BOREAL FORESTS AND TUNDRA

The circumpolar boreal biomes cover ca. 2 109 ha of the northern hemisphere and contain ca. 800 Pg C in biomass, detritus, soil, and peat C pools. Current estimates indicate that the biomes are presently a net C sink of 0.54 Pg C yr-1. Biomass, detritus and soil of forest ecosystems (including ca. 419 Pg peat) contain ca. 709 Pg C and sequester an estimated 0.7 Pg C yr-1. Tundra and polar regions store 60-100 Pg C and may recently have become a net source of 0.17 Pg C yr-1. Forest product C pools, including landfill C derived from forest biomass, store less than 3 Pg C but increase by 0.06 Pg C yr-1. The mechanisms responsible for the present boreal forest net sink are believed to be continuing responses to past changes in the environment, notably recovery from the little ice-age, changes in forest disturbance regimes, and in some regions, nutrient inputs from air pollution. Even in the absence of climate change, the C sink strength will likely be reduced and the biome could switch to a C source. The transient response of terrestrial C storage to climate change over the next century will likely be accompanied by large C exchanges with the atmosphere, although the long-term (equilibrium) changes in terrestrial C storage in future vegetation complexes remains uncertain. This transient response results from the interaction of many (often non-linear) processes whose impacts on future C cycles remain poorly quantified. Only a small part of the boreal biome is directly affected by forest management and options for mitigating climate change impacts on C storage are therefore limited but the potential for accelerating the atmospheric C release are high. © 1993 Kluwer Academic Publishers.