LITTER MASS-LOSS RATES IN LATE STAGES OF DECOMPOSITION IN A CLIMATIC TRANSECT OF PINE FORESTS - LONG-TERM DECOMPOSITION IN A SCOTS PINE FOREST .9.

We investigated rate-regulating factors for decomposition rates of Sco ts pine needle litter at 22 sites over a 2000-km long transect ranging from the Arctic Circle in Scandinavia to northern continental Europe. We found very different patterns for rate-regulating factors in the e arly stages of decomposition as compared to later stages (>20% accumul ated mass loss). The initial decomposition rates (measured over the 1s t year) ranged from about 10.9%/year close to the Arctic Circle to abo ut 43.7%/year in south Sweden. The dominant rate-regulating factor was climate (average annual temperature, and actual evapotranspiration), and none of the substrate-quality factors was significant. In the late r stages, the annual mass loss varied from 2.2%/year to 41.5%/year. Th e rate-regulating factors were climate and the litter's concentration of lignin. We found that the effect of lignin concentration on litter mass-loss rate varied with site and this relative effect was negativel y related with actual evapotranspiration. The effect of lignin concent ration on mass-loss rates near the Arctic Circle was thus low (at low values for actual evapotranspiration) whereas in Southern Sweden and o n the continent the rate-regulating effect of lignin was higher.