Spatial patterns of species and plant traits in response to 20 years of grazing exclusion in subalpine grassland communities
Patrons spatiaux des espèces et des traits des plantes en réponse à 20 ans d'exclusion du pâturage en prairies subalpines
Résumé
Question: Does long-term grazing exclusion affect spatial patterns of canopy height, plant species and traits in subalpine grassland communities? Are spatial patterns of species and traits similarly affected by grazing exclusion? Are changes in spatial patterns of species associated with changes in species abundances? Location: Subalpine grasslands, Vercors and Oisans Mountains, Alps (France). Methods: Spatial sampling of vegetation and measurements of plant traits were carried out within nine pairs of grazed and ungrazed 10m10-m plots in three species-rich communities with different productivities. We estimated within-plot spatial patterns of canopy height, species and aggregated trait values by measuring the extent (or patch size) and intensity of spatial dependence with Moran's I. Abundance-weighted averages for species patch size and intensity of spatial dependence were calculated across all species per plot and across species per life form. Such measures derived from analysis of spatial dependence were considered spatial traits. Results: Response of spatial patterns to grazing exclusion was only detected in patch size, whereas intensity of spatial dependence was not affected. Changes in spatial patterns were community-dependent because spatial traits based on patch size of canopy height and species increased following grazing exclusion only in the less productive community. Unexpectedly, changes in spatial patterns of species did not support changes in spatial patterns of trait values. Abundances and patch sizes of several life forms were significantly affected by grazing exclusion. However, at the scale investigated, changes in abundance of life forms did not correspond to changes in their spatial patterns and vice versa. Conclusion: In species-rich communities, grazing alters spatial spread of species (i.e. patch size) rather than intra-specific aggregation (i.e. intensity of spatial dependence). Results revealed possible mechanisms of species spatial reorganisation that are independent of abundance variation. Therefore, it is important to consider changes in spatial patterns in addition to changes in mean values of vegetation features when assessing impacts of grazing management.