Lower competition in grasslands and higher facilitation in forests as potential drivers of the higher species richness of calcareous than siliceous communities

We tested the hypothesis that, in calcareous and siliceous ecosystems from the wet temperate climate of southwest France, competition primarily explains differences in community composition and species richness in grasslands but that facilitation is more determinant in forests. In a first experiment, we transplanted, in dry and wet years, grass species of contrasting light requirements in gaps and forest understories to compare on the two bedrocks the effects of herbaceous neighbors in gaps to the effects of adult trees on understory species. In a second experiment, we manipulated light and soil nutrient fertility in the same system and transplanted six understory species to disentangle the direct drivers of the positive effect of trees for understory species. In the first experiment, competition was higher in gaps of siliceous than calcareous bedrocks during wet years and facilitation by canopy trees was higher on calcareous than siliceous bedrocks during dry years. In addition, facilitation in calcareous forests was very intense for species from the shadiest origins, while competition was only significant for species from the sunniest origins. In the second, experiment we found that both higher microclimates buffering ability of forest canopies and higher soil fertility explained the higher facilitation found in calcareous forests. Synthesis : the higher importance of competition in grasslands and facilitation in forests is mostly due to the contrasting adaptations to light conditions of grassland and understory species. The higher facilitation in calcareous forests appeared to be due to higher buffering effect of tree canopies and higher soil fertility in calcareous than siliceous soils. The major role of facilitation in forest communities from favorable conditions has been strongly overlooked in the literature, although it might explain the higher species richness of calcareous than siliceous forests.