Tang, J., P.V. Bolstad, A.R. Desai, J.G. Martin, B.D. Cook, K.J. Davis, and E.V. Carey, accepted. Ecosystem respiration and its components in an old-growth northern forest, Agricultural and Forest Meteorology.

Ecosystem respiration and its components are sensitive to age, species, stand structure, and environmental conditions, and substantially influence net ecosystem productivity. We measured ecosystem respiration and component respirations including soil, woody debris, stem and leaf respiration in old-growth hardwood-dominated and hemlock-dominated stands in northern Michigan, USA in 2002 and 2003. Respiration was mainly controlled by temperature, peaked in July-August and reached minimums in January-March. Total ecosystem respiration averaged 1013 g C m^-2 y^-1 in the hardwood stand and 922 g C m^-2 y^-1 in the hemlock stand. Cumulative annual soil respiration, coarse woody debris respiration, stem respiration, and leaf respiration were 724, 43, 131, 115 g C m^-2 y^-1, respectively, accounting for 72%, 4%, 13%, and 11% of total ecosystem respiration in the hardwood stand, and 614, 29, 207, 72 g C m^-2 y^-1, respectively, accounting for 67%, 3%, 22%, and 8% in the hemlock stand. Ecosystem respiration and its components except for leaf respiration in 2002 were larger than year 2003 due primarily to lower temperature in 2003. Component respirations except for stem respiration were higher in the hardwood stand than the hemlock stand. Daily mean ecosystem respiration upscaled from chamber measurements agreed well with eddy covariance measurements, with r² of 0.96. By comparing respiration from the old-growth with a nearby young and a mature second-growth forest based on chamber measurements, we found that both age class and species are important in determining the magnitude and proportion of component respiration. Total ecosystem respiration generally increased from the young forest to the mature forest, and then decreased from the mature to the old-growth forest.