Identity rather than richness drives local neighbourhood species composition effects on oak sapling growth in a young forest

Abstract

In light of global biodiversity loss, the influence of species composition on ecosystem functioning has attracted increasing attention. However, questions remain whether neighbourhood species richness or identity is more important and what mechanisms drive these composition effects. We investigated the role of local species richness and identity on oak sapling growth in young forest stands, and attempted to link growth with two commonly expected mechanisms: oak powdery mildew (PM), a host-specific leaf pathogen, and degree of shading. Furthermore, we studied how such effects might interact with reduced rainfall.We established a reduced rainfall gradient on oak (Quercus) saplings planted within two sites of a young tree diversity experiment (FORBIO). We found no effect of species richness on growth, whereas presence of particular species had a significant impact. In one site, we found an identity effect on growth through negative effects of oak (Q. robur) presence, which was linked to increased PM infection severity. Such link was less clear in the other site, where the impact of PM and oak (Q. petraea) presence was lower and there was no relation between the two. Other identity effects were related to levels of shading, which were themselves influenced by fast early growth rates of the neighbouring trees. We found no consistent effects of reduced rainfall on growth. Host-mediated effects through oak PM and functional size-mediated effects through shading were the most important composition effects in our young forest experiment, with no support for species richness effects through local environmental conditions or natural enemies.