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Abstract:
Aim: Biodiversity loss and climate-driven ecosystem modification are leading to
substantial changes in forest structure and function. However, the effects of
diversity on demographic responses to the environment are poorly understood.
We tested the diversity hypothesis (measured through functional diversity) and
the mass ratio hypothesis (measured through functional identity) in relation to
tree growth, tree mortality and sapling abundance. We sought to determine
whether functional diversity underlies demographic responses to environmental
variation in European forests.
Location: Europe (Spain, Germany, Wallonia, Finland and Sweden).
Methods: We used data from five European national forest inventories from
boreal to Mediterranean biomes (c. 700,000 trees in 54,000 plots and 143 tree
species) and the main forest types across Europe (i.e. from needle-leaved
evergreen forests to broad-leaved deciduous forests). For each forest type, we
applied maximum likelihood techniques to quantify the relative importance of
stand structure, climate and diversity (i.e. functional diversity and functional
identity) as determinants of growth, mortality and sapling abundance. We also
tested whether demographic responses to environmental conditions (including
stand density, evapotranspiration and temperature anomalies) varied with
functional diversity.
Results: Our results suggest that functional diversity has a positive effect on
sapling abundance and growth rates in forests across Europe, while no effect
was observed on tree mortality. Functional identity has a strong effect on
mortality and sapling abundance, with greater mortality rates in forests
dominated by needle-leaved individuals and a greater abundance of saplings
in forests dominated by broad-leaved individuals. Furthermore, we observed
that functional diversity modified the effects of stand density on
demographic responses in Mediterranean forests and the influence of
evapotranspiration and temperature anomalies in forests widely distributed
across Europe.
Main conclusion: Our results suggest that functional diversity may play a key
role in forest dynamics through complementarity mechanisms, as well as by
modulating demographic responses to environmental variation.
