Abstract:

Abstract Aims The two main methods for estimating CO₂ flux from forests are the eddy covariance micrometeorological method and the ecophysiological component summation method. Eddy covariance is a standard method for long-term, direct measurement of forest CO₂ and is used in studying large-scale terrestrial carbon budgets, while the ecophysiological method can estimate each component (e.g., stem , leaves, branches, roots, as well as soil microbes) of total CO₂ flux of forests. Because forest CO₂ flux study, including eddy covariance measurement, is a recent development in China, it is important to compare results from these two methods for understanding scaling-up of forest carbon budgets. We did a preliminary comparison during a typical month of the strongest sink capacity (June 2002). Our aim was to determine how the methods differed in carbon budget estimation and evaluate implications for future research.
Methods A micrometeorological tower with the eddy covariance system was used to directly estimate net ecosystem exchange of a larch (Larix gmelinii) plantation at Laoshan station (45°20′N, 127°34′E). Ecophysiological measurements by a Li_ 6400 system were used to measure leaf photosynthesis and respiration of the tree canopy and herbaceous understory, stem respiration, branch respiration and soil respiration. Root respiration, soil microbe respiration and litter respiration were measured by the pre-installed trenched box and litter exclusion method. We converted each photosynthesis and respiration value from an organ-area base to a soil-area base using leaf area index measured by LAI_2000 and stem area index and branch area index estimated by standard tree sampling.
Important findings Energy balance was estimated to be 75% using half-hourly flux data, but improved when 5 days of accumulated data were used, indicating that the eddy covariance method is suitable for this site. In relative cloudy weather (mean photosynthetic active radiation, PAR<400μmol•m^-2•s^-1) , light use efficiency was much higher than on days with a mean PAR>500μmol•m^-2•s^-1. This may be related to diffuse light on cloudy days. Expressed on a soil area base, gross primary productivity (GPP) of the larch plantation was20-50μmol•m^-2•s^-1 estimated by the eddy covariance method. This value was
much higher than the total photosynthetic capacity of dominant canopy leaves of 9.8-23.4μmol•m^-2•s^-1 (mean of 16.2　μmol•m^-2•s^-1); however, it was equivalent to the summation of dominant canopy and understory photosynthesis, indicating the critical importance of under story photosynthesis in the carbon balance of the studied plantation. Ecosystem respiration estimated by eddy covariance on a windy night was 3-9μmol•m^-2•s^-1, which is about 50% lower than estimated by the ecophysiological method (14.2　μmol•m^-2•s^-1). This large discrepancy between the two methods would lead to a large difference in carbon sink estimation. Therefore, methods of estimating respiration need additional study.