Functional composition of grassland communities: The interplay of regional climate and local land management

Background/Question/Methods

Trait-based community ecology has been primarily developed to reveal and quantify two key niche-based assembly processes: Habitat filtering and limiting similarity. These processes are not mutually exclusive, and their relative importance is likely to vary across environmental conditions and the traits under scrutiny. The use of a management gradient in grassland ecology allows one to highlight the change in the functional structure of plant communities in response to increasing human activities. Surprisingly, the interplay of climate and management factors that drive the functional diversity of grassland communities is poorly studied. Here we asked how regional and local environmental factors act together to shape the functional composition of grassland communities. We used data from eight sites across France representing a large climatic gradient. Within each site, from 6 to 22 grassland communities are distributed along two quantified gradients, one related to soil fertility and the other to disturbance intensity. We used 8 plant traits representing the main axes of plant ecological strategies, including vegetative and reproductive traits, and a null model approach to detect patterns of functional convergence (habitat filtering) or divergence (limiting similarity) along both regional level climate  and local level management  gradients.

Results/Conclusions

Habitat filtering was often predominant, although both habitat filtering and limiting similarity were detected in all sites. The magnitude of niche-based assembly processes depended greatly on the trait considered and the environmental conditions. Further, the regional climatic gradient and the local land management gradients often interacted together to drive community assembly. For instance, when analyzing the time to first flowering within communities, habitat filtering was detected under low fertilization and low disturbance in sites characterized by unfavorable climatic conditions whereas the opposite pattern, i.e. a significant effect of limiting similarity, was found in sites with more favorable growth conditions. Our results thus indicate that the functional assembly processes are very much function-specific as well as environment-specific. This calls for a careful selection of traits to be used in the study of trait-based community assembly and for a wide representation of functional dimensions in order to disentangle the relative importance of different assembly processes. Our results also highlight the importance of regional and local drivers of community assembly, which need both to be better considered if we are to understand the patterns of habitat filtering or limiting similarity detected, in particular in the case of managed ecosystems.