Respiratory carbon losses and the carbon-use efficiency of a northern hardwood forest, 1999-2003

(.)Quantitative assessment of carbon (C) storage by forests requires an understanding of climatic controls over respiratory C loss. Ecosystem respiration can be estimated biometrically as the sum (R-Sigma) of soil (R-s), leaf (R-l) and wood (R-w) respiration, and meteorologically by measuring above-canopy nocturnal CO2 fluxes (F-cn). (.)Here we estimated R-Sigma over 5 yr in a forest in Michigan, USA, and compared R-Sigma and F-cn on turbulent nights. We also evaluated forest carbon-use efficiency (E-c = P-NP/P-GP) using biometric estimates of net primary production (P-NP) and R-Sigma and F-cn-derived estimates of gross primary production (P-GP). (.)Interannual variation in R-Sigma was modest (142 g C m(-2) yr(-1)). Mean annual R-Sigma was 1425 g C m(-2) yr(-1); 71% from R-s, 18% from R-l, and 11% from R-w. Hourly R-Sigma was well correlated with F-cn, but 11 to 58% greater depending on the time of year. Greater R-Sigma compared with F-cn resulted in higher estimated annual P-GP and lower annual E-c (0.42 vs 0.54) using biometric and meteorological data, respectively. (.)Our results provide one of the first multiyear estimates of R-Sigma in a forested ecosystem, and document the responses of component respiratory C losses to major climatic drivers. They also provide the first assessment of E-c in a deciduous forest using independent estimates of P-GP. (c) New Phytologist (2005).