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.