Ecosystem Water Use Efficiency Research Articles

CO2 enrichment accelerates alpine plant growth via increasing water-use efficiency

Phenological changes in global vegetation are often attributed to climate warming. However, climate warming and elevated atmospheric CO2 concentration (eCO2) are two co-occurring global change factors, and how eCO2 would affect vegetation phenology has received less attention. The partial pressure of atmospheric CO2 on the Tibetan Plateau (TP) is lower than that in regions of lower altitudes. Consequently, the growth and phenology of alpine plants in this region could be more sensitive to eCO2, but this hypothesis is not yet supported by empirical evidence. Here we explored the effect of eCO2 on plant phenology (including phenophases of green-up, budding, and flowering) through a 5-year field manipulation experiment in a high-altitude (4600 m above sea level) alpine grassland on the TP. Our results showed that eCO2 significantly advanced the spring phenology of an early-flowering species (Kobresia pygmaea), while it had no impact on the phenology of two mid-flowering species (Potentilla saundersiana and Potentilla cuneata). Compared to other low-altitude regions, plant phenology on the TP underwent greater alterations under eCO2, which supports our hypothesis that the growth of high-altitude plants is more sensitive to eCO2. Furthermore, we found that eCO2 significantly reduced the overlapping of flowering between contrasting plant species, mainly due to the phenological advancement of the K. pygmaea induced by eCO2. The observed advancement of the spring phenology in K. pygmaea under eCO2 was associated with increasing ecosystem water-use efficiency (WUE), thereby advancing its subsequent phenological development, such as budding and flowering. Our findings provide experimental evidence that atmospheric CO2 enrichment can accelerate plant growth processes in high-altitude regions, and suggest that large-scale model simulations should consider the effects of elevated atmospheric CO2 concentration on plant growth and phenology.

Modeling the Ecosystem Water Use Efficiency of Croplands Using Machine Learning and MODIS Data

The ecosystem water use efficiency (WUE), which is the ratio of carbon gain (gross primary productivity, GPP) to water consumption (evapotranspiration, ET), is widely regarded as a crucial link in coupling the carbon and water cycles of terrestrial ecosystems on a global scale. The estimation of cropland WUE is considerably important for improving agricultural management, simulating the carbon–water cycle processes of croplands, and enhancing crop yields. Currently, the estimate of WUE primary relies on the modeled GPP and ET data. However, differences in model structures and parameter uncertainties cause remarkable differences in the accuracy of the estimated WUE among different models. The eddy covariance technology enables the continuous observation and research of carbon and water fluxes at a site scale, ensuring simulation accuracy. However, the technology is constrained by its limited spatial coverage. In this study, we integrated data from 20 global cropland sites and built 7 machine learning models based on remote sensing to directly estimate cropland WUE, thereby expanding the spatial scope of WUE simulations, omitting the simulation process for obtaining GPP and ET, and ensuring a certain degree of precision. Results show that, compared with other machine learning algorithms, the ensemble learning model has the strongest ability to reproduce cropland WUE, with a determination coefficient of 0.90 and a root mean square error of 1.54–1.75 . These results indicate that a combination of machine learning methods and remote sensing can reasonably simulate the WUE of agricultural ecosystems.

Comparison of carbon and water fluxes and the drivers of ecosystem water use efficiency in a temperate rainforest and a peatland in southern South America

Abstract. The variability and drivers of carbon and water fluxes and their relationship to ecosystem water use efficiency (WUE) in natural ecosystems of southern South America are still poorly understood. For 8 years (2015–2022), we measured carbon dioxide net ecosystem exchange (NEE) and evapotranspiration (ET) using eddy covariance towers in a temperate rainforest and a peatland in southern Chile. NEE was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco), while ET was partitioned into evaporation (E) and transpiration (T) and used to estimate different expressions of ecosystem WUE. We then used the correlation between detrended time series and structural equation modelling to identify the main environmental drivers of WUE, GPP, ET, E and T. The results showed that the forest was a consistent carbon sink (−486 ± 23 g C m−2 yr−1), while the peatland was, on average, a small source (33 ± 21 g C m−2 yr−1). WUE is low in both ecosystems and likely explained by the high annual precipitation in this region (∼ 2100 mm). Only expressions of WUE that included atmospheric water demand showed seasonal variation. Variations in WUE were related more to changes in ET than to changes in GPP, while T remained relatively stable, accounting for around 47 % of ET for most of the study period. For both ecosystems, E increased with higher global radiation and higher surface conductance and when the water table was closer to the surface. Higher values for E were also found with increased wind speeds in the forest and higher air temperatures in the peatland. The absence of a close relationship between ET and GPP is likely related to the dominance of plant species that either do not have stomata (i.e. mosses in the peatland or epiphytes in the forest) or have poor stomatal control (i.e. anisohydric tree species in the forest). The observed increase in potential ET in the last 2 decades and the projected drought in this region suggests that WUE could increase in these ecosystems, particularly in the forest, where stomatal control may be more significant.

Response of ecosystem water-use efficiency to global vegetation greening

Global vegetation has been greening during the past two decades. However, relatively less is known concerning the change of carbon–water cycle coupling in response to vegetation greening. A key indicator that can characterize this coupling is water-use efficiency (WUE) of ecosystems. Therefore, we explored the impact of global vegetation greening on ecosystem WUE using Theil-Sen trend analysis, Mann-Kendall (M−K) test, stepwise regression and Lindeman Merenda Gold model (LMG) model based on the remote sensing data from 1982 to 2018. Our results showed that global vegetation greening had a positive effect on the gross primary productivity (GPP), evapotranspiration (ET) and WUE during the past thirty years. The WUE in 70 % of these vegetation greening areas, including India, China, Russia, Japan, North Korea, Europe and North America, was increased, and the WUE was increased by 0.002 g C kg H2O yr−1, which was twice as much as that of the global vegetation cover areas. Results of stepwise regression analysis showed that vegetation greening contributed 80 % of the changes in WUE, much higher than the contribution of the climate factors. GPP contributes approximately 81 % to the changes in WUE as a result of vegetation greening. In addition, we also found that, in the greening areas around the world, leaf area index (LAI) explained 80 % of the changes in WUE, and that vegetation has a positive effect WUE. Therefore, the WUE should be considered in the design of ecological restoration programs in the future, and vegetation of low water consumption would be favored to realize sustainable ecosystem services and regional development.

Earthworms do not increase greenhouse gas emissions (CO2 and N2O) in an ecotron experiment simulating a three-crop rotation system

Earthworms are known to stimulate soil greenhouse gas (GHG) emissions, but the majority of previous studies have used simplified model systems or lacked continuous high-frequency measurements. To address this, we conducted a 2-year study using large lysimeters (5 m2 area and 1.5 m soil depth) in an ecotron facility, continuously measuring ecosystem-level CO2, N2O, and H2O fluxes. We investigated the impact of endogeic and anecic earthworms on GHG emissions and ecosystem water use efficiency (WUE) in a simulated agricultural setting. Although we observed transient stimulations of carbon fluxes in the presence of earthworms, cumulative fluxes over the study indicated no significant increase in CO2 emissions. Endogeic earthworms reduced N2O emissions during the wheat culture (− 44.6%), but this effect was not sustained throughout the experiment. No consistent effects on ecosystem evapotranspiration or WUE were found. Our study suggests that earthworms do not significantly contribute to GHG emissions over a two-year period in experimental conditions that mimic an agricultural setting. These findings highlight the need for realistic experiments and continuous GHG measurements.

High spatial variability in water use efficiency of terrestrial ecosystems throughout China is predominated by biological factors

Water use efficiency (WUE) in terrestrial ecosystems is a crucial indicator of regional resource use efficiency and the water-carbon balance relationship. However, spatial variability casts uncertainties in evaluating the water-carbon coupling, leaving the spatial patterns and underlying driving mechanisms of WUE obscure. To address this issue, our study conducted a comprehensive analysis of China's WUE from 2003 to 2020, utilizing 83 observed ecosystem fluxes and micrometeorological data from ChinaFLUX. We discovered the multi-year measured average WUE value was 2.13 ± 0.88 g C/kg H2O, with forest and cropland ecosystems demonstrating superior WUE (2.59 and 2.58 g C/kg H2O, respectively) compared to other ecosystem types. China's WUE exhibited marked spatial heterogeneity, characterized by geographic and climatic gradients, mainly decreasing from southeast towards northwest. Intriguingly, South China potentially exhibits maximal WUE theoretically. Yet, we observed an “oversaturation” effect in its WUE concerning gross primary productivity (GPP), peaking when GPP surpassed approximately 2200 g C m−2 yr−1. A clear dividing line between high- and low- WUE regions in China was observed at 400–500 mm isohyets. Our structural equation model, based on the "geography-climate-biology-flux coupling-WUE" cascade relationship, explained 93 % of WUE spatial variability. Here, GPP emerges as the primary direct factor of WUE variation, with climate conditions primarily affecting WUE through GPP. Our study provides valuable insights into the regional pattern of resource use efficiency and enriches the understanding of the integrated water-carbon cycle and carbon sinks.

Observed divergence in the trends of temperature controls on Chinese ecosystem water use efficiency

How have plants addressed the trade-off between carbon gain and water loss in this warming world? Ecosystem water-use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is a key indicator of the carbon–water relationship. WUE is expected to change due to climate change, yet the extent to which GPP or ET affects WUE changes remains unclear. Moreover, the potential time-varying variations in the WUE responses to recent warming have been overlooked. In this study, we assessed the relative contributions of GPP and ET to WUE using variance decomposition and investigated trends in temperature controls on WUE using moving windows and partial correlation analysis. Our results include: (1) the national multi-year average WUE in China increased significantly at a rate of 0.0174 gC·kg-1H2O·a-1, with 86.88% of the study area exhibiting increased trends. Forest ecosystems, except for ENF, had relatively higher WUE, and the highest value was observed in the deciduous broad-leaf forest (3.77 gC·kg-1H2O), while grassland ecosystems had the lowest WUE of only 1.05 gC·kg-1H2O. (2) GPP, rather than ET, primary drove WUE variations in most areas, except in Northeast China and Southwest China. GPP always contributed more to WUE than ET in each land cover type. In forests, ET contributed more to WUE than in other land cover types. (3) WUE exhibited significantly positive correlations with temperature and radiation, with a positive correlation with temperature in 63.73% (14.83% significant in level of p < 0.05 and the same as below) of the study area and positive correlation with radiation in 74.12% (20.71% significant) of the study area. The control of precipitation on the WUE was complex, with a positive correlation with precipitation in 54.29% of the study area (9.38% significant). (4) The correlation coefficients with temperature exhibited an increasing trend in 52.26% of the study areas and a decreasing trend in 47.74% of the study areas, with a notable divergence in the spatial distribution of the trend of the temperature controls on WUE. Warming is projected to enhance ecosystem functioning in northern regions but may have adverse effects in the south. These findings shed light on the dynamic response of ecosystem functioning to ongoing warming, and would improve our understanding of the terrestrial carbon–water cycle.

Spatial variations in water use efficiency across global terrestrial ecosystems

Ecosystem water use efficiency (WUE) is an important property for understanding the coupling between ecosystem carbon and water cycles and for constraining land models, whereas the mechanism of its spatial variations remains unclear. Here we proposed a framework to decompose ecosystem WUE into component processes, with which the physiological and non-physiological processes controlling ecosystem WUE were included. Further, with the data of measurements by eddy covariance at global Fluxnet sites, we investigated the spatial patterns and determinants of ecosystem WUE with the framework proposed. Our results showed that crop exhibited the highest value of WUE, followed by forest, grassland, wetland, and shrub. The cross-site difference in WUE was mainly determined by that of gross primary productivity (GPP) and the physiological component, i.e., the ratio of GPP to plant transpiration. Spatially, ecosystem WUE showed an overall pattern of increase with air temperature, peaking at ca. 10 ℃ and decreased thereafter. Mean annual precipitation was positively correlated with WUE when < 1000 mm. Moreover, leaf area index (LAI) and the ratio of ambient to intercellular CO2 concentration at canopy level (Ci/Ca) were the two key factors dominating the spatial ecosystem WUE patterns. LAI affected ecosystem WUE mainly through biophysically controlling the fraction of soil water evaporation. Our findings highlight the importance of LAI and canopy level Ci/Ca on controlling the spatial variations in ecosystem WUE, which warrants more attentions and investigations in ecosystem models.

Threshold response of ecosystem water use efficiency to soil water in an alpine meadow

Ecosystem water use efficiency (WUE) is a coupled index of carbon (gross ecosystem productivity, GEP) and water fluxes (transpiration, Tr or evapotranspiration, ET), reflecting how ecosystem uses water efficiently to increase its carbon uptake. Though ecosystem WUE is generally considered to decrease with increasing precipitation levels, it remains elusive whether and how it nonlinearly responds to extreme water changes. Here, we performed a 5-year precipitation halving experiment in an alpine meadow, combined with extremely interannual precipitation fluctuations, to create a large range of soil water variations. Our results showed that WUETr and WUEET consistently showed a quadratic pattern in response to soil water. Such quadratic patterns were steadily held at different stages of growing seasons, with minor changes in the optimal water thresholds (25.0–28.4 %). Below the water threshold, more soil water stimulated GEP but reduced Tr and ET by lowering soil temperature, resulting in a positive response of ecosystem WUE to soil water. Above the threshold, soil water stimulated GEP less than Tr (ET), leading to a negative response of ecosystem WUE to soil water. However, biological processes, including plant cover and belowground biomass as well as vertical root biomass distribution, had less effect on ecosystem WUE. Overall, this work is among the first to reveal the nonlinearity and optimal water thresholds of ecosystem WUE across a broad range of soil water, suggesting that future extreme precipitation events will more frequently surpass the water threshold and differently change the coupling relationships of carbon and water fluxes in alpine grasslands.

Spatial and Temporal Variation in Water Use Efficiency and Ecosystem Photosynthetic Efficiency in Central Asia

Ecosystem water use efficiency (WUE) and ecosystem photosynthetic efficiency (EPE) are key indicators in studies of the carbon–water cycle in terrestrial ecosystems. Analyses of WUE and EPE can enhance our understanding of the relationship between ecosystem light use efficiency and WUE. Although several studies of individual indexes (i.e., either WUE or EPE) have been conducted, analyses of variation in both WUE and EPE, as well as their relationships, have rarely been conducted. Here, we analyzed spatial and temporal variation in WUE and EPE in Central Asia. Specifically, time trend analysis was conducted to characterize temporal and spatial changes in WUE and EPE in Central Asia from 2001 to 2020 at different altitudes and latitudes. Pearson correlation analysis was used to characterize the effects of precipitation and temperature on WUE and EPE. WUE decreased and EPE increased in Central Asia over the 20-year study period; this might have been due to interannual variations in precipitation and temperature. WUE was highest in August, and EPE was highest in June and July. Substantial spatial heterogeneity in WUE and EPE was observed; WUE was highly variable in Central Asia as well as in western and southern Central Asia. Major changes in EPE were observed in northern, eastern, and southern Central Asia. We also found that both WUE and EPE decreased with the increase in altitude. WUE was positively correlated with temperature and negatively correlated with precipitation, whereas EPE was positively correlated with both temperature and precipitation. The increase in photosynthetic efficiency might be one of the main factors contributing to increases in ecosystem productivity in arid environments. The temporal and spatial variation in WUE and EPE observed in our study will aid ecosystem research, providing a reliable theoretical basis for ecosystem research in areas with scarce large-scale data, integrated water resources management, and ecosystem restoration efforts. Our findings also enhance our understanding of the terrestrial carbon–water cycle and have implications for predicting ecosystem responses to climate change. The results of this study provide insights that will aid studies of the terrestrial carbon–water cycle under the background of climate change. It is of great significance to further study the carbon water cycle in the future.

Divergent impacts of VPD and SWC on ecosystem carbon-water coupling under different dryness conditions

Ecosystem water use efficiency (WUE) is an indicator of carbon-water interactions and is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). However, it is currently unclear how WUE responds to atmospheric and soil drought events in terrestrial ecosystems with different dryness conditions. Additionally, the contributions of GPP and ET to the WUE response remain poorly understood. Based on measurements from 26 flux tower sites distributed worldwide, the binning method and random forest model were employed to separate the sensitivities of daily ecosystem WUE, GPP, and ET to vapor pressure deficit (VPD) and soil water content (SWC) under different dryness conditions (dryness index = potential evapotranspiration/precipitation, DI). Results showed that the sensitivity of WUE to VPD was negative at humid sites (DI < 1), while the sensitivity of WUE to SWC was positive at arid sites (DI > 2). Furthermore, the contribution of GPP to VPD-induced WUE variability was 63 % at humid sites, and the contribution of ET to SWC-induced WUE variability was 68 % when SWC was less than the 60th percentile at arid sites. Consequently, one increasing VPD-induced decrease in GPP was generally linked to a decrease in WUE at humid sites, and one drying soil moisture-caused decrease in ET was linked to a WUE increase under low SWC conditions at arid sites. Finally, VPD had a stronger effect on WUE than SWC when VPD was less than the 90th percentile or SWC was greater than the 50th percentile. Our findings underscore the importance of considering ecosystem dryness when investigating the impacts of VPD and SWC on ecosystem carbon-water coupling.

Recent change in ecosystem water use efficiency in China mainly dominated by vegetation greening and increased CO2

Over the last two decades, the significant vegetation increase caused by the execution of several ecological restoration programs has modified the integrated carbon and water cycles. However, the underlying mechanisms of ecosystem water-use efficiency (EWUE) change remain unclear. The current study estimates the annual and seasonal variations in the spatiotemporal patterns of EWUE from 2000 to 2018 and quantifies its driving factors to distinguish the impacts of vegetation and environmental factors across China. The contribution of driving factors to EWUE dynamics is quantified using PML_V2 gross primary productivity (GPP) and evapotranspiration (ET) products by employing the partial derivative (PD) equation. The results reveal that the leaf area index (LAI) is the major contributor in altering the EWUE of China, followed by CO2. The mean seasonal contribution of LAI is dominated by summer (0.0044 gC mm−1H2O yr−1) and spring (0.0035 gC mm−1H2O yr−1), followed by winter (0.0029 gC mm−1H2O yr−1) and autumn (0.0013 gC mm−1H2O yr−1), while the annual contribution is calculated to be 0.0017 gC mm−1H2O yr−1. The rate of CO2 contribution to EWUE was highest in spring (0.0022 gC mm−1H2O yr−1), followed by winter (0.0012 gC mm−1H2O yr−1), autumn (0.001 gC mm−1H2O yr−1) and summer (0.0007 gC mm−1H2O yr−1), while the annual rate was calculated to be 0.001 gC mm−1H2O yr−1. The relative contribution of climatic factors is the most considerable in the summer season, with 8.6%. The negative contribution of CO2 to EWUE along south China coast may not be influenced by CO2, rather because of the seasonal variations in GPP and ET trends. Our findings suggest that vegetation greening taken place in the last couple of decades has significantly enhanced EWUE trends in China and could help to develop future ecological restoration programs considering EWUE variations for effective ecosystem management and efficient water use.

Water use efficiency of China's karst ecosystems: The effect of different ecohydrological and climatic factors

Water use efficiency (WUE) is an important indicator for understanding the coupled ecosystem carbon and water cycles. However, the effect and contributions of factors on WUE variations in China's karst ecosystems for different climatic conditions have not been extensively studied. Our studies on WUE variations of China's karst ecosystems from 2001 to 2021 based on evapotranspiration and net primary productivity (NPP) from Moderate-resolution imaging spectroradiometer revealed the contributions of soil moisture (SM), leaf area index (LAI), precipitation (P), temperature (T), vapor pressure deficit (VPD), and CO2 concentration (CO2). Results showed that the trend of WUE was similar to that of NPP in terms of the latitude, longitude, and elevation, and WUE started abruptly decreasing after an elevation >3000 m until it reached 0 at 4500 m. WUE was primarily “slightly increased” in the humid region (H) and “slightly decreased” in the semi-humid region (SH), arid and semi-arid regions (ASA), and Qinghai–Tibet plateau region (QTP). CO2 (0.34), LAI (0.60), P (0.58), and LAI (0.55) exhibited the strongest positive direct effects on WUE in H, SH, ASA, and QTP, while VPD exhibited the strongest negative direct effect. VPD (0.26), VPD (0.28), SM (0.47), and P (0.39) had the strongest positive indirect effect, while T (−0.24), T (−0.18), VPD (−0.35), and P (−0.03) had the strongest negative indirect effect on WUE. The positive contributions of WUE variations in H, SH, ASA, and QTP were dominated by T (47.96 %), CO2 (26.36 %), P (8.81 %), and CO2 (52.97 %), whereas the negative contributions were dominated by P (−7.95 %), LAI (−26.57 %), CO2 (−35.98 %), and VPD (−9.59 %), respectively. This study quantifies the spatial and temporal distribution patterns of WUE in China's karst ecosystems and the regional differences between the multiple ecohydrological factors, thereby facilitating in-depth understanding and effective regulation for the carbon and water cycles in karst ecosystems.

Variability of spring ecosystem water use efficiency in Northeast Asia and its linkage to the Polar-Eurasia pattern

Given that water use efficiency (WUE) is an important indicator to measure the trade-off between carbon uptake and water consumption within the ecosystem, better understanding the variation of ecosystem WUE and related driving factors is of great interest. In this study, the variability of spring ecosystem WUE in Northeast Asia (NEA) was investigated. The results show that its primary mode exhibits a monosign variation. This mode is directly controlled by the variability of gross primary productivity. The climate conditions also play remarkable roles, featuring that warm surface air temperature (high soil moisture) favors enhanced ecosystem WUE in northern (southern) NEA. Further analysis reveals that the Polar-Eurasia (POL) pattern can significantly impact the variability of spring ecosystem WUE in NEA through changing surface air temperature and soil moisture. When the POL pattern lies in the positive phase during spring, anticyclonic circulation anomalies with an equivalent barotropic structure prevail over northern NEA, concurrent with anomalous easterlies over southern NEA and a weakening of the East Asian jet (EAJ). Accordingly, anomalous downward motion is introduced over northern NEA, resulting in higher surface air temperature which is beneficial for the increase of local ecosystem WUE. Meanwhile, the easterly anomalies help to increase water vapor transport into southern NEA and the weakened EAJ can induce anomalous ascending over southern NEA, favoring the increase of precipitation and hence soil moisture, which consequently enhances the ecosystem WUE in southern NEA.

The impact of tillage practices on daytime CO2 fluxes, evapotranspiration (ET), and water-use efficiency in peanut

Peanut (Arachis hypogaea L.) growers use different tillage systems in the Southeastern United States, the impact of which needs to be assessed with regard to evapotranspiration (ET), carbon uptake, and water-use efficiency (WUE). The eddy-covariance method was used to measure these fluxes in peanut in two common tillage systems (strip tillage vs. conventional tillage) over the course of three consecutive growing seasons (2019–2021). Results suggest that during the dry year of 2019 with rainfall of only 30 cm, strip tillage peanut had a significantly higher daytime ecosystem WUE, 105%, 51%, and 32% higher than that of the conventional tillage in early, mid, and late growth stages, respectively. In 2020, with mean rainfall the overall difference in average WUE was nonsignificant between the tillage systems. Heavy rainfall of 112 cm in 2021 led to waterlogged conditions in the conventional tillage field due to poorer infiltration. This likely reduced the CO2 uptake. Waterlogging did not occur in the strip tillage field due to improved infiltration. As a result, in 2021, 18%, 33%, and 48% greater ecosystem WUE in strip tillage during early, mid, and later stages was found. Thus, this study suggests that strip tillage fields can achieve higher net CO2 uptake and WUE in Georgia during dry or very wet years. However, no difference in WUE was found between different tillage systems in a typical year with average rainfall for Georgia. The present study has implications for regions characterized by long growing seasons and low rainfall.

The non-uniform time-lag and cumulative responses of terrestrial ecosystem water use efficiency to climate change in Lake Victoria Basin, East Africa

Revealing the response mechanisms of carbon–water cycle to maximum and minimum temperatures and daytime and nighttime warming is important for managing fragile ecosystems exposed to severe climate warming and extreme weathers. Taking water use efficiency (WUE) characterizing the coupled carbon–water cycle as the research object, and maximum and minimum temperatures as the indicators of temperature change, this study explored the response of ecosystem WUE to temperature change in Lake Victoria Basin during distinct rainy and dry seasons. The results revealed the time-lag and cumulative multiple effects of antecedent maximum and minimum temperatures on current ecosystem WUE, which marked differences between the rainy and dry seasons. The monthly average ecosystem WUE in the rainy seasons (0.99 g C kg−1 H2O) were higher than those in the dry seasons (0.83 g C kg−1 H2O) during 2001–2018. WUE were more sensitive to temperature change in the dry seasons, with significant negative correlation with minimum temperature and positive correlation with cumulative maximum temperature. In rainy seasons, WUE showed more significant time-lag and cumulative effects, with response to maximum temperature for lagging approximate 3 months, to minimum temperature for lagging 0–2 months, to cumulative maximum temperature for lagging 1 month, and to cumulative minimum temperature lagging 3–4 months, respectively. The results add new evidence for differential non-instantaneous responses of WUE to maximum and minimum temperatures under dry and wet conditions.

Changes in Water-Use Efficiency of Eucalyptus Plantations and Its Driving Factors in a Small County in South China

Ecosystem water-use efficiency (WUE) has been central in revealing the variability in terrestrial carbon and water cycles. Short-rotation plantations such as Eucalyptus plantations can simultaneously impact net primary production (NPP) and actual evapotranspiration (ETa), components of WUE, resulting in changes in terrestrial carbon and water cycles. However, there are few detailed studies on the changes in the WUE of Eucalyptus plantations at the catchment scale with high spatial remote sensing imagery. Here, we present the changes in the WUE of Eucalyptus plantations and its driving factors (i.e., NPP and ETa) using satellite-based models combined with 5-m spatial resolution RapidEye imagery in a small county in South China. The increases in ETa of Eucalyptus plantations are primarily the result of climate warming and result in low WUE of Eucalyptus plantations. The management practice used (short rotation in this study) can enhance the effect of climate warming on WUE by varying the NPP of Eucalyptus plantations. A high value of NPP leads to a high WUE of Eucalyptus plantations at the end of a short rotation, while a low value of NPP results in a low WUE at the beginning of another short rotation. Changes in the WUE of Eucalyptus plantations indicated large spatial and temporal variability, associated with climate warming and short-rotation practices.

Models overestimate ecosystem water use efficiency for northern permafrost regions

Understanding the carbon-water coupling over permafrost regions is essential to projecting global ecosystem carbon sequestration and water dynamics. Ecosystem water use efficiency (EWUE), defined as the ratio of gross primary productivity (GPP) and evapotranspiration (ET), reflects plant acclimation strategies with varying ecosystem functioning against environmental stress. Yet EWUE change and its potential drivers across the northern permafrost regions remain poorly quantified, hampering our understanding of permafrost carbon−climate feedback. Here, we compared and analyzed the difference using satellite observations and process-based models to estimate the spatio-temporal variations of EWUE in 1982–2018 over northern permafrost regions. Using flux measurements as truth data, satellite-derived EWUE was more reliable than model-based EWUE. Satellite-derived EWUE showed biome-dependent spatial patterns, with a steady temporal trend (0.01 g C mm−1 decade−1, P > 0.05) for spatially averaged EWUE over northern permafrost regions. Carbon dioxide (CO2) concentration and nitrogen deposition positively affected interannual variations of EWUE, while vapor pressure deficit and other climatic factors (i.e., temperature, precipitation, and radiation) negatively controlled EWUE. Compared to satellite-derived EWUE, we found that EWUEs derived from an ensemble of process-based carbon cycle models are overestimated in seven out of ten models, with an increasing trend of 0.11 g C mm−1 decade−1 (P < 0.001) for spatially averaged EWUE of the ensemble mean. The relationships between climatic factors and EWUE are partially misinterpreted in model estimates, especially with overstated CO2 sensitivity and the opposite temperature effect. The fluctuating sensitivities to climate over time and the diminishing effect of CO2 fertilization on gross primary productivity (GPP) may partially explain the discrepancy observed between satellite-derived and model-based estimates of EWUE. Thus, this study calls for caution concerning model-based EWUE and aids in understanding permafrost-climate feedbacks and projections of carbon and water cycles.

Influence of planting pattern on peanut ecosystem daytime net carbon uptake, evapotranspiration, and water-use efficiency using the eddy-covariance method

Peanut is planted in a pattern of either single or twin rows in Georgia, USA. However, limited attention has been paid to the impact of planting pattern on the carbon footprint and how the net carbon uptake is intertwined with the amount of water used to determine the ecosystem water-use efficiency (WUE) in peanut. This paper reports on the relationship between the amount of carbon produced to the amount of water used in peanut, carbon dioxide flux, and crop evapotranspiration of peanut in a single- or in a twin-row planting pattern measured using the eddy-covariance method. To the best of our knowledge, the present study is unique in that it examines for the first time the effect of planting pattern on the net carbon uptake and WUE. The two-year study took place in contrasting weather conditions with the 2016 year experiencing a higher vapor pressure deficit and lower precipitation than in the 2018 year. In this study, field-scale daytime net carbon ecosystem exchange (CO2 fluxes), ET and WUE of single- and twin-row peanut were compared using the eddy-covariance technique. Results showed that in 2018, both the net carbon uptake from the atmosphere and the WUE of twin-row peanut were significantly greater than those in the single-row peanut by 7-10% and ~9% respectively, for pod filling and seed maturity growth stages (aGDD 1000-2000 and aGDD &amp;gt; 2000). In 2016, the net daytime carbon uptake and WUE of peanut were similar for both planting patterns during pod filling (aGDD 1000-2000). Higher precipitation and lower VPD in 2018 likely resulted in greater peanut yield in twin-row as compared to single-row with abundant precipitation. Owing to the fast canopy growth rate in twin-row peanut, results suggest that during the vegetative stage (aGDD&amp;lt;500) in 2016, both daytime net carbon uptake from the atmosphere and WUE were considerably greater in twin-row than single-row by 32% and 27%, respectively. Given that in both years, the ET from both planting patterns was similar, it appears that the determination of WUE in both planting patterns was more impacted by changes in daytime net carbon uptake than evapotranspiration. The results of this study suggest the possibility that the higher WUE at the critical stages of twin-row peanut in 2018 are likely to lead to greater yield than single-row peanut. This should be confirmed with further year-to-year investigations.

Responses of ecosystem water use efficiency to drought in the Lancang–Mekong River Basin

The ecosystem water use efficiency (WUE), a crucial indicator of how climate change will affect terrestrial ecosystems, depicts the coupling of the carbon gain and water loss in terrestrial ecosystems. In this study, the spatiotemporal variations in the WUE and its responses to drought in the Lancang–Mekong River Basin (LMRB) from 1982 to 2018 were investigated using the gross primary productivity (GPP) and evapotranspiration (ET) data acquired from the Global Land Surface Satellite (GLASS) products. The analyses revealed that: (1) the mean yearly WUE for the LMRB was 1.63 g C kg−1 H2O, with comparatively higher values in forests and warm temperate climatic types. The interaction of temperature and leaf area index was the main factor affecting the spatial distribution of WUE. The yearly WUE for the entire region exhibited a decreasing trend with a rate of −0.0009 g C kg −1 H2O·yr−1, and the spatially significantly decreasing area accounted for 41.67% of the total area. (2) The annual WUE was positively correlated with drought in the humid regions, accounting for 66.55% of the total area, while a negative relationship mainly occurred in the high-altitude cold region. (3) The ecosystem WUE lagged behind the drought by 3 months in most regions. The lag effect was more apparent in the grassland-dominated upstream region and the cropland-dominated Mekong Delta. (4) The resilience analysis revealed that the ecosystems in forests and temperate climate types were strongly resistant to drought, while the grassland and high-altitude regions with a dry and cold climate had relatively poor resilience. The results of this study shed light on how the WUE responds to drought across diverse land use types, climate types, and elevation gradients, uncovering fresh insights into the potential mechanisms behind the impact of drought on water and carbon cycles within ecosystems.