Ecosystem Water Use Efficiency Research Articles

Machine learning-based investigation of forest evapotranspiration, net ecosystem productivity, water use efficiency and their climate controls at meteorological station level

Evapotranspiration (ET), net ecosystem productivity (NEP), and ecosystem water use efficiency (EWUE) of forests are changing due to climate change. Traditional models using coarse-scale climate reanalysis data fail to capture local meteorological and hydrological conditions accurately. This study combines in situ meteorological observations, remote sensing, and advanced datasets (forest age, rooting depth, soil moisture) to estimate ET, NEP, and EWUE at forest meteorological stations via machine learning. About 60.6 % of stations showed a decrease in NEP from 2003 to 2010 to 2011–2019, while 63.9 % showed an increase in ET, and 58.9 % showed a decrease in EWUE. NEP and EWUE significantly declined in forests older than 60 years, with younger forests exhibiting higher NEP. EWUE in different forest types is driven by varying mechanisms, with DBF sites influenced by VPD and ENF sites by RSDN. EWUE of regions with inconsistent VPD data between site and reanalysis, such as northwestern North America, showed divergences from previous reanalysis-based studies but aligned more with atmospheric inversion findings. Slight summer VPD increases boosted NEP, especially in high-latitude areas, while early spring phenology and increased spring VPD reduced summer water availability. Incorporating more site-specific observations, such as plant traits, could enhance understanding of climate-plant-ecosystem relationships. This study underscores the potential of meteorological station-level data to improve forest carbon and water flux dynamics understanding, aiding forest management for carbon neutrality and climate adaptation.

Impact of extreme seasonal drought on ecosystem carbon‒water coupling across China

Water use efficiency (WUE) is a critical evaluation indicator of ecosystem responses to climate change and its extremes. However, the influence of the timing of extreme drought on the variation of ecosystem WUE under severe water stress has not been studied extensively. In particular, the modulation of drought impacts on WUE under regional hydro-climatic conditions and biome types is poorly understood. Considering observation-based ecosystem flux and drought index datasets, this study examined the impact of extreme seasonal drought on WUE in China and attempted to reveal the underlying drivers of seasonal variations in drought impacts. Results showed that the direction and magnitude of drought impacts on WUE depend on the occurrence time of extreme drought and the seasonal dynamics of regional ecological and climatic conditions. Across the vegetated regions in China, the most widespread reduction in WUE under extreme drought conditions was observed in summer, whereas approximately 60% of the study area showed positive changes in WUE under extreme drought conditions in spring. Furthermore, the co-regulation of drought characteristics and background environmental conditions in determining the impacts of extreme seasonal drought on ecosystem WUE is highlighted. Classification and regression tree analysis results illustrate that leaf area index (LAI) and drought timing dominated ecosystem WUE variation in response to extreme drought in China. Regions with lower LAI experienced more serious reductions in WUE under extreme drought. These findings indicate the importance of accounting for the interaction between drought seasonality and biome features in assessing drought impacts, thus contributing to improving the modelling of terrestrial ecosystem responses to climate extremes under global warming.

Soil water content and vapor pressure deficit affect ecosystem water use efficiency through different pathways

Ecosystem water use efficiency (WUE) quantifies the amount of carbon assimilated per unit of water consumed and elucidates the trade-off between water loss and carbon gain at the ecosystem level. Numerous studies have highlighted the significant impact of low soil water content (SWC) and high vapor pressure deficit (VPD) on WUE. However, the underlying mechanisms driving these effects differ markedly. The intensifying coupling between land and atmosphere accentuates the distinctive co-occurrence of low SWC and high VPD, posing challenges in disentangling the respective influences of these two water stressors. Consequently, a clear understanding of the independent responses of ecosystem WUE to SWC and VPD remains elusive. Using meteorological and flux data from FLUXNET’s eddy-covariance sites, we investigated the impact of SWC and VPD on ecosystem carbon–water fluxes and proposed potential pathways through which SWC and VPD influence WUE. We deconstructed WUE into intrinsic WUE of vegetation (GPP/T) and the ratio of transpiration to evapotranspiration (T/ET). Our findings revealed that WUE exhibited greater sensitivity to VPD dynamics compared to SWC. The sensitivity of WUE to SWC stemmed from the responsiveness of T/ET to SWC, especially under high VPD conditions. Conversely, the sensitivity of WUE to VPD emanated from the impact on GPP/T. SWC and VPD exerted distinct effects on ecosystem WUE through different pathways.

Vegetation factors and atmospheric dryness regulate the dynamics of ecosystem water use efficiency in a temperate semiarid shrubland

Ecosystem water use efficiency (WUE), a key indicator of the coupling between carbon and water cycle, has been widely used to quantify ecosystem responses to climate change. However, large uncertainties remain regarding the dynamics and driving factors of WUE in temperate semiarid shrublands. Using eddy-covariance measurements, we investigated the role of vegetation and hydrometeorological factors in affecting the temporal dynamics of WUE (GPP/ET, where GPP and ET represent gross primary production and evapotranspiration, respectively) over a semiarid shrubland of northern China during 2012–2016. Daily WUE was lower (<1.5 g C kg−1 H2O) in early-spring and late-autumn, and higher (2.0–7.0 g C kg−1 H2O) in summer months (June–August). Annual (January–December) WUE (1.43–1.78 g C kg−1 H2O) was higher than most values previously reported for semiarid and arid shrubland ecosystems. Vegetation factors such as surface conductance (gs) and normalized difference vegetation index (NDVI) played a primary role in promoting monthly WUE during the warm and moist growing-season (April–October), while high vapor pressure deficit (VPD) and low root-zone soil water content (SWC) were key hydrometeorological factors that imposed constraints on daily WUE. Using both a machine-learning model and a data-binning algorithm, we showed that NDVI and VPD were the most important factors regulating WUE dynamics. Ongoing climate warming and increasingly frequent extreme events (e.g., droughts and heatwaves) are expected to induce high VPD, which in turn could reduce ecosystem WUE and undermine ecosystem carbon sequestration in semiarid shrublands. In addition, increasing vegetation cover in the studied area owing to continued restoration efforts is expected to enhance ecosystem WUE, and may result in stronger vegetation controls on WUE dynamics.

Ecosystem level carbon and moisture fluxes from a high biomass fibre producing jute crop (Corchorus olitorius L): An eddy covariance-based analysis

ContextJute (Corchorus olitorius L) is the second-largest natural fibre producing crop in the world and plays a key role in the rural economy of South Asia particularly India and Bangladesh. It has diverse use due to its long length, lusture, biodegradability, high tensile strength, heat resistance and bendability. Jute is also fast-growing high biomass crop and quantification of the carbon / moisture exchanges is need of the hour towards its sustainability. ObjectiveThe study aims at quantitative estimation of the ecosystem level CO2 and H2O exchanges at diurnal, daily, and seasonal scales from jute crop cultivated in the alluvial soil of the Gangetic delta under a tropical hot and moist sub-humid climate during the years 2021–23. MethodsAn eddy covariance flux tower with suite of meteorological sensors were established at the jute growing farm to assess the high frequency ecosystem level exchanges of CO2, H2O and biometeorological parameters. Concurrent crop bio-physical parameters were also measured. The flux data were processed, quality checked, gap-filled and partitioned to construct seamless time-series of half-hourly fluxes for further analysis towards its dynamics, driving parameters, photosynthetic response and derived eco-physiological parameters. ResultsThe half-hourly CO2 fluxes exhibited significant variations across different growth stages of the jute crop, reaching peak values during the fibre development to maturity stages [mean Net Ecosystem CO2 Exchange (NEE) ranging from −20 to −22 µmol m−2 s−1; mean Gross Primary Production (GPP) ranging from 25 to 30 µmol m−2 s−1; and mean Ecosystem Respiration (Reco) ranging from 8 to 10 µmol m−2 s−1]. Seasonal GPP values was found to be high (902–1038 gC m−2) with moderate NEE value of −150 to −223 gC m−2. Seasonal mean Ecosystem Water Use Efficiency (EWUE) was high (2.5–2.7 gC kg−1 H2O) compared to other C3 crop. The daily Crop Coefficient (Kc) peaked at 1–1.2 during 90–110 Days After Sowing (DAS), indicative of high physiological activities. ConclusionDespite relatively short crop duration but high biomass (peak Leaf Area Index, LAI ∼6 m2m−2), the jute crop proved to be a robust gross CO2 sink. Due to the rapid growth, the jute crop allocates a significant portion of GPP as Reco (78–84 %), resulting in a relatively lower seasonal NEE. The high EWUE coupled with a net positive CO2 exchange positioned jute as a potential climate-resilient crop. This study provides valuable insights into the ecosystem exchanges of the jute crop, which in turn contribute to the global carbon and moisture budget.

The impact of extreme precipitation on water use efficiency along vertical vegetation belts in Hengduan Mountain during 2001 and 2020

In the context of climate change, extreme precipitation events are continuously increasing and impact the water‑carbon coupling of ecosystems. The vertical vegetation zonation, as a characteristic of mountain ecosystems, reflects the differences in vegetation response to climate change at different elevations. In this study, we used the water use efficiency (WUE) as an indicator to evaluate the water‑carbon relationship. By using MODIS data, we analyzed the spatiotemporal patterns of gross primary productivity (GPP), evapotranspiration (ET), and WUE from 2001 to 2020, as well as the responses of WUE to extreme wetness factor Number of precipitation days (R0.1), extreme dryness factor Consecutive dry days (CDD), and meteorological factors under the vertical vegetation zonation. Our results showed that annual GPP and ET displayed a significant increasing trend between 2001 and 2020, whereas WUE showed a weak decreasing trend. Spatially, GPP and WUE decreased with increasing elevation. Analyzing the WUE of mountainous ecosystems as a unified whole may not precisely capture the reactions of vegetation to severe rainfall occurrences. In fact, across different vegetation belts in mountainous areas, there exists a negative correlation between WUE and R0.1, and a positive correlation with CDD. In terms of meteorological factors, the temporal variation of GPP was primarily associated with vapor pressure deficit (VPD) and temperature (Ta), while those of ET was mainly related to soil water content (SWC). WUE was affected by a combination of meteorological factors and had a certain degree of variation between different altitude intervals. These findings contribute to a better understanding and prediction of the relationship between extreme rainfall climate and water‑carbon coupling in mountainous areas.

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.

Quantifying the water use efficiency of karst ecosystems and response to environmental factors

Study regionThe study area is located in Guizhou province, the centre of the karst region in southwest China. Study focusAs a typical karst region, Guizhou has long faced challenges with water scarcity. Quantifying water use efficiency (WUE) is crucial for the efficient use of water resources and sustainable management. This study examined the spatio-temporal evolution patterns and its driving factors on WUE in Guizhou using Theil-Sen's slope, partial correlation methods, and the geographic detector model. New hydrological insights for the region(1) The multi-annual mean WUE of non-karst region (NKR, 2.75 gC kg−1 H2O yr−1) is higher than that of karst region (KR, 2.10 gC kg−1 H2O yr−1) during 2001–2020. (2) The rate of increase of WUE was three times higher in KR than in NKR. (3) Precipitation and temperature were identified as the key factors influencing WUE. Interactions between each environmental factors were observed to have both bi-variate and non-linear enhancement effects. Notably, in the NKR, soil moisture, vegetation type and leaf area index were the dominant factors influencing WUE, with interactions between environmental factors had both enhanced and weakened effect on WUE. (4) The synergistic effects of these interactions exceeded the individual or cumulative effects of the two environmental factors. These findings contribute to our understanding of the environmental factors driving changes in WUE in the KR.

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.