Tree diversity is key for promoting the diversity and abundance of forest‐associated taxa in Europe

Plant diversity is an important driver of diversity at other trophic
levels, suggesting that cascading extinctions could reduce overall
biodiversity. Most evidence for positive effects of plant diversity comes
from grasslands. Despite the fact that forests are hotspots of
biodiversity, the importance of tree diversity, in particular its relative
importance compared to other management related factors, in affecting
forest‐associated taxa is not well known. To address this, we used data
from 183 plots, located in different forest types, from Mediterranean to
Boreal, and established along a climatic gradient across six European
countries (FunDivEUROPE project). We tested the influence of tree
diversity, tree functional composition (i.e. functional trait values),
forest structure, climate and soil on the diversity and abundance/activity
of nine taxa (bats, birds, spiders, microorganisms, earthworms, ungulates,
foliar fungal pathogens, defoliating insects and understorey plants) and
on their overall diversity and abundance/activity
(multidiversity,multiabundance/activity). Tree diversity was a key driver
of taxon‐level and overall forest‐associated biodiversity, along with tree
functional composition, forest structure, climate and soil. Both tree
species richness and functional diversity (variation in functional trait
values) were important. The effects of tree diversity on the
abundance/activity of forest‐associated taxa were less consistent.
Nonetheless, spiders, ungulates and foliar fungal pathogens were all more
abundant/active in diverse forests. Tree functional composition and
structure were also important drivers of abundance/activity: conifer
stands had lower overall multidiversity (although the effect was driven by
defoliating insects), while stands with potentially tall trees had lower
overall multiabundance/activity. We found more synergies than trade‐offs
between diversity and abundance/activity of different taxa, suggesting
that forest management can promote high diversity across taxa. Our results
clearly show the high value of mixed forest stands for multiple
forest‐associated taxa and indicate that multiple dimensions of tree
diversity (taxonomic and functional) are important.