A method for deriving net primary productivity and component respiratory fluxes from tower-based eddy covariance data: a case study using a 17-year data record from a Douglas-fir chronosequence
2007
SCHWALM, CHRISTOPHER R. | BLACK, T ANDREW | MORGENSTERN, KAI | HUMPHREYS, ELYN R.
Conventional gap-filling procedures for eddy covariance (EC) data are limited to calculating ecosystem respiration (RE) and gross ecosystem productivity (PG) as well as missing values of net ecosystem productivity (FNEP). We develop additional postprocessing steps that estimate net primary productivity (PN), autotrophic (Ra), and heterotrophic respiration (Rh). This is based on conservation of mass of carbon (C), Monte Carlo (MC) simulation, and three ratios: C use efficiency (CUE, PN to PG), Ra to RE, and FNEP to RE. This procedure, along with the estimation of FNEP, RE, and PG, was applied to a Douglas-fir dominated chronosequence on Vancouver Island, British Columbia, Canada. The EC data set consists of 17 site years from three sites: initiation (HDF00), pole/sapling (HDF88), and near mature (DF49), with stand ages from 1 to 56 years. Analysis focuses on annual C flux totals and C balance ratios as a function of stand age, assuming a rotation age of 56 years. All six C balance terms generally increased with stand age. Average annual PN by stand was 213, 750, and 1261 g C m⁻² yr⁻¹ for HDF00, HDF88, and DF49, respectively. The canopy compensation point, the year when the chronosequence switched from a source to a sink of C, occurred at stand age ca. 20 years. HDF00 and HDF88 were strong and moderate sources (FNEP=-581 and -138 g C m⁻² yr⁻¹), respectively, while DF49 was a moderate sink (FNEP=294 g C m⁻² yr⁻¹) for C. Differences between sites were greater than interannual variation (IAV) within sites and highlighted the importance of age-related effects in C cycling. The validity of the approach is discussed using a sensitivity analysis, a comparison with growth and yield estimates from the same chronosequence, and an intercomparison with other chronosequences.
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