Carbon sequestration Policy, science and economics

Although much carbon is removed each year from the earth’s atmosphere by forest growth, the CO2 content of the air has been increasing over the past number of centuries. To calculate the rate of carbon storage in forest ecosystems a number of models have been developed. The models developed in Britain generally combine tree production functions with carbon retention curves for various products. Attempts have been made to apply these models to forest ecosystems in Ireland. However, while preliminary estimates indicate that Irish forests may, on average, fix 3.36 tonnes of carbon ha yr, many of the figures used to calculate this estimate, particularly that for the so-called biomass expansion factors, need much more refinement before more accurate estimates can be made available. Introduction The increasing concentration of carbon dioxide (CO2) in the atmosphere has intensified interest in research on cycling of carbon (C) at the global level. Atmospheric CO2 concentrations have increased 30% from the pre-industrial level of about 280 parts per million (ppm) to close to 370 ppm today. Forest ecosystems play an important role in the global carbon cycle. Trees remove CO2 from the atmosphere through photosynthesis during growth and development processes. The carbon is stored in wood, bark, needles/leaves and in roots. Much of the carbon passes to the forest floor and forms a litter layer, which is composed of dead foliage and twigs (Figure 1). As the litter layer decomposes part of it passes to the soil as organic matter. Thus, the soil is an important carbon reservoir, and accounts for a large proportion of stored carbon (Figure 2). The standing biomass and the forest products pool, at least under Irish conditions, account for smaller proportions of the carbon pool. The rate of carbon sequestration in forests varies with species and with yield class (YC). In general, carbon storage rates increase rapidly with increasing yield class, but long-term storage levels are not as sensitive to growth rates (Table 1).