Dynamics of carbon stocks in soils and detritus across chronosequences of different forest types in the Pacific Northwest, USA

We investigated variation in carbon stock in soils and detritus (forest floor and woody debris) in chronosequences that represent the range of forest types in the US Pacific Northwest. Stands range in age from <13 to >600 years. Soil carbon, to a depth of 100 cm, was highest in coastal Sitka spruce/western hemlock forests (36+/-10 kg C m(-2)) and lowest in semiarid ponderosa pine forests (7+/-10 kg C m(-2)). Forests distributed across the Cascade Mountains had intermediate values between 10 and 25 kg C m(-2). Soil carbon stocks were best described as a linear function of net primary productivity (r(2)=0.52), annual precipitation (r(2)=0.51), and a power function of forest floor mean residence time (r(2)=0.67). The highest rates of soil and detritus carbon turnover were recorded on mesic sites of Douglas-fir/western hemlock forests in the Cascade Mountains with lower rates in wetter and drier habitats, similar to the pattern of site productivity. The relative contribution of soil and detritus carbon to total ecosystem carbon decreased as a negative exponential function of stand age to a value of similar to35% between 150 and 200 years across the forest types. These age-dependent trends in the portioning of carbon between biomass and necromass were not different among forest types. Model estimates of soil carbon storage based on decomposition of legacy carbon and carbon accumulation following stand-replacing disturbance showed that soil carbon storage reached an asymptote between 150 and 200 years, which has significant implications to modeling carbon dynamics of the temperate coniferous forests following a stand-replacing disturbance.