As globally important storehouses of carbon, forests play a critical role in influencing the Earth's climate. Forest plants and soils drive the global carbon cycle by sequestering carbon dioxide through photosynthesis and releasing it through respiration. Although carbon uptake by photosynthesis eventually declines as trees age, many mature forests continue to sequester carbon in their soils.

Yet, in many parts of the world, forests are being rapidly cleared for agriculture or pasture, destructively logged and mined, and degraded by human-set fires. When forests are degraded or cleared, their stored carbon is released back to the atmosphere during harvest and through respiration, thus these forests are net contributors of carbon to the atmosphere. Tropical deforestation is responsible for approximately 20% of total human-caused carbon dioxide emissions each year, and is a primary driver of extinction of forest species (see graph below).

Forest and land-use measures have the potential to reduce net carbon emissions by the equivalent of 10-20% of projected fossil fuel emissions through 2050.

Sustainable forest management

Expanding forest area by promoting regeneration of native trees, allowing trees to grow larger, employing harvesting methods that reduce damage and waste, and establishing conservation set-asides within production forests can all increase the average long-term quantity of stored carbon. These management options also tend to have beneficial effects on biodiversity, and on other key ecosystem services such as maintaining watersheds.

Restoring forests also tends to improve habitat quality, especially for wide-ranging forest birds and mammals. Allowing trees to grow larger before harvesting generally increases a forest's structural diversity and provides habitat for a broader range of forest species. Healthy forests that retain their natural complexity and diversity in age and habitat structure generally have greater stability and resilience to withstand disturbances associated with climate change.

Trees grow quickly when they are young, but growth slows as they mature. To increase average carbon storage over time, harvests should occur after the annual growth rate falls below the average growth rate. Because timber companies have a strong economic incentive to harvest when prices are most favourable, however, many forests are harvested well before this optimal age. Lengthening the time between harvests or retaining older trees through successive harvests could significantly increase the carbon stores in the Pacific Northwest and Southeast. Establishing a carbon market and a sound regulatory framework could provide financial incentive to lengthen harvest cycles. Reducing damage to non-harvested trees and disturbance of forest soils during logging operations can also substantially reduce CO2 emissions.

Carbon in Forests: Growth, Harvest, Decay

Preserve the integrity of mature forests when managing for timber or biomass

There is a widespread and misguided belief that logging or clearing mature forests and replacing them with fast-growing younger trees will benefit the climate by sequestering atmospheric CO2. While younger trees grow and sequester carbon quickly, the fate of stored carbon when mature forests are logged must also be considered. When a forest is logged, some of its carbon may be stored for years or decades in wood products. But large quantities of CO2 are also released to the atmosphere - immediately through the disturbance of forest soils, and over time through the decomposition of leaves, branches, and other detritus of timber production.

In addition, voluntary measures will serve to galvanize greater recognition of the role of land-use in climate change. Building confidence in this approach requires that rules be sufficiently rigorous to ensure that voluntary actions result in both measurable net reductions in atmospheric carbon as well as other environmental benefits. Without a true economy-wide cap on carbon emissions, however, such interim voluntary measures are not likely to ameliorate the failure of the market for forest products and services.