Above-ground biomass and structure of 260 African tropical forests

Abstract

We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha⁻¹ (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha⁻¹) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha⁻¹ greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus-AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.

Keywords: Congo Basin; climate; east Africa; soil; west Africa; wood density.

Figure 1.

Above-ground biomass (AGB), basal area (BA), basal area-weighted wood mass density (WMD_BA), and stem density for 260 plots in closed-canopy tropical forest. Green represents ‘closed forest’ and ‘flooded forest’ categories from the 300 m resolution European Space Agency Globcover (v. 2.3) map for the year 2009. (Online version in colour.)

Figure 2.

Above-ground biomass (AGB) plotted against basal area, basal area-weighted wood mass density, and stem density for 260 plots in closed-canopy tropical forest. OLS lines are, AGB = −78.6 + 15.6 × BA (r² = 0.71); AGB = −82.4 + 755 × WMD_BA (r² = 0.18). (Online version in colour.)

Figure 3.

Above-ground biomass (AGB), basal area (BA), basal area-weighted wood mass density (WMD_BA), and stem density for 260 plots versus latitude in decimal degrees. Quadratic fits are AGB = 451.6–3.57 × latitude² (r² = 0.31, p < 0.001); BA = 32.7–0.150 × latitude² (r² = 0.18, p < 0.001); WMD_BA = 0.641–0.00051 × latitude² (r² = 0.02, p = 0.02). (Online version in colour.)

Figure 4.

Bivariate plots of AGB and (a) temperature (top; mean annual temperature, temperature coldest month, temperature in warmest quarter, temperature of coefficient variation, left to right), (b) rainfall (middle; mean annual rainfall, rainfall in driest month, rainfall wettest quarter, rainfall coefficient of variation, left to right) and (c) soil and fragmentation (bottom; PCA axis two, silt to clay texture, sum of bases in topsoil, carbon to nitrogen ratio in topsoil, distance to nearest forest edge and clearing, left to right; note log scale). Dashed regression lines indicate a significant relationship before accounting for spatial autocorrelation, solid lines after accounting for spatial autocorrelation (full details and equivalent graphs for BA, WMD_BA and stem density in electronic supplementary material). CV is coefficient of variation. (Online version in colour.)