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Carbon dioxide (CO2) storage flux in the air space below measurement height of eddy covariance is very important to correctly evaluate net ecosystem exchange of CO2 (NEE) between forest ecosystem and atmosphere. This study analyzed the dynamic variation of CO2 storage flux and its effects on the carbon budget of a temperate broad-leaved Korean pine mixed forest at Changbai Mountains, based on the eddy covariance flux data and the vertical profile of CO2 concentration data. The CO2 storage flux in this forest ecosystem had typical diurnal variation, with the maximum variation appeared during the transition from stable atmospheric layer to unstable atmospheric layer. The CO2 storage flux calculated by the change in CO2 concentration throughout a vertical profile was not significantly different from that calculated by the change in CO2 concentration at the measurement height of eddy covariance. The NEE of this forest ecosystem was underestimated by 25% and 19% at night and at daytime, respectively, without calculating the CO2 storage flux at half-hour scale, and was underestimated by 10% and 25% at daily scale and annual scale, respectively. Without calculating the CO2 storage flux in this forest ecosystem, the parameters of Michaelis-Menten equation and Lloyd-Taylor equation were underestimated, and the ecosystem apparent quantum yield (alpha) and the ecosystem respiration rate (Rref) at the reference temperature were mostly affected. The gross primary productivity (GPP) and ecosystem respiration (Re) of this forest ecosystem were underestimated about 20% without calculating the CO2 storage flux at half-hour, daily scale, and annual scale.

Abstract: Continuous measurements of carbon dioxide using the eddy covariance (EC) technique were made in 2003 at three ChinaFLUX forest sites, including a temperate mixed forest (CBS), a subtropical evergreen coniferous plantation (QYZ) and a subtropical evergreen broad-leaved forest (DHS) along the North–South Transect of Eastern China (NSTEC). The three forest sites are influenced by Asian monsoon climate to varying degrees. The environmental controls on the seasonal variations of ecosystem apparent quantum yield (α), maximum photosynthesis rate (P max) and ecosystem respiration (R e) were investigated in the three forest ecosystems. At the CBS site, temperature was found to be the dominant factor controlling the seasonal patterns of ecosystem α and P max, while the phenology, which was mainly embodied by the development of LAI, also displayed important influence. The seasonal patterns of α and P max at QYZ were mainly ascribed to vapor pressure deficit (VPD). Ecosystem α decreased with increasing VPD and P max decreased below and above the optimum VPD as a result of the Asian subtropical monsoon climate which resulted in the extremely high temperature and low moisture. At the DHS site, ecosystem α only appeared weak negative correlation with temperature, while there was no obvious dominant factor controlling the seasonal pattern of ecosystem P max. The ecosystem respiration of the three forest ecosystems could be estimated well by temperature with an exponential equation. During the growing season, the ecosystem α, P max and R e at CBS were much stronger than that of QYZ and DHS. The R e at CBS was more sensitivity to temperature than that of QYZ and DHS, so its response to changing climate deserved more attention in the evaluation of the carbon budget of such ecosystem. [Copyright &y& Elsevier]

Abstract: As a weather element, clouds can affect CO2 exchange between terrestrial ecosystems and the atmosphere by altering environmental conditions, such as solar radiation received on the ground surface, temperature, and moisture. Based on the flux data measured at five typical ecosystems of China during mid-growing season (June–August) from 2003 to 2006, we analyzed the responses of net ecosystem exchange of carbon dioxide (NEE), light use efficiency (LUE, defined as Gross ecosystem photosynthesis (GEP)/Photosynthetically active radiation (PAR)), and water use efficiency (WUE, defined as GEP/Evapotranspiration (ET)) to the changes in cloudiness. The five ecological sites included Changbaishan temperate mixed forest (CBS), Dinghushan subtropical evergreen broad-leaved forest (DHS), Xishuangbanna tropical rainforest (XSBN), Inner Mongolia semi-arid Leymus chinensis steppe (NMG), and Haibei alpine frigid Potentilla fruticosa shrub (HB). Our analyses show that cloudy sky conditions with cloud index (k t) values between 0.4 and 0.6 increased NEE, LUE, and WUE of the ecosystems at CBS, DHS, NMG and HB from June to August. The LUE of tropical rainforest at XSBN was higher under cloudy than under clear sky conditions, but NEE and WUE did not decrease significantly under clear sky conditions from June to August. The increase in GEP with increasing diffuse radiation received by ecosystems under cloudy skies was the main reason that caused the increases in LUE and net carbon uptake in forest ecosystem at CBS, DHS, and alpine shrub ecosystem at HB, compared with clear skies. Moreover, for the ecosystem at CBS, DHS, and HB, when sky condition became from clear to cloudy, GEP increased and ET decreased with decreasing VPD, leading to the increase in WUE and NEE under cloudy sky conditions. The decrease in R e with decreasing temperature and increase in GEP with decreasing VPD under cloudy skies led to the increase in LUE, WUE, and net carbon uptake of semi-arid steppe at NMG, compared to clear skies. These different responses among the five ecosystems are attributable to the differences in canopy characteristics and water conditions. From June to August, the peaks of the k t frequency distribution in temperate ecosystems (e.g., CBS, NMG, and HB) were larger than 0.5, but they were smaller than 0.4 in subtropical/tropical forest ecosystems (e.g., DHS and XSBN). These results suggest that the pattern of cloudiness during the years from 2003 to 2006 in the five ecosystems was not the best condition for their net carbon uptake. This study highlights the importance of cloudiness factor in the prediction of net carbon absorption in the Asia monsoon region under climate change. [Copyright &y& Elsevier]

Abstract: Variation in temperature accounts for most of the seasonal fluctuation of terrestrial ecosystem respiration. However, other factors, such as soil moisture, also influence ecosystem respiration. In this study, continuous measurement of carbon dioxide exchange was made over a subtropical Pinus plantation of southeastern China using the eddy covariance (EC) technique. The effect of soil water content on ecosystem respiration and its sensitivity to temperature (Q 10) were examined during the unusual dry summer of 2003. The results indicate that soil water content significantly affected the dynamics of respiration rate and its relationship with temperature in the drought-stressed ecosystem. The effect of soil water content on the Q 10 value of ecosystem respiration is described best by a quadratic function, instead of the commonly used multiplicative model. The regression model analysis revealed that ecosystem respiration was more sensitive to soil water content than is estimated by the multiplicative model. The multiplicative model led to an overestimation of response of the respiration to warming under the dry soil condition. Sensitivity of the ecosystem respiration to temperature was found to vary with air temperature and soil water content. This, to a considerable extent, precludes accurate estimates of the seasonal dynamics of ecosystem respiration. [Copyright &y& Elsevier]