PRELIMINARY STUDY OF CO<sub>2</sub> FLUX OF A LARCH FOREST BY EDDYCOVARIANCE AND ECOPHYSIOLOGICAL METHODS

doi:10.17521/cjpe.2007.0015

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<400 μmol·m^-2·s^-1), light use efficiency was much higher than on days with a mean PAR>500 μmol·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 20-50 μmol·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 9.8-23.4 μmol·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 understory photosynthesis in the carbon balance of the studied plantation. Ecosystem respiration estimated by eddy covariance on a windy night was 3-9 μmol·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.

Key words: larch forest, CO₂ flux, energy flux, non-photosynthetic organs, photosynthesis, respiration, eddy covariance method, ecophysiological method of component summation

WANG Wen-Jie, ZU Yuan-Gang, WANG Hui-Min, YANG Feng-Jian, Saigusa Nobuko, Koike Takayoshi, Yamamoto Susumu. PRELIMINARY STUDY OF CO<sub>2</sub> FLUX OF A LARCH FOREST BY EDDYCOVARIANCE AND ECOPHYSIOLOGICAL METHODS[J]. Chin J Plant Ecol, 2007, 31(1): 118-128.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2007.0015

https://www.plant-ecology.com/EN/Y2007/V31/I1/118

Figures/Tables 8

Fig.1 Energy balance at Laoshan flux site using half-hourly data in June 2002

Fig.2 Temporal variation of latent energy (LE), sensible energy (E) CO2 fluxes (NEE) and corresponding environmental variables including air temperature (Ta), photosynthetic active radiation (PAR) and vapor pressure deficit (VPD) in June 2002

Fig.3 Different responses of gross primary productity (GPP) to photosynthetic active radiation (PAR) in different weather condition

Fig.4 Temperature (Ta) dependence of ecosystem respiration rate(RE) at windy night (friction velocity U*>0.20 m·s-1) Part of the data was measured in winter

Fig.5 Light response curves of canopy species of Larix gmelinii, Franxinus mandshurica and Betulla platyphylla in June 2002

Fig.6 Soil-microbe, litter decomposition, and root respiration contribution to total soil respiration

Fig.7 Leaf, branch and stem respiration of three dominant trees in study site

Table 1 Comparison on the GPP and ecosystem respiration of the larch plantation estimated by eddy-covariance method and ecophysiological method at an instant level

碳平衡 Carbon budget	涡度协方差法 Eddy covariance method	生理生态学方法 Ecophysiological method
碳平衡 Carbon budget	涡度协方差法 Eddy covariance method	叶片水平 Leaf level	器官面积指数 Organ area index (m²·m^-2)	生态系统水平 Ecosystem level
总初级生产力Gross primary productivity (GPP)(μmol·m^-2·s^-1)	20~50	林冠 Canopy:4.9~11.7(平均mean:8.1)	2.0	9.8~23.4 (平均mean:16.2)
		草本 Understory herbs: 2.3~12.3	2.2	5.0~25.0
		合计 Sum		14.8~48.4
系统呼吸 Ecosystem respiration (μmol·m^-2·s^-1)	3~9	树叶 Leaves 0.9~2.0(平均mean:1.9)	2.0	1.8~4.0(平均mean:3.8)
		树枝 Branches 0.8~2.2(平均mean:1.1)	0.5	0.4~1.1(平均mean:0.5)
		树干 Stems 4.1~5.6(平均mean:4.3)	0.7	2.9-~3.9(平均mean:3.0)
		根系 Roots 2.4	1.0	2.4
		土壤微生物 Soil microbes 2.8	1.0	2.8
		枯枝落叶 Litters 1.8	1.0	1.8
		合计 Sum		14.2

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