Tree species richness increases ecosystem carbon storage in subtropical forests

Abstract

Forest ecosystems are an integral component of the global carbon cycle as they take up and release large amounts of C over short time periods (C flux) or accumulate it over longer time periods (C stock). However, there remains uncertainty about whether and in which direction C fluxes and in particular C stocks may differ between forests of high versus low species richness. Based on a comprehensive dataset derived from field-based measurements, we tested the effect of species richness (3-20 tree species) and stand age (22-116 years) on six compartments of above- and below-ground C stocks and four components of C fluxes in subtropical forests in southeast China. Across forest stands, total C stock was 149 ± 12 Mg ha^-1 with richness explaining 28.5% and age explaining 29.4% of variation in this measure. Species-rich stands had higher C stocks and fluxes than stands with low richness; and, in addition, old stands had higher C stocks than young ones. Overall, for each additional tree species, the total C stock increased by 6.4%. Our results provide comprehensive evidence for diversity-mediated above- and below-ground C sequestration in species-rich subtropical forests in southeast China. Therefore, afforestation policies in this region and elsewhere should consider a change from the current focus on monocultures to multi-species plantations to increase C fixation and thus slow increasing atmospheric CO₂ concentrations and global warming.

Keywords: BEF-China; carbon flux; carbon storage; ecosystem functioning; evergreen broad-leaved forest; forest biodiversity.

Figure 1.

Benchmarking map of carbon stocks (solid white boxes) and fluxes (dashed grey boxes) averaged across all 27 forest stands. Numbers represent means and standard errors. Different colours of arrows indicate effects of tree species richness (red) and stand age (blue). Directions of arrows show positive (upward) and negative (downward) relationships of richness and age with respective C stocks and fluxes. No arrows are shown when effects were not significant.

Figure 2.

Hierarchical partitioning of the variation explained for each component and layer of carbon stocks and fluxes by species richness, stand age and environmental variation. The term for environmental variation was obtained as principal component axis (environmental PC1) of an ordination that incorporated elevation, slope, eastness of aspect, northness of aspect and soil pH at the 27 plots. (Online version in colour.)

Figure 3.

Relationships between C stocks and tree species richness for (a) six components and (b) three layers and total C. Each dot represents a plot. Significant R²-values from linear regressions are shown (***p < 0.001, **p < 0.01, *p < 0.05). Partial regression lines for three levels of stand ages (young, medium, old) are shown in different colours. Logarithmic scales are used for all dependent variables (y-axes). (Online version in colour.)

Figure 4.

Relationships between the four C fluxes and tree species richness. Each dot represents a plot. Significant R²-values from linear regressions are shown (***p < 0.001, **p < 0.01). Partial regression lines for three levels of stand ages (young, medium, old) are shown in different colours. Logarithmic scales are used for all dependent variables (y-axes). (Online version in colour.)