Decoupling facilitative effects in a temperate subhumid grassland: photosynthetic metabolism matters

Background: Abiotic microhabitat amelioration and protection from herbivores are two of the main mechanisms associated with facilitative interactions in plant communities. Aims: We investigated the effect of the shrub Acanthostyles buniifolius on the herbaceous community of a subhumid grassland in order to disentangle the protective role of the shrub from its effect on ameliorating the abiotic conditions of microhabitats. Methods: In two contiguous areas, one continuously grazed and the other excluded from livestock for 3 years, we determined floristic composition and species cover under the shrub canopy and in paired open sites. We calculated the Relative Interaction Index (RII) for cover of plant functional types and species. Results: Under grazing, C3 grass richness was higher under the shrub canopy than in open plots. Also, the RII was higher in the grazed area for most functional types. Inside the exclosure, the cover of C3 grasses was higher in the presence of the shrub. Conversely, the richness and cover of C4 grasses was lower under the shrub canopy. Conclusions: We found evidence of facilitation in a subhumid grassland. The shrub may protect grasses from grazers and may benefit C3 grasses through microclimatic amelioration. Therefore, photosynthetic pathway should be considered when analysing facilitative interactions in mesic environments.