Determinants of ecosystem stability in a diverse temperate forest

Understanding how diversity affects ecosystem stability is crucial for
predicting the consequences of continued habitat and biodiversity loss on
ecosystem functions and services. Long-term productivity stability in
plant communities is often associated with greater species, phylogenetic
or functional diversity, more complex size and age structures, or higher
asynchrony in species fluctuations (compensatory dynamics), all
potentially increasing community resistance to perturbations. However, the
relative importance of these stabilizing pathways is still poorly
understood, especially in old-growth species-rich forests. Here we explore
how compensatory dynamics and multiple facets of diversity underpin
temporal stability of wood biomass production over forty years in a
Japanese temperate forest, based on more than 45,500 stem increments from
15 species. Whereas the effect of species richness and phylogenetic
diversity was small, the old-growth structural attributes markedly
increased community stability via increased asynchrony in the performance
of co-occurring species. Greater standing tree volume, stem density and
interspecific variation in growth rates enhanced productivity stability
both directly and indirectly via increased asynchrony. This corroborates
the predictions of increased compensatory dynamics with increased
asymmetric competition for light in a more productive environment.
Asymmetric competition in old-growth patches, between dominant oaks and
sub-canopy shade-tolerant firs and maples, is a major driver of
productivity stability over time via compensatory dynamics. Overall
productivity remains relatively constant in old-growth patches, as
abundant firs and maples in the lower canopy layers compensate for biomass
losses in canopy oaks caused by aging, wind and snow disturbances. Younger
forest patches, composed of fast-growing, shade-intolerant species, had a
lower stability of productivity, with reduced stem basal area and tree
density due to higher understory bamboo coverage preventing tree
regeneration and growth. We provide new insights into mechanisms
underlying the stability of ecosystem functioning in diverse forest
ecosystems, and emphasize the importance of preserving and supporting
old-growth forests and their structural complexity.