Role of forest regrowth in global carbon sink dynamics

Abstract

Although the existence of a large carbon sink in terrestrial ecosystems is well-established, the drivers of this sink remain uncertain. It has been suggested that perturbations to forest demography caused by past land-use change, management, and natural disturbances may be causing a large component of current carbon uptake. Here we use a global compilation of forest age observations, combined with a terrestrial biosphere model with explicit modeling of forest regrowth, to partition the global forest carbon sink between old-growth and regrowth stands over the period 1981-2010. For 2001-2010 we find a carbon sink of 0.85 (0.66-0.96) Pg year^-1 located in intact old-growth forest, primarily in the moist tropics and boreal Siberia, and 1.30 (1.03-1.96) Pg year^-1 located in stands regrowing after past disturbance. Approaching half of the sink in regrowth stands would have occurred from demographic changes alone, in the absence of other environmental changes. These age-constrained results show consistency with those simulated using an ensemble of demographically-enabled terrestrial biosphere models following an independent reconstruction of historical land use and management. We estimate that forests will accumulate an additional 69 (44-131) Pg C in live biomass from changes in demography alone if natural disturbances, wood harvest, and reforestation continue at rates comparable to those during 1981-2010. Our results confirm that it is not possible to understand the current global terrestrial carbon sink without accounting for the sizeable sink due to forest demography. They also imply that a large portion of the current terrestrial carbon sink is strictly transient in nature.

Keywords: carbon sink; demography; disturbance; forest; regrowth.

Fig. 1.

Fraction of forest defined as regrowth (less than 140-y-old) in the age dataset for the year 2010. The blank area in southern Australia occurs because no data for this area exists in the GFAD dataset.

Fig. 2.

The 2001–2010 mean carbon sink in global forests partitioned between old-growth and regrowth forests, as calculated by LPJ-GUESS forced by GFAD. (A) Total uptake in old-growth and regrowth forest. Dark green shows the fraction of the regrowth sink that would have occurred in the absence of any environmental change since 1870 (CF), while the light green bar shows the additional flux including all environmental forcing (FF). (B) Total forest area in old-growth and regrowth categories. (C) Uptake rate per area. Results from sensitivity studies are illustrated with additional symbols. The blue square shows the sensitivity to assumptions about the fate of cleared material (Methods, S1), the orange square to assumptions about land-use type before forest regrowth (S2) and the red square to the assumed rate of disturbance in spin-up (S3). The downward pointing arrow is forced by the 5% confidence limit of the stand age distribution and the upwards pointing arrow the 95% confidence limit. For regrowth forest, these sensitivity simulations are shown both for CF (left of regrowth bar) and FF (right of regrowth bar). By definition, the sink in old-growth forest is only driven by changes in environmental forcing (FF) and hence has no CF component.

Fig. 3.

(A) The 2001–2010 mean carbon sink in global forests partitioned between old-growth and regrowth forests, as calculated by LPJ-GUESS forced by GFAD. The sink is split by forest type (for forest type distribution, see SI Appendix, Fig. S9). Coloring and symbols as for Fig. 2. (B) The 2001–2010 mean carbon sink in regrowth forests, forced by the LUH2 land-use dataset, as calculated for three different DGVMs. More intense colors show the sink in CF simulations, and lighter shades additional sink due to environmental change (FF). Numbers above the bars in A and B show the total regrowth forest area in each classification in units of million square kilometers. (C) Regrowth forest sink as estimated from combining the GFAD and LUH2 datasets best estimates (see text), coloring as for A. Forest types are: MX, broadleaved-needleleaved mixed forest; ND, needleleaved deciduous; NE, needleleaved evergreen; Other, other forest; OTR, other tropical forest; TeBD, temperate broadleaved deciduous; TeBE, temperate broadleaved evergreen; TrBD, tropical broadleaved deciduous; TrBE, tropical broadleaved evergreen. Forest type classification was based on ESA CCI land cover (Methods).

Fig. 4.

(A) Percentage difference between biomass in regrowth forest (2010 values) and the biomass that would exist at that location if the forest was in equilibrium with the mean 1981–2010 forest disturbance rate, averaged over each forest type and based on LPJ-GUESS simulations forced by GFAD. (B) Total missing biomass carbon for each forest type, found by differencing the carbon densities of old-growth and regrowth stands in 2010 and multiplying by regrowth area, based on the CF simulation (dark green) and the FF simulation (light green). The symbols show sensitivity simulations, as in Fig. 2. Difference between CF and FF in A was minimal, and thus only CF is shown.