Carbon Flux Partitioning in an Old-Growth Forest: Study of Seasonal and Interannual Variation

We investigate the decompostion of eddy flux measurements of net ecosystem exchange (NEE) into gross primary productivity (GPP), ecosystem respiration (R_eco) and the temporal dynamics of component fluxes for 4 « years of data from long-term measurements of carbon fluxes above and within Old-growth Forest at the Wind River Canopy Crane AMERIFLUX site. Trees at the site are up to 500 years old and 65 meters tall. The forest structure at the site is complex for a temperate conifer stand with seven gymnosperm and two angiosperm tree species in the 2.3 ha crane circle, large standing biomass and large amounts of woody debris on the forest floor. Soil respiration is a major contributor to the carbon budget at the site with roughly 75% of total respiration on average. The annual estimates of net ecosystem exchange (NEE) of carbon range from a strong sink (2.1 tC/ha per year) to a source (-0.5 tC/ha per year). Summers are usually warm and dry (1998, 2001) but relatively wet and cool ones have been observed (1999). Precipitation levels throughout the observation period varied from 1600 to 2600 mm with the latter close to the 25-year average. The main period of maximum carbon uptake is limited to the months March through May when ecosystem respiration and water stress are low. Stand-level light response functions show optima for low temperatures and diffuse light conditions. R_eco also shows a clear seasonal pattern but lags significantly behind NEE with a maximum in late summer and has a range of 13 - 16 tC/ha per year. GPP shows a similar lag against NEE but is relatively invariant on an annual basis (16 tC/ha).