CARBON STORAGE IN FOREST SOIL OF FINLAND .1. EFFECT OF THERMOCLIMATE

A total of 30 coniferous forest sites representing two productivity cl asses, forest types, were investigated on a temperature gradient (effe ctive temperature sum using +5 degrees C threshold 800-1300 degree-day s and annual mean temperature -0.6-+3.9 degrees C) in Finland for stud ying the effect of thermoclimate on the soil C storage. Other soil for ming factors were standardized within the forest types so that the var iation in the soil C density could be related to temperature. Accordin g to the applied regression model, the C density of the 0-1 m mineral soil layer increased 0.266 kg m(-2) for every 100 degree-day increase in the temperature sum, and the layer contained 57% and 28% more C und er the warmest conditions of the gradient compared to the coolest in t he less and more productive forest type, respectively. Accordingly, th is soil layer was estimated to contain 23% more C in a new equilibrium with a 4 degrees C higher annual mean temperature in Finland. The C d ensity of the organic layer was not associated with temperature. Both soil layers contained more C at the sites of the more productive fores t type, and the forest type explained 36% and 70% of the variation in the C density of the organic and 0-1 m layers, respectively. Within th e forest types, the temperature sum accounted for 33-41% of the variat ion in the 0-1 m layer. These results suggest that site productivity i s a cause for the large variation in the soil C density within the bor eal zone, and relating the soil C density to site productivity and tem perature would help to estimate the soil C reserves more accurately in the boreal zone.