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.