Forest age improves understanding of the global carbon sink

One-fifth of anthropogenic greenhouse gas emissions are sequestered by the terrestrial biosphere, where forests serve as an important “natural solution” to climate change (1). Forests are expected to persist as a substantial carbon sink, dampening future rises in atmospheric CO₂ levels (2–4). However, a significant part of this carbon uptake occurs in forests regrowing from past land-use changes or natural disturbances. A clear example is the landscape history of central New England in the United States, where significant transformations have occurred: from pristine old-growth forests to clear-cut areas giving way to agriculture, to old-field succession, and to expansion of regrowth forests (Fig. 1). As more forests approach old-growth conditions, their rate of carbon uptake may begin to decline. To anticipate the future of the global carbon sink, Pugh et al. (5) show that it is necessary to account for forest regrowth and demography and to consider the broader issue of the terrestrial biosphere’s ultimate capacity to sequester carbon.

Fig. 1.

History of changing forests in New England. (A) Presettlement forests in 1700 consisted of old-growth stands dominated by conifers and broadleaf trees. (B) At the height of agriculture in 1830, most of the forests were cleared for tillage and pasture. (C) Farming advanced to the Midwest and declined in New England, setting the stage for regrowth forests; this started with conifers in 1910, followed by hardwoods. (D) The modern forest landscape is characterized by an expanse of maturing trees in a state of regrowth. The historical perspective shows that forests carry legacies of human activities and help inform the prediction and management of future forests and the terrestrial carbon sink. Images courtesy of Harvard Forest.

In PNAS, Pugh et al. (5) utilize a new global database of forest age to inform a vegetation model and find that regrowth forests constitute a carbon sink that is even greater than that of old-growth forests. Nearly half of the carbon uptake in regrowth forests, however, can be attributed to changes in forest demography instead of environmental change. Pugh et al. (5) demonstrate that this demographic approach estimates a greater global regrowth sink but a smaller tropical regrowth sink in comparison with the traditional land-use–change approach. Combining the unique and shared strengths of forest age and land-use datasets significantly improves our understanding of carbon sink estimates, especially for regrowth forests. Projecting into the future, Pugh et al. (5) calculate the total amount of carbon in live biomass that is missing in forests relative to a world in which forests were allowed to readjust to a business-as-usual disturbance rate. Their assessment suggests that the current forest carbon sink is largely transient in nature.

This study has several implications for our understanding and predictions of the global carbon sink. First, Pugh et al. (5) highlight the importance of present-day forest regrowth in driving carbon sequestration. Many studies so far have attributed the recent increase in the forest carbon sink to environmental changes such as CO₂ fertilization, nitrogen deposition, and climate change (6, 7). Those findings suggest that forest growth may continue to buffer the negative impacts of anthropogenic carbon emissions. However, more researchers are starting to find that the forest carbon sink might be dominated by regrowth and driven by postdisturbance recovery (8, 9). Using a global forest age dataset that informs disturbance history, Pugh et al. (5) show that the regrowth process alone drives about one-fourth of the carbon uptake, while the remainder is driven by environmental change. This is a great step forward in recognizing the key factors causing the changes in carbon uptake at a global scale.

Second, the new forest age dataset allows Pugh et al. (5) to explicitly partition the forest carbon sink into old-growth and regrowth stands, aligning with a growing recognition that demography should play a central role in terrestrial biosphere modeling (10). Using this approach, Pugh et al. (5) estimate that regrowth forests contribute 50% more than old-growth forests to the global carbon sink. However, the size of the regrowth sink is found to be substantially smaller than previous estimates made using limited forest age information (11, 12). Contrasting with prevailing notions, Pugh et al. (5) uncover a greater regrowth sink in the northern midhigh latitudes than in the tropics. Such deviations reveal the large uncertainties in identifying regrowth forests and estimating the regrowth sink based on land-use changes alone. These results point to a new research direction that emphasizes the age and size distributions of forests and how demographic changes drive carbon accumulation (13).

Third, by highlighting the prominent role of forest regrowth in sequestering carbon, Pugh et al. (5) challenge the traditional equilibrium view of the ecological system and promote a transient view (14). Forest ecosystems are constantly shaped by natural and human-induced disturbances over short and long timescales. Because the regrowth sink is gained through past recovery and may be lost in future disturbances, a large portion of the current global forest carbon sink is strictly transient. The transient nature of carbon cycle implies the limitations of focusing on the long-term behavior and emphasizes the need to understand disturbance–recovery dynamics in forest ecosystems (15). Considering forest age, as Pugh et al. (5) do in their study, offers a starting point to characterize forest disturbance and recovery. Furthermore, ongoing and future global changes are shifting disturbance regimes into uncharted territory, complicating carbon sink predictions. Quantifying transient dynamics and disturbance regime shifts should be important elements in evaluating the carbon sink capacity of the terrestrial biosphere.

Finally, Pugh et al.’s (5) thought-provoking finding on the limited potential of the forest carbon sink motivates continued discussion of climate change mitigation strategies. Stabilization of global temperature at any level requires net zero CO₂ emissions, implying equal CO₂ emission and sequestration. The size of the terrestrial carbon sink and its interactions with the atmosphere have always contributed to great uncertainty in the global carbon cycle (16). An improved prediction of the future forest carbon sink will therefore help to identify a more strategic mitigation pathway in the effort to limit global warming. In light of Pugh et al.’s (5) estimate of a smaller carbon sink than previously anticipated, even more immediate action on CO₂ emission reduction may be justified. Existing forests will continue to sequester a significant amount of carbon, but this sink will gradually saturate as regrowth forests age. Pugh et al. (5) add to the increasing lines of evidence that forests may have limits to growth in sequestering carbon in Europe (17), South America (18), and North America (13). In addition, the possibility of a transient carbon sink in global forests and unknown future disturbances raises concerns that historical emissions already sequestered might eventually be released (15). Disturbance management that mitigates carbon losses in existing forests (19) and reforestation of formerly high carbon density forests (20) might be some ways to sustain the growing forest carbon sink.

Pugh et al. (5) provide a unique perspective of regrowth forest and invite new assessments of global forest carbon dynamics. Regrowth forests have been sequestering a sizeable amount of carbon, on par with old-growth forests, and this regrowth sink is attributable to not only environmental change but also demographic change. This latest understanding predicts a limited sequestration potential of a transient carbon sink. The challenges and opportunities in analyzing forest regrowth and demography call for improved evaluation of forest carbon storage as an emerging land-based climate solution.