Modeling CO2 and water vapor exchange of a temperate broadleaved forest across hourly to decadal time scales

Fluxes of carbon dioxide, water and energy between a temperate deciduous fo rest and the atmosphere were quantified across time scales of hours, days, seasons, years and decades. This exercise was performed using stand-level e ddy covariance flux measurements and a biophysical model, CANOAK. The CANOA K model was tested with measurements of carbon dioxide, water vapor and ene rgy flux densities we have been collecting since October 1994. Model calcul ations reproduced 80% of CO2 and water vapor flux variance that are contain ed in a year-long time series, when the model was forced with hourly weathe r data and seasonal information on plant structure and physiological capaci ty. Spectral analysis of measured and computed time series revealed that pe ak time scales of flux variance have periods of a day, half-week, season an d year. We examined questions relating to inter-annual variability of mass and energy exchange by forcing the validated model with a decade-long meteo rological record. Theoretical estimates of year-to-year variability of net ecosystem CO2 exchange were on the order of 200 gC m(-2) year. We also dedu ced that significant variance of water vapor and CO2 exchange occurs on the time scale of 5-6 years, the time scale associated with El Nino phenomena. Sensitivity tests performed with the model examined issues associated with model complex and parameterization issues. Of particular importance were t he effects of leaf clumping and length of the growing season on canopy phot osynthesis and net ecosystem CO2 exchange. Ignoring the effects of leaf clu mping caused an error as large as 50% in the estimation of annual biosphere -atmosphere net carbon exchange. Each incremental day change in the length of the growing season altered the net ecosystem CO2 exchange by 5.9 gC m(-2 ). (C) 2001 Elsevier Science B.V. All rights reserved.