The higher species richness of calcareous than siliceous communities is explained by lower competition in grasslands but higher facilitation in forests

We aim (i) at testing the hypothesis that higher species richness of calcareous than siliceous communities is due to lower competition in open habitats but to higher facilitation in forests, (ii) at disentangling the direct drivers of the higher facilitation of calcareous than siliceous forest canopies. To address these two aims, we conducted two transplant experiments on the two rock types in the South-west of France. In the first experiment, we transplanted, in dry and wet years, six understory species of contrasting light requirements, with and without neighbors in gaps and forest understories. In the second experiment, we transplanted six understory species of the two bedrock origins, in calcareous closed and semi-closed forests, in siliceous control and fertilized forests, and in calcareous and siliceous gaps. 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 less intense and 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 species richness of calcareous than siliceous communities might be explained by lower competition in open calcareous habitats and higher facilitation in calcareous forests. 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.