Climate and mycorrhizae mediate the relationship of tree species diversity and carbon stocks in subtropical forests

1. It is increasingly being recognized that tree species diversity has
positive effects on forest ecosystem carbon (C) stock. However, at broad
spatial scales this relationship may depend on climate conditions and
species mycorrhizal associations. 2. Here, observations from 667 forest
plots in subtropical China, were used to investigate the effects of
species diversity, mean annual precipitation (MAP), mean annual
temperature (MAT) and mycorrhizal type (arbuscular or ectomycorrhizal) on
the forest C stock and its components (tree C stock, shrub layer C stock,
herb layer C stock, litter layer C stock, root C stock and soil C stock).
3. We found positive effect of tree species diversity on total forest C
stock. MAP had positive effects on total forest C stock and its
components, while MAT had consistently negative effects on total forest C
stock and most of its components. Different levels of MAP and MAT did
modulate the strength of effect of species diversity on forest C stock and
its components. In addition, species diversity, MAT and MAP showed a
significant positive relationship with arbuscular mycorrhiza-associated
tree C stock but had no or negative relationship with
ectomycorrhiza-associated tree C stock. 4. Synthesis. Our results indicate
that maintaining high level of species diversity may support the buffering
of negative effects resulting from climate warming. Furthermore, under
climate warming the specific C stock of AM trees can increase, which can
potentially promote forest C stock. Taken together, our study suggests
that afforestation policies should consider not only tree species
diversity to increase forest C stock but also the effects of different
tree mycorrhizal types.