Long-term carbon sink in Borneo's forests halted by drought and vulnerable to edge effects

Abstract

Less than half of anthropogenic carbon dioxide emissions remain in the atmosphere. While carbon balance models imply large carbon uptake in tropical forests, direct on-the-ground observations are still lacking in Southeast Asia. Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha^-1 per year (95% CI 0.14-0.72, mean period 1988-2010) above-ground live biomass. These results closely match those from African and Amazonian plot networks, suggesting that the world's remaining intact tropical forests are now en masse out-of-equilibrium. Although both pan-tropical and long-term, the sink in remaining intact forests appears vulnerable to climate and land use changes. Across Borneo the 1997-1998 El Niño drought temporarily halted the carbon sink by increasing tree mortality, while fragmentation persistently offset the sink and turned many edge-affected forests into a carbon source to the atmosphere.

Fig. 1

Locations of 71 long-term forest monitoring plots shown on the 2010 Borneo forest cover map. Intact and logged forest cover are based on Gaveau et al. 2014, accessed at http://www.cgiar-csi.org/portfolio-items/forests-of-borneo. All plots were located in 2010 intact forest areas. Plot symbols overlap, with some obscuring the small forest fragments containing the plots

Fig. 2

Anthropogenic edge impact on plot above-ground live biomass change in structurally intact forests of Borneo. a A hockey-stick model (solid line; break-point at 448 m, 1.14 Mg ha⁻¹ per year) and asymptotic model (dashed line, asymptote 1.26 Mg ha⁻¹ per year) showing saturating edge effect on AGB change. b Histograms of plot AGB change, weighted by the cube root of monitoring length, in forest interior (≥448 m, n = 49; mean 1.01 Mg ha⁻¹ per year, green dashed line) and edge plots (<448 m, n = 22; mean −0.36 Mg ha⁻¹ per year, orange dashed line)

Fig. 3

Above-ground live biomass change rates in pan-tropical structurally intact forests. Results for Borneo (this study) were based on linear mixed effects (LME) model estimates for the mean period of 1988–2010 (spanning 1958–2015), showing contrast between forest interior plots and edge-affected plots. The most recent published estimates for similar periods for Amazon and tropical Africa are shown for comparison. All estimates are based on direct, ground measurements, with the number of long-term inventory plots and total monitoring effort (in ha years) indicated. Bars are 95% CIs. Only the difference between Borneo interior and edge plots was tested statistically where asterisk indicates significant difference (P = 0.034)

Fig. 4

Contrasting forest dynamics observed in forest interior and close to anthropogenic edges in Borneo. Above-ground wood productivity (AGWP, a), Above-ground live biomass (AGB) mortality (b), stem recruitment (c) and mortality (d) rates are long-term means (mean monitoring period 1988–2010) estimated using linear mixed effects (LME) models. Bars are 95% CIs. Asterisks indicate significant difference (Stem mortality: P = 0.006; Stem recruitment: P < 0.001). Individual plot values for these variables are presented as histograms in Supplementary Fig. 5 showing variation in both interior and edge plots

Fig. 5

Mean changes in stand basal area and basal area weighted mean wood density in forest interior and close to anthropogenic edges in Borneo.Stand basal area (BA, a) and BA weighted mean wood density (WD, b) were calculated on a proportional basis relative to values of the initial censuses of long-term plots in Borneo. Plots were classified as edge affected if within 448 m from anthropogenic edges. Values represent long-term trends (mean monitoring period 1988–2010) estimated using linear mixed effects (LME) models. Bars are 95% CIs. Asterisk indicates significant difference (P = 0.011). Individual plot values for these variables are presented as histograms in Supplementary Fig. 5 showing variation in both interior and edge plots

Fig. 6

Biomass dynamics in 19 forest interior plots in Borneo that were monitored over the 1997–1998 El Niño. Above-ground wood productivity (AGWP, a), above-ground live biomass (AGB) mortality (b) and AGB change (c) were estimated for three mean time intervals: before drought 1978.6–1996.5, during drought 1996.5–2000.0 and after drought 2000.0–2011.1. Values shown are estimated from linear mixed effects (LME) models with 95% CIs (bars) for the 19 plots. Asterisks indicate significant difference based on non-overlapping CIs. Before–during–after trajectories for individual plots for each variable are presented in Supplementary Fig. 6