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 … [↵][1]1To whom correspondence should be addressed. Email: kai.zhu{at}ucsc.edu. [1]: #xref-corresp-1-1