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Highlights • Gross primary productivity was more depressed by water stress than ecosystem respiration. • Water use efficiency increased under moderate and severe water-stressed conditions. • Cloudy sky conditions were beneficial to water use efficiency. Abstract Water stress can cause the structural and physiological changes of plants to prevent excessive water loss according to species-specific water use strategies. CO 2 and water vapor fluxes were measured by the eddy covariance in a mixed plantation during a 4–year period. The potential evapotranspiration (PET) was obtained by the Shuttleworth–Wallace model. Evapotranspiration (ET) versus PET was used to determine the plant water stress index (PWSI). The objective of this study is to quantify the effect of PWSI on net ecosystem productivity (NEP), gross primary productivity (GPP) and water use efficiency (WUE). The relationship between PWSI and soil water content in sunny and cloudy skies was examined. Mean seasonal PWSI values varied substantially among years. The average annual value of PWSI was 0.49 ± 0.07. PWSI was strongly correlated with vapor pressure deficit, illustrating that the moisture limitation through high evaporative demand was more remarkable than water supply limitation. Water stress can substantially reduce canopy conductance and leaf area index, and hence decreasing photosynthesis. Water stress suppressed GPP while having little influence on ecosystem respiration, resulting in the increase of net carbon uptake. NEP decreased as a reaction to water stress when PWSI was more than 0.4, whereas WUE was enhanced under moderate and severe water stress (PWSI > 0.6). Compared to NEP, the difference of ET between cloudy and sunny sky conditions was more evident at the same PWSI value. Therefore, cloudy sky conditions were beneficial to enhancing WUE. Water stress restricted the mixed plantation in the period of spring-early summer. At the annual scale, the mixed plantation was exposed to moderate water stress. The PWSI approach is a promising tool quantifying water stress in forest ecosystems. [ABSTRACT FROM AUTHOR]

Croplands play a vital role in regional carbon budgets. We hypothesized that biophysical factors would be important for soil respiration in a wheat-maize rotation cropping system. Soil CO 2 efflux was measured using the closed chamber method, and net CO 2 exchange between the cropland and the atmosphere obtained by the eddy covariance technique in a winter wheat-summer maize double cropping system over four years (Oct 2002–Oct 2006). In addition to soil temperature, soil respiration was controlled by leaf area index and soil moisture in the wheat field and soil moisture in the maize field. Temperature sensitivity ( Q 10 ) of soil respiration was 2.2 in the wheat and maize growing seasons. In the wheat field, the Q 10 value during the sowing–returning green period (4.9) was more than that during the returning green–ripening period (2.0). On a monthly time scale, soil respiration was controlled by gross primary productivity in the wheat field, indicating that soil respiration was coupled with ecosystem photosynthesis. Annual soil respiration was 825 ± 73g C m −2 in the wheat–maize rotation system in 2003–2006. Over a 4–year average, soil respiration was 355 ± 50 g C m −2 in the wheat growing season and 470 ± 67 g C m −2 in the maize growing season, which accounted for 43% and 57% of the annual value respectively. At an annual time scale, soil respiration contributed to 72% of ecosystem respiration in the winter wheat–summer maize double cropping system. [ABSTRACT FROM AUTHOR]

Accurate characterization of plant phenology is of great importance for monitoring global carbon, water, and energy cycling. Remotely sensed satellite observations have been widely used to estimate land surface phenology across multiple spatial scales in the last three decades. Recent development on satellite solar-induced chlorophyll fluorescence (SIF) observations have opened an opportunity to monitor the seasonality of plant growth from the perspective of photosynthesis phenology. The SIF observations from the TROPOspheric Monitoring Instrument (TROPOMI) with high spatial resolution (up to 7 km × 3.5 km pixels) and near-daily global coverage provide unprecedented opportunity to observe photosynthetic and land surface phenology from space. However, the performance of TROPOMI SIF-derived phenology has not been systematically evaluated. In this study, we used flux tower gross primary productivity (GPP) and PhenoCam green chromatic coordinate (Gcc) data as the benchmark to verify phenology metrics derived from satellite observations. The phenology metrics including the start (SOS), end (EOS), length of growing season (LOS), and the peak of growing season (POS) were estimated from TROPOMI SIF, normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), near‐infrared reflectance of vegetation (NIRv), and Global Vegetation Phenology product (MCD12Q2), and the latter four were obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) across six vegetation types over North America during the period of 2018–2020. We found that the overall agreements of SIF (R2 ranging from 0.30 to 0.63) against GPP-estimated phenology were stronger than NDVI (0.17–0.41), EVI (0.19–0.39), NIRv (0.23–0.62), and MCD12Q2 (0.19–0.48) derived phenological events. In reference to GPP and Gcc-estimated phenology, SIF also generally has the least error and bias compared to other satellite remote sensing-derived phenology metrics. No significant differences were found between SIF and GPP-derived phenology (P>0.05, two-tailed t -test). In addition, we found that the spatial distribution of SIF-derived phenology reflected the expected latitudinal patterns in phenology dates. SOS, EOS, LOS, and POS observed by MCD12Q2 appeared to be earlier, later, longer, and earlier than TROPOMI SIF-derived phenology, respectively. SIF-based phenological transition dates more closely tracked GPP-based phenology dates, indicating TROPOMI SIF could be a great measure to track photosynthesis seasonality and land surface phenology. [ABSTRACT FROM AUTHOR]

Highlights: [•] The seasonal and interannual variations of ecosystem WUE were analyzed. [•] The impacts of climate variables on WUE were discussed. [•] Cloudiness can improve photosynthesis and enhance WUE. [•] The major driver of interannual variability in WUE was soil water content in May. [Copyright &y& Elsevier]

Abstract: In recent decades, forest area in China increased rapidly by afforestation and reforestation, especially in its temperate parts. However, lack of information on carbon exchange in temperate plantations in China reduced the accuracy of estimation on regional carbon budget. In this study, CO2 flux was measured using the eddy covariance method over a broadleaf dominant mixed plantation in the lithoid hilly area of the North China. The results showed that annual maximum photosynthetic capacity (A max) varied from 0.81 to 1.22 mg CO2 m−2 s−1 and annual initial light use efficiency (α) from 0.014 to 0.026. Net CO2 uptake was depressed when vapor pressure deficit (VPD) was more than 2.5 kPa. Annual temperature sensitivity coefficient (Q 10) for ecosystem respiration, ranged from 1.84 to 2.35, was negatively correlated with base ecosystem respiration (R 0) (P < 0.05). Annual R 0 decreased but Q 10 increased evidently when winter drought occurred. From 2006 to 2010, annual net ecosystem carbon exchange (NEE), Gross primary productivity (GPP) and ecosystem respiration (R ec) were −355 ± 34, 1196 ± 21 and 841 ± 43 g C m−2 yr−1, respectively. The warm-temperate mixed plantation in the lithoid hilly area of the North China was a strong carbon sink of the atmosphere, which was usually weaken when spring drought happened. [Copyright &y& Elsevier]