EFFECTS OF THE AGE CLASS DISTRIBUTIONS OF THE TEMPERATE AND BOREAL FORESTS ON THE GLOBAL CO2 SOURCE-SINK FUNCTION

The role of the temperate and boreal forests as a global CO2 source or sink is examined, both for the present time and for the next hundred years. The results of the Forest Resource Assessment for 1990 of the E conomic Comission for Europe and the Food and Agricultural Organisatio n of the United Nations (1992) serve as the main database in this stud y. Out of the estimated total area of approximately 20 . 10(6) km(2) o f forests and wooded lands in tile temperate and boreal zone only appr oximately fifty percent is documented within the category of exploitab le Forests, which are examined in detail here. In this study, a genera l formalism of the time evolution of an ensemble of forests within an ecological province is developed using the formalism of the Leslie mat rix. This matrix can be formulated if the age class dependent mortalit ies which arise from the disturbances are known. A distinction is made between the natural disturbances by fire, wind throw and insect infes tations and disturbances introduced through harvesting of timber. Thro ugh the use of Richards growth function each age class of a given biom e is related to the corresponding biomass and annual increment. The da ta reported on the mean net annual increment and on the mean biomass s erve to calibrate the model. The difference of the reported nut annual increment and annual fellings of approximately 550 . 10(6) m(3) round wood correspond to a sink of 210-330 Mt of carbon per year excluding a ny changes in the soil balance. it could be shown that the present dis tribution of forest age classes for the United States, Canada, Europe, or the former Soviet Union does not correspond to a quasi-stationary state, in which biomass is accumulated only due to a stimulated growth under enhanced atmospheric CO2 levels. The present CO, sink function will not persist in the next century, if harvesting rates increase wit h 0.5% annually or even less. The future state will also be influenced by the effect of the greenhouse climate, the impact of which may rang e from a stimulating effect on growth. which is calculated by the Fran kfurt biosphere model, up to a transitional negative effect through a shift in vegetation zones.