Data from: The functional trait space of tree species is influenced by the species richness of the canopy and the type of forest

Analysing how species modify their trait expression along a diversity
gradient brings insight about the role that intraspecific variability
plays over species interactions, e.g. competition versus complementarity.
Here, we evaluated the functional trait space of nine tree species
dominant in three types of European forests (a continental-Mediterranean,
a mountainous mixed temperate and a boreal) growing in communities with
different species richness in the canopy, including pure stands. We
compiled whole-plant and leaf traits in 1719 individuals, and used them to
quantify species trait hypervolumes in communities with different tree
species richness. We investigated changes along the species richness
gradient to disentangle species responses to the neighbouring environment,
in terms of hypervolume size (trait variance), shape (trait relative
importance) and centroid translation (shifts of mean trait values) using
null models. Our main results showed differences in trait variance and
shifts of mean values along the tree diversity gradient, with shorter
trees but with larger crowns in mixed stands. We found constrained
functional spaces (trait convergence) in pure stands, suggesting an
important intraspecific competition, and expanded functional spaces (trait
divergence) in two-species admixtures, suggesting competition release due
to interspecific complementarity. Nevertheless, further responses to
increasing species richness were different for each forest type, waning
species complementarity in sites with limiting conditions for growth. Our
results demonstrate that tree species phenotypes respond to the species
richness in the canopy in European forests, boosting species
complementarity at low level of canopy diversity and with a site-specific
pattern at greater level of species richness. These outcomes evidence the
limitation of functional diversity measures based only on traits from pure
stands or general trait database values.