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Part VII. Logging Releases Significant Amounts of Carbon.
Part VII of “The Straight Facts on Forests, Carbon, and Global Warming,” an Oregon Wild report.
Not surprisingly, logging accelerates the transfer of carbon to the atmosphere by killing trees that would otherwise continue to capture and store carbon through photosynthesis and growth. Killing trees also stops them from pumping carbon into the soil where much of the carbon in forests is stored1. Logging actually accelerates the rate of decomposition of wood via several mechanisms. By removing the forest canopy and exposing the soil to more sunlight, logging raises soil temperature which increases the rate of decay. Logging also breaks up woody material in the forest thereby decreasing the average piece size and increasing the surface area exposed to microbial decomposition. Finally, logging debris is often burned on site or as part of an industrial process.
Traditional logging also increases the risk of disturbances. Logging increases wind damage by creating exposed edges and increasing wind speeds within forest stands. Logging often increases the wildfire hazard by making the stand hotter, dryer, and windier; by moving the most flammable small fuels from the forest canopy to the forest floor (i.e., logging slash) where they are more available for combustion; and by initiating the establishment of dense stands of young trees with interlocking branches (resinous fuels) close to the ground. Logging roads also increase the risk of human-caused fire ignitions and spread tree diseases like Port Orford cedar root disease that kill trees and release carbon.
Scientists estimate that a large fraction of all the carbon transferred to the atmosphere by humans has been released due to forest exploitation2. In recent decades CO2 emissions resulting from human-induced changes to forests exceed CO2 emissions from all motor vehicle sources combined, but forest releases are less than total emissions from all uses of fossil fuels3. After logging an old-growth forest, the site remains a net source of carbon for more than 20 years, and depending on the conditions, the site does not rebuild pre-logging carbon stores for a century or more. As a result of widespread clearcutting and aggressive slash burning, the Pacific Northwest has contributed huge quantities of carbon to the atmosphere4.
[1] Forests store massive amounts of carbon in the soil in the form of live and dead roots, woody debris, charcoal, and the vast below-ground ecosystem supported by photosynthate received from trees. Logging cuts off the food supply for the below-ground ecosystem which rapidly dies and decomposes.
[2] G. M. Woodwell, J. E. Hobbie, R. A. Houghton, J. M. Melillo, B. Moore, B. J. Peterson, and G. R. Shaver. 1983. Global Deforestation: Contribution to Atmospheric Carbon Dioxide. Science 9 December 1983: Vol. 222. no. 4628, pp. 1081 – 1086. http://www.sciencemag.org/cgi/content/abstract/222/4628/1081
[3] The Scottish Forest Alliance. Factsheet: Human influences on forest carbon flows. July 2002. http://www.scottishforestalliance.org.uk/carbon/fs_human_influences.pdf
[4] "Mass balance calculations indicate that the conversion of 5 x 106 hectares of old growth forests to younger plantations in western Oregon and Washington in the last 100 years has added 1.5 x 109 to 1.8 x 109 megagrams of carbon to the atmosphere." Harmon, M., Ferrell, W., and J. Franklin. 1990. Effects on Carbon Storage of Conversion of Old-Growth to Young Forests. Science. 9 February 1990.
Warren B. Cohen, Mark E. Harmon, David 0. Wallin, and Maria Fiorella. 1996. Two Decades of Carbon Flux from Forests of the Pacific Northwest - Estimates from a new modeling strategy. BioScience 46(11):836-844. http://www.humboldt.edu/~storage/pdfmill/Batch%203/carbonflux.pdf