Forests affect climate, and climate affects forests, with carbon linking the two. Forests contain three-fourths of the earth's plant biomass, about half of which is carbon. Consequently, forests play a key role in the global carbon cycle by capturing, storing, and cycling carbon.
Forests can either be a carbon "sink" or a "source" of atmospheric carbon. Trees absorb, or "uptake," carbon dioxide (CO2) from the atmosphere during photosynthesis, emitting oxygen while using carbon to build woody stems, branches, roots, and leaves. This carbon is stored in carbon "pools." Trees release CO2 during respiration and after they die through decomposition or when they burn. When carbon uptake in a forest exceeds respiration and other carbon losses, forest carbon pools are increasing and carbon "sequestration" is occurring. Young forests sequester carbon faster than old forests because CO2 uptake greatly exceeds respiration, but old forests store more total carbon than young ones. In very old and undisturbed forests, respiration may exceed uptake, and such forests have switched from being sinks to sources of atmospheric carbon. In forests with very long-lived species, many forests that are hundreds of years old have been measured to still be carbon sinks. In all forests, the carbon dioxide emissions from microbial respiration on dead and downed wood, including both logging residues and natural mortality, can be a significant component of the net carbon balance from any specific forest area.
Carbon sequestration is the capture and storage of atmospheric carbon in other carbon pools, such as forest vegetation. From 1990 to 2005, U.S. forests sequestered an annual average of 179 million tons of carbon, enough to offset about 10% of the nation's CO2 emissions. Increased use of wood products and wood energy represent part of the solution to reducing greenhouse gases. When trees are harvested, carbon is extracted from the forest but not necessarily returned directly to the atmosphere. If trees are used to make wood products, a portion of the sequestered carbon is stored in solid form for several or more decades in the wood products carbon pool or even longer in the landfill carbon pool. If wood is used to produce energy, the carbon released through combustion offsets or displaces carbon that otherwise would have been released through the burning of fossil fuels.
by Bill Stewart, University of California Berkeley
A global review of net ecosystem carbon exchange concluded that: “in general, 77 gC m-2 year-1 of carbon is lost by ecosystem respiration for every 100 gC m-2 year-1 gained by gross photosynthesis when an ecosystem has not experienced recent and significant disturbance.” (Baldocchi 2008). A synthesis study from 10 studies in forests worldwide found that about half the respiration came from the forest floor and soil, and half came from leaf respiration in the canopy (Misson et al. 2007). In addition, the increase in respiration from forests disturbed by harvests, fires, severe drought, or storm blowdown will increase in proportion to how much residue is left on site to decompose (Albani et al. 2006). Post-disturbance studies have shown that respiration exceeds photosynthesis for the first ~ 10 years, while the leaf area increases and the residues on the forest floor are decomposed by microbes. While the logging residues and natural mortality are not considered as part of the growing stock in forests, they do have a significant impact in increasing overall forest respiration and thereby decreasing the net carbon storage at the forest-wide level.
Albani, M., D. Medvigy, G.C. Hurtt, and P.R. Moorcroft 2006. The contributions of land-use change, CO2 fertilization, and climate variability to the Eastern U.S. carbon sink. Global Change Biology. 12: 2370-2390.
Baldocchi, D. 2008. "Breathing" of the terrestrial biosphere: Lessons learned from a global network of carbon dioxide flux measurement systems. Australian Journal of Botany. 56: 1-26.
Mission, L., D.D. Baldocchi, T.A. Black, P.D. Blanken, Y. Brunet, J. Curiel Yuste, J.R. Dorsey, M. Falk, A. Granier, M.R. Irvine, N. Jarosz, E. Lamaud, S. Launiainen, B.E. Law, B. Longdoz, D. Loustau, M. McKay, U. Paw, T. Vesala, D. Vickers, K.B. Wilson, and A. H. Goldstein. 2007. Partitioning forest carbon fluxes with overstory and understory eddy-covariance measurements: A synthesis based on FLUXNET data. Agricultural and Forest Meteorology. 144: 14-31.