change significantly although species evenness decreased in the highly fertilized treatments (NPK), particularly in the most productive grassland. Here, functional changes were largely driven by intraspecific trait variability. Both functional divergence and convergence were found for different traits. In NPK, grassland assemblages converged toward taller plants while SLA diverged. Conclusions: In our upland study system the different components of grassland diversity responded differently, in terms of direction or rate of change and mostly under extreme modifications of management. Following abandonment, the observed patterns suggest a prominent and consistent role of competitive exclusion, as expected under the DEM. However, our findings also suggest the role of additional processes, especially the importance of niche differentiation, for our mown grassland under NPK. Timing and net effects of these combined assembly drivers were sensitive to even slight differences in initial productivity and species composition.