Mature and old-growth trees and stands are powerhouses of carbon absorption and carbon storage throughout their lives and well after they die. Older and larger trees store significant amounts of carbon, sequestered over decades to centuries of growth. Importantly, they maintain their carbon sequestration abilities as they age, continuing to pull carbon dioxide from the atmosphere and storing the carbon in the trunk, branches, leaves, roots, and soil while returning the oxygen to the atmosphere. Upon death, this carbon transfers from live to dead carbon accounting pools, where it slowly decomposes as new vegetation maintains and increases the carbon balance.
On the scale of an individual tree, research increasingly indicates that the rate of carbon accumulation continuously increases as the tree grows older and larger. The mechanisms behind this phenomenon are the tree’s leaves (or needles) providing greater leaf area. More leaf area means more light can be intercepted via photosynthesis, which means more atmospheric carbon is absorbed. Moreover, the increase in the rate of carbon accumulation occurs even as a tree’s overall growth becomes less efficient. As a recent study concluded: “[L]arge, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.”
Although carbon dynamics operate differently at the stand-level, the rate of carbon accumulation in a stand of trees does not suddenly collapse as forests mature, absent major disturbance (e.g., by insect infestation). As a stand of trees ages, that rate will peak around the time the canopy closes. The timing of the peak varies based on species and growing conditions (e.g., climate, competition). This peak in carbon sequestration has been linked to the age of maturity by different studies, with one of these studies providing a correlation between stand age and diameter at breast height, making these measures of maturity operational in the field. Following the sequestration peak, the rate of accumulation in some conditions will decelerate toward equilibrium (carbon in equaling carbon out); in others, it will remain relatively constant, decelerating gradually, if at all. But, as a general matter, the rate of carbon accumulation remains robust well into a stand’s lifespan, and this rate is often still higher for older stands than much younger stands. Studies at the Forest Service’s Wind River research facility in southwester Washington documented that old-growth forests continue to perform net carbon sequestration even after 500 years. Additionally, a recent study shows that in eastern forests, unmanaged stands are equally if not more effective at storing carbon than managed (logged) stands.
Older trees and forests can store their accumulated carbon for centuries. As a healthy tree ages and continues to absorb carbon, the total amount of its stored carbon increases. Older, larger trees can hold a substantial portion of a forest’s total above-ground carbon even though they account for a relatively small percent of the trees. Further, research indicates that, once dead, such trees often decay more slowly than smaller, younger trees, holding onto their stored carbon for decades to centuries as they decay in the forest. Even then not all carbon is lost to the atmosphere—much is absorbed into the forest soil. Logging removes trees that would otherwise become snags and dead wood, therefore virtually all managed forests have less dead wood and less carbon storage per capita than unmanaged mature and old-growth forests.
Moreover, the relationship between stand or tree age and susceptibility to natural disturbances is not simple. Some species of trees become more vulnerable to certain disturbances as they age—mature lodgepole pine, for instance, tends to be susceptible to mountain pine beetle. But older trees often possess features that make them more resistant to fire than younger trees, such as the thicker bark that comes with increasing age and size, and lower branch self-pruning in some species that limits fire crown spread. Further, relatively little of the carbon held by older trees is combusted. Fires—including severe ones—consume mostly the duff, understory vegetation, and live foliage, but not the tree stem where most of the carbon is stored. And, as noted, even if disturbance kills them, the carbon of dead older and larger trees can persist in the forest for extremely long periods.
It is when such trees are removed from the forest that they start to rapidly lose carbon. The logging, manufacture, use, and decay of wood products emits stored carbon over a relatively short time. But carbon accounting—including by the Forest Service—often neglects logging-related emissions, overstates substitution benefits, and ignores the lost sequestration potential of letting trees grow.
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Ibid.
Ibid.
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