Box 23F Forest as a carbon sink?

Recently, the issue of climate change has started a debate about the possible role of forests for carbon dioxide sequestration (MacKenzie, 1994). When they are burned fossil fuels produce CO2; part of the CO2 is absorbed by the oceans but most remains in the atmosphere having the potential to influence world climate. However, the increase in CO2 in the atmosphere is far less marked than expected. One reason for this could be due to northern forests (boreal and temperate) acting as a sink for the carbon by way of afforestation and atmospheric pollution, absorbing CO2 and turning it into wood (IPCC, 1992). Indeed, because of the increase in atmospheric CO2 and fertilising nitrogen deposited by pollutants such as nitrogen oxides, it has been observed that some trees are growing faster, thus absorbing more carbon (Kauppi and Tomppo, in press). In the next few decades, consequent changes in the structure of the forest ecosystems and in their water and nutrient requirements are likely to influence both the interaction of the forest with the climate and the sensitivity to pollutant stress.

European forests contain about 2.8 Gt C in trees and litter, plus 3 to 4 Gt C in forest soils; to give a comparison, 2.8 Gt C is the amount of carbon emitted from fossil fuels in the EU in about 4 years (Cannell et al, 1992). This carbon represents only a small fraction of the total carbon in world temperate forests (200 to 250 Gt C). Using forests to sequester carbon dioxide from the atmosphere has been discussed for some time. To store most carbon, most rapidly, short rotation, fast-growing trees should be planted with wood products that last longer than one rotation, and in soils with initially small organic carbon contents. However, such measures cannot be used as the main strategy to reduce CO2 atmospheric levels, since the flux through forests is simply too small. One estimate is that forests currently sequester around 85 to 120 Mtonnes of carbon each year (Kauppi et al, 1992), that is about 5 per cent of emissions. At such rates, this would imply vast plantations of monoculture of fast-growing trees, causing important impacts on the environment (landscape degradation, loss of biodiversity, etc). Furthermore, what is acting as a sink now may become a source later. Before 1890, boreal forests were a source of CO2, mainly because of forest fires and tree felling (deforestation). After 1920, a steep increase in tree growth outstripped carbon losses, turning the boreal forest into a carbon sink, and storing CO2 from fossil fuel burning. If forest loss and tree damage degradation are not controlled, instead of forming a continuous and possibly increasing carbon storage, these forests might begin to revert to sources of carbon.