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Ecosystem light use efficiency (LUE) is a critical parameter in estimating CO2 uptake by vegetation from climatological and satellite data. However, the spatiotemporal dynamics and biophysical regulations of ecosystem‐level LUE are not well understood, resulting in large uncertainties in the estimation of gross primary productivity (GPP) using LUE‐based models. In this study, we used eddy covariance to explore spatiotemporal variations and controls of LUE in four contrasting forest ecosystems (savanna, tropical rainforest, subtropical evergreen forest, and subalpine coniferous forest). Based on 27 site years of data, we found that (1) the multiyear mean LUE was 0.063, 0.251, 0.247, and 0.140 g C· mol photon−1 in the four contrasting ecosystems, respectively; (2) the LUE in the wet season (May–October) was higher than that in the dry season in all studied ecosystems; (3) the leaf area index controlled GPP and LUE significantly and explained 74%, 29%, 54%, and 36% of the variation in GPP and 51%, 19%, 41%, and 54% of the variation in LUE in the four contrasting ecosystems, respectively; (4) path analysis revealed the critical roles of GPP and vapor pressure deficit in controlling LUE in these four forest ecosystems; and (5) under warming scenarios, LUE may decrease in savanna but increase in the other three ecosystems, while decreasing precipitation (P) may reduce LUE in the ecosystems studied. This study improves our understanding of the influence of biophysical factors on LUE and demonstrates how LUE changes with variations in temperature, soil moisture, and leaf area index, thereby improving estimations of large‐scale carbon exchange/cycling. Key Points: Evergreen broad‐leaved forest (EBF) ecosystem shows higher LUE than subalpine coniferous/savanna ecosystemsThe vapor pressure deficit (VPD) and leaf area index (LAI) largely determine the variability in LUEWarming might decrease LUE in the savanna ecosystem and may increase LUE in the other three ecosystems [ABSTRACT FROM AUTHOR]

Net ecosystem productivity (NEP) was continuously measured using the eddy covariance (EC) technique from 2003 to 2005 at three forest sites of ChinaFLUX. The forests include Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical coniferous plantation (QYZ), and Dinghushan subtropical evergreen broad-leaved forest (DHS). They span wide ranges of temperature and precipitation and are influenced by the eastern Asian monsoon climate to varying extent. In this study, we estimated ecosystem respiration (RE) and gross ecosystem productivity (GEP). Comparison of ecosystem carbon exchange among the three forests shows that RE was mainly determined by temperature, with the forest at CBS exhibiting the highest temperature sensitivity among the three ecosystems. The RE was highly dependent on GEP across the three forests, and the ratio of RE to GEP decreased along the North–South Transect of Eastern China (NSTEC) (i.e. from the CBS to the DHS), with an average of 0.77 ± 0.06. Daily GEP was mainly influenced by temperature at CBS, whereas photosynthetic photon flux density was the dominant factor affecting the daily GEP at both QYZ and DHS. Temperature mainly determined the pattern of the interannual variations of ecosystem carbon exchange at CBS. However, water availability primarily controlled the interannual variations of ecosystem carbon exchange at QYZ. At DHS, NEP attained the highest values at the beginning of the dry seasons (autumn) rather than the rainy seasons (summer), probably because insufficient radiation and frequent fog during the rainy seasons hindered canopy photosynthesis. All the three forest ecosystems acted as a carbon sink from 2003 to 2005. The annual average values of NEP at CBS, QYZ, and DHS were 259 ± 19, 354 ± 34, and 434 ± 66 g C m−2 yr−1, respectively. The slope of NEP that decreased with increasing latitude along the NSTEC was markedly different from that observed on the forest transect in the European continent. Long-term flux measurements over more forest ecosystems along the NSTEC will further help verify such a difference between the European forest transect and the NSTEC and provide insights into the responses of ecosystem carbon exchange to climate change in China. [ABSTRACT FROM AUTHOR]

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: 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]