The accumulation of carbon within a forest growth cycle can be considered in four stages. The initial establishment stage involves low global carbon cycle accumulation and may even experience net carbon loss (particularly from soil) as a result of site preparation and low biomass and bioenergy inputs. A rapid uptake of carbon is then experienced during the second phase, known as the full vigor stage, which subsequently levels off as the stand reaches the mature stage. Finally, the forest reaches old growth and the carbon is in steady state with accumulation associated with new growth balanced by mortality and disturbances.
Modeling management for forest conservation in the United Kingdom indicates that the maximum accumulation of carbon, and the subsequent removal of CO2 from the atmosphere, over the life cycle of the stand is approximately 200 tCha_ 1. Forest stands managed for commercial production experience periodic harvesting and generally have lower carbon stocks than stands that are not harvested. Under such conditions, commercial stands rarely experience growth phases past the full vigor phase. Over a typical 50-year rotation, commercially managed Sitka spruce stands yield average carbon accumulation rates of 70 tCha_ 1 (Fig. 4b). Although these figures indicate that the carbon stock increases even under periodic harvesting, the longer term C stock depends on a balance between the impacts of harvesting and the rate of forest regeneration. Consideration of the long-term average C stocks in these two examples indicates that even though young, emissions from a 30 MW power station burning fossil fuels. In contrast, approximately 11,250 ha of dedicated energy crops could supply such a power plant and indefinitely replace fossil fuels, providing permanent GHG emission reductions.
There may be potential to promote carbon sinks globally over the next 100 years or more to the extent of sequestering 90 GtC, but increasing carbon stocks in vegetation will ultimately reach ecological or practical limits and other measures will need to be adopted. Forests have a finite capacity to remove CO2 from the atmosphere. The provisions of the Kyoto Protocol with respect to sinks can be seen as a valuable incentive to protect and enhance carbon stock now, while possibly providing the biomass resources needed for continued substitution of fossil fuels into the future.
