The following is from:http://www.ornl.gov/ORNLReview/rev28_2/text/bio.htm

Top: Growing forest accumulates carbon until it achieves, over time, a balance between the carbon taken up in photosynthesis and the carbon released back to the atmosphere from respiration, oxidation of dead organic matter, and fires and pests. In the meantime, fossil fuels are used to meet society's energy needs. Bottom: In productive forests, trees can be harvested for use in producing heat or power. Although harvesting may result in less carbon stored in standing biomass and forest soils, biomass fuels replace some of the fossil fuel that would otherwise be burned. The carbon in that fossil fuel remains stored in the ground rather than being released to the atmosphere. In both scenarios there are some energy needs for gathering the resource and converting it into useful energy, but, as the arrows on the transportation system suggest here, these are generally comparatively small. Arrows provide a qualitative indication of the magnitude and direction of carbon flows.