Relative influence of environmental, stand factors and functional traits on allocation of forest productivity during the restoration of subtropical forests in central China

Many studies have examined plant biomass allocation among leaf, stem and root organs in relation to environmental and ontogenetic factors. But the drivers for allocation of stand-level productivity across restoration stages, and the potential role of functional traits, are still poorly understood. Here we investigated 24 plots across four restoration stages in subtropical forests on Mt. Shennongjia, central China. We examined the relative effects of environment (slope, aspect and canopy openness), stand factors (forest height, total basal area, stand density and age), and community-weighted mean (CWM) of key functional traits (wood density and specific leaf area), on the allocation of forest productivity to stems, leaves and roots. The results showed that forest productivity, and the stem fraction of stand productivity, both increased during forest restoration; while leaf and root productivity fractions showed an overall decrease. A higher fraction of productivity was allocated to leaves and roots while lower fraction was allocated to stems towards harsher site condition. Stem fraction was positively related to stand age and forest height, whereas root fraction showed a converse pattern. None of the stem, leaf or root fraction was significantly correlated with the proxies of competition (stand density and total basal area). As for the effect of functional traits, stem productivity fraction was negatively related to the CWM of wood density, while leaf fraction was positively related, which can be explained by the optimal theory and growth strategy together. However, CWM of specific leaf area has no significant effect on productivity allocation. Multivariate analyses showed that topography (slope) and stand age were main drivers for the productivity allocation to woody organs (stem and root), and wood density was also important for stem productivity fraction. Meanwhile, leaf fraction is mainly affected by wood density and aspect. Our results suggest that, in addition to environmental and stand factors, functional traits (e.g. wood density) are also important in affecting the allocation of stand productivity. However, there are still many equivocal results about the influence of functional traits. Allocation of productivity among organs, and key functional traits (e.g. wood density and specific leaf area), are both indispensable parameters in ecosystem processes models. Further studies are needed to explore the mechanisms how functional traits and stand factors affect productivity allocation, to better predict forest carbon balance under future global change, and to better guide forest management to increase ecosystem services and carbon sink.