Variation In Water Use Efficiency Research Articles

Dynamics of Water Use Efficiency of Coniferous and Broad-Leaved Mixed Forest in East China

The aim of our study is to understand the patterns of variation in water use efficiency (WUE) in coniferous and broad-leaved mixed forest ecosystems across multiple scales and to identify its main controlling factors. We employ the eddy covariance method to gather data from 2017, 2018, and 2020, which we use to calculate the gross primary productivity and evapotranspiration of these forests in East China and to determine WUE at the ecosystem level. The mean daily variation in WUE ranges from 4.84 to 7.88 gC kg−1 H2O, with a mean value of 6.12 gC kg−1 H2O. We use ridge regression analysis to ascertain the independent effect of environmental factors on WUE variation. We find that WUE responds differently to environmental factors at different time scales. In mixed conifer ecosystems, temperature and relative humidity emerge as the most significant environmental factors influencing WUE variability. Especially at the seasonal scale, temperature and relative humidity can explain more than 51% of the WUE variation. Our results underscore the varied effects of environmental factors on WUE variation across different time scales and aid in predicting the response of WUE to climate change in coniferous and broad-leaved mixed forest ecosystems.

Transition from opportunistic to conservative water use strategy with the growth of Robinia pseudoacacia in a typical watershed in the semiarid Loess Plateau

Understanding water use strategy is essential to evaluate plant adaptability in water-limited regions, but whether this strategy changes with plant growth needs further investigation. Robinia pseudoacacia is an exotic pioneer plantation species widely planted in the semiarid Loess Plateau of China to restore degraded ecosystems. Plant water uptake proportion from different soil layers and the water uptake pattern calculated through soil water uptake niche breadth (B index) were determined for R. pseudoacacia in 11-, 25-, 33–, and 43-yr plantations. Leaf physiological parameters and carbon stable isotopic composition (δ13C) were measured concurrently to calculate short-term and long-time integrated water use efficiency (WUE). Then, the water use strategy for R. pseudoacacia at different plantation ages was assessed through water uptake patterns, stomatal conductance (gs), and WUE variation under varied soil water conditions. Results showed that R. pseudoacacia maintained high gs and low short-term WUE and δ13C through a relative equal water uptake pattern and absorbed water within 0 − 400 cm soil depths in 11- and 25-yr plantations. However, this species exhibited an exclusive water uptake pattern and absorbed water mainly from 50 − 400 cm soil depth, adopted low gs and high short-term WUE and δ13C in 33– and 43-yr plantations. Therefore, R. pseudoacacia transited from an opportunistic to a conservative water use strategy with plant growth. The short-term WUE and δ13C were mainly and significantly influenced by gs and specific leaf area, respectively. Furthermore, the significant influence of B index on gs, δ18OΔ, short-term WUE, and δ13C indicated that R. pseudoacacia coordinated both water uptake pattern and water use behavior to contend with varied soil water conditions. This study suggested that changes in age-related water use strategy were essential for the adaptability of R. pseudoacacia in water-limited conditions.

Dynamics of carbon and water vapor fluxes in three typical ecosystems of Heihe River Basin, Northwestern China

Understanding the dynamics of carbon and water vapor fluxes in arid inland river basin ecosystems is essential for predicting and assessing the regional carbon-water budget amid climate change. However, studies aiming to unravel the mechanisms driving the variations and coupling process of regional carbon-water budget in a changing environment in arid regions are limited. Here, we used the eddy covariance technique to analyze the relationship between CO2 and H2O fluxes in three typical ecosystems across the upper, middle, and lower reaches of an arid inland river basin in Northwestern China. Our results showed that all ecosystems acted as carbon sinks, with the alpine swamp meadow, cropland, and desert shrubland sequestrating −300.2 ± 0.01, −644.8 ± 2.9, and − 203.7 ± 22.5 g C m−2 yr−1, respectively. Air temperature (Ta) primarily controlled daily gross primary productivity (GPP) and net ecosystem CO2 exchange (NEE) in the irrigated cropland during the growing season, while soil temperature (Ts) and vapor pressure deficit (VPD) regulated these parameters in the alpine swamp meadow and desert shrubland. Additionally, Ta and net radiation (Rn) controlled daily evapotranspiration (ET) in cropland, while Ts and Rn regulated ET at other sites. Consequently, carbon and water vapor fluxes of all three ecosystems tended to be energy-limited during the growing season. The differential responses of carbon and water vapor fluxes in the upper, middle, and lower reaches of these ecosystems to biophysical factors determined their distinct coupling and variations in water use efficiency. Notably, the desert shrub ecosystem in the lower reach of the basin maintained a stable balance between carbon gain and water loss, indicating adaptation to aridity. This study provides valuable insights into the underlying mechanisms behind the changes in carbon and water vapor fluxes and water-use efficiency in arid river basin ecosystems.

Biological factor controls the variations in water use efficiency of an alpine meadow during the growing season in a permafrost region of the Qinghai-Tibet Plateau

The alpine meadow located in permafrost is crucial for ecosystem services of the Qinghai-Tibet Plateau (QTP), which is experiencing precipitation changes in most areas. Water use efficiency (WUE) can quantify the inextricable link between carbon assimilation and water loss in terrestrial ecosystems. However, the temporal variations in WUE and its driving factors across different precipitation years still need to be clarified in alpine meadows on the QTP. Therefore, 4-year carbon and water flux data were used to elucidate the mechanisms behind seasonal and interannual variations in WUE of an alpine meadow in the hinterland of the QTP. Noticeable seasonal variations in WUE were observed during the studied period, with the highest value (1.38±0.38 g C kg−1 H2O) occurring during the mid-growing season (MG, starting around 166 DOY), approximately 2 and 3 times those during the late-growing season (LG, starting around 256 DOY and ending around 282 DOY) and early-growing season (EG, starting around 140 DOY), respectively. Standardized total effects in the structural equation models from NDVI to WUE were highest in all seasons, indicating that NDVI was the primary controlling factor for daily WUE variations. Additionally, energy factors (temperature and solar radiation) also significantly influenced daily WUE variations. The highest mean daily WUE (1.23±0.65 g C kg−1 H2O) was in the mild dry year (2016). However, no significant differences were noted in mean daily WUE in severe dry (2015) and wet (2019) years compared to the normal year (2020) during GS. This can be attributed to the varying sensitivity of carbon assimilation and water loss to biotic and abiotic changes across divergent precipitation years, with WUE exhibiting a greater sensitive to gross primary productivity than to evapotranspiration. These findings suggest that alpine meadows have endured in its harsh environment and have adapted to climatic fluctuations through long-term evolution.

Agronomic adaptations to heat stress: sowing summer crops early in late winter.

Abstract CONTEXT: Summer crops are exposed to heat and drought stresses at critical stages during and after flowering, and their intensity and frequency are likely to increase with climate change. Agronomic stress avoidance offers the opportunity to temporally separate critical crop stages from heat and drought events, though it might require sowing cold sensitive summer crops early in late winter into colder than recommended soil temperatures. There is a need to understand how cold is too cold to sow summer crops early in late winter as well as what are the yield benefits and risks. OBJECTIVE: Here we quantify the likely benefits and trade-offs of sowing sorghum, a summer cereal, early in late winter as an adaptation to the increased frequency and intensity of heat and water stresses during flowering and grain filling. METHODS: Two seasons of multi-environment ( n =32) genotype by management trials were conducted across sub-tropical and semi-arid regions of eastern Australia. Environments (E) consisted of the combination of seasons, sites, three times of sowing (early, spring, and summer), and the use of supplementary irrigation. At each E a factorial combination of four plant populations (M) and eight commercial sorghum hybrids (G) were sown with three replications. Crop growth and yield components were measured, and the APSIM model was used to simulate all trials and treatments to quantify risks, and derive insights into functional relationships between simulated and measured environmental co-variates, and measured crop traits. RESULTS: The tested hybrids showed small differences in cold tolerance during crop establishment. Across the tested environments the G×M combinations produced up to 60% variation in treatment yields across environment yields, that varied between <0.5 to about 10t ha -1 , this translated into a ~8-fold variation in water use efficiency. Significant G×E and M×E interactions were observed for grain yield components. No G×M or G×E×M interactions were observed on yield or yield components. Early sowing was associated to a reduced risk of heat stress, and water use transfer from vegetative to reproductive stages. Early sowing in late winter or early spring resulted in no significant yield gain or yield loss when all sites and seasons are included in the analysis, and between 1-2 t ha -1 yield gains when the hottest sites and seasons are considered separately. The likelihood and severity of terminal drought stresses was also reduced with late winter and early spring sowings, improving yield quality parameters. CONCLUSIONS: Early sowing of sorghum can reduce the likelihood of heat stresses around flowering as well as the likelihood of terminal drought stresses. Advantages include reduced yield losses in the hottest seasons, and a transfer of water use to grain filling stages that resulted in increased grain yield and improved grain quality parameters. IMPLICATIONS: Agronomic adaptations offer opportunity to quickly adapt to the increase in the frequency and intensity of extreme hot events during critical crop stages. However, for the practice to be de-risked there is need to increase cold and chilling tolerance in sorghum, and or identify interventions that enhance seed germination and seedling vigour when the crop is sown early into cold soils.

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.

The Contributions of Climate and Human Activities to Water Use Efficiency in China’s Drylands

In the context of global warming, terrestrial ecosystems have undergone significant variations. China has implemented a variety of ecological engineering methods to enhance carbon stocks. However, understanding the spatial and temporal dynamics of carbon and water in drylands under climate change remains limited. Here, our research elucidates carbon and water dynamics in China’s drylands over the last two decades, with a focus on understanding spatial–temporal changes and the effects of ecological engineering on the carbon–water cycle. Furthermore, this study investigates the relationships among climate change, water use efficiency (WUE), and its components—Gross Primary Productivity (GPP) and Evapotranspiration (ET)—identifying key climatic drivers and assessing possible directions for enhancing WUE under changing climate conditions. Our research indicates that both GPP and ET have significantly increased over the past 20 years, with growth rates of 4.96 gC·m−2·yr−1 and 4.26 mm·yr−1, respectively. Meanwhile, WUE exhibited a slight declining trend, at a rate of −0.004 gC·mmH2O·yr−1. This confirms the positive impact of vegetation restoration efforts. We found that fluctuations in interannual WUE were influenced by human activities and climate change. Precipitation (Prec) was the key climatic factor driving the GPP increase. Both solar radiation (Solra) and Prec were crucial for the interannual variation of WUE. Interestingly, WUE was the main factor affecting GPP development. The decline in WUE in drylands is linked to interannual variability in WUE and increased Vapor Pressure Deficit (VPD) due to warming. Seasonal variations in how WUE responds to climatic factors were also observed. For instance, fall rainfall increased WUE, while spring rainfall decreased it. Fall WUE was highly sensitive to VPD. Spatially, we found higher WUE in China’s eastern and Xinjiang regions and lower in inland areas and the Tibetan Plateau. Geomorphologic factors and soil conditions were the main drivers of this spatial variability in WUE. Temperature (Tem), Solra, VPD, and relative humidity (Relah) also played significant roles. Our results show a generalized inverse persistence in WUE variability. This suggests a potential for increased WUE in the eastern regions and a risk of decreased WUE on the Tibetan Plateau. Addressing the threat of vegetation decline in arid regions, particularly within the Tibetan Plateau, is crucial. It is essential to adapt forestry practices to complement the carbon and water cycles in these landscapes.

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.

Climate gradient and leaf carbon investment influence the effects of climate change on water use efficiency of forests: A meta-analysis.

Forest ecosystems cover a large area of the global land surface and are important carbon sinks. The water-carbon cycles of forests are prone to climate change, but uncertainties remain regarding the magnitude of water use efficiency (WUE) response to climate change and the underpinning mechanism driving WUE variation. We conducted a meta-analysis of the effects of elevated CO2 concentration (eCO2 ), drought and elevated temperature (eT) on the leaf- to plant-level WUE, covering 80 field studies and 95 tree species. The results showed that eCO2 increased leaf intrinsic and instantaneous WUE (WUEi, WUEt), whereas drought enhanced both leaf- and plant-level WUEs. eT increased WUEi but decreased carbon isotope-based WUE, possibly due to the influence of mesophyll conductance. Stimulated leaf-level WUE by drought showed a progressing trend with increasing latitude, while eCO2 -induced WUE enhancement showed decreasing trends after >40° N. These latitudinal gradients might influence the spatial pattern of climate and further drove WUE variation. Moreover, high leaf-level WUE under eCO2 and drought was accompanied by low leaf carbon contents. Such a trade-off between growth efficiency and defence suggests a potentially compromised tolerance to diseases and pests. These findings add important ecophysiological parameters into climate models to predict carbon-water cycles of forests.

Quantifying the variation in water use efficiency across climates and biomes on the Qinghai-Tibetan Plateau

Water use efficiency (WUE) is an important indicator in linking the carbon and water cycles of ecosystems. Previous studies performed in alpine and cold regions have contributed greatly to understanding the WUE response of single ecosystem or specific species in small-scale regions to climatic factors; however, studies focusing on differences in the WUE response among ecosystems or species are insufficient, which has limited the understanding of the climatic adaptation of WUE in alpine and cold regions. Here, based on the 134 leaf δ13C records measured for the 46 dominant species in two main ecosystems on the Qinghai-Tibetan Plateau, variation in the response of WUE among biomes was analyzed by grouped linear regression, redundancy analysis (RDA), and structural equation models (SEMs). The results showed that (1) photosynthetically active radiation (PAR0), moisture index (MI) and vapor pressure deficit (VPD) were significantly correlated with the variation in WUE; (2) compared with the WUE of alpine grasslands, alpine desert grasslands was more sensitive to climatic factors, which presenting a steeper slope that varied with modified growing degree days (MGDD0), PAR0, MI, VPD and CO2; (3) composition of genera distributions explained 32.22% of the variation in WUE, while the WUE of Stipa was more sensitive compared to other species to most climatic factors; (4) climate (composition of genera) and their joint effects explained 58.98% (63.53%) of the variation in WUE of alpine desert grasslands (alpine grasslands), while altitude indirectly controlled the WUE variation in alpine grasslands and MI directly controlled the WUE variation in alpine desert grasslands. This study quantified the driving effects of climates and biomes on the variation in WUE and contributed to the understanding of vegetation adaptation in alpine and cold regions.

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.

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.

Tree-ring isotopic composition reveals intraspecific variation in water use efficiency of Pinus pinaster Ait. provenances grown in common gardens

Key messageThe physiological responses expressed by variation in carbon and oxygen stable isotopes and iWUE in five provenances of maritime pine grown in four common gardens were primarily determined by genotype differences in phenotypic plasticity and secondarily by genotype.Given the impacts of climate change on forest resources and considering the slowness of evolutionary processes in trees, a need arises to understand the interplay between tree species adaptation to climate, genetic variation, and their impact on tree growth and productivity. Broadening knowledge of the capacity of tree populations to respond to climate-related disturbances is a prerequisite for the development of resilience strategies, including assisted migration and climate-smart forestry. This study tests the physiological ability of different maritime pine provenances, comparing Mediterranean (Corsica, Sardinia, and Tuscany) and Atlantic (Portugal) provenances, to adapt to progressively drier conditions that have occurred in the last thirty years. Four provenance trials with randomized blocks of the five maritime pine provenances were used as test sites in Sardinia (Italy). Wood cores were collected from the 40-year-old plants. Cores were split into five-year segments to determine provenance-related variations in carbon and oxygen stable isotopes and provide information on long-term patterns in intrinsic water use efficiency (iWUE). The provenance × site interaction was the most important source of variation, meaning that the genotypes responded differently to the planting sites. Considering the main effects, both genotype and environmental conditions at the planting sites influenced stable isotope composition in tree rings. This suggests that iWUE was determined by phenotypic plasticity that differed among genotypes. In contrast, provenance responses were stable with time, and the provenance × site interaction was stable across time periods. These findings suggest that provenance selection to improve iWUE in maritime pine may need to consider site conditions but point more to soil conditions than to climate. In any case, they limit our ability to recommend maritime pine provenances based on iWUE until the missing site factors can be identified.

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.

Integrated spatial and temporal variability of the system water use efficiency in a lower Baro River watershed, Ethiopia

Abstract The Baro Akobo River is representative of lower Baro watersheds with lost soils. Under eight landscapes, the geospatial and temporal variability of system water use efficiency (sWUE) were examined in a total area of 20,325 km2. This study used GIS, RS, Cropwat8.0, and EasyFit software. The anticipated irrigation requirement for the selected crop's driest five months of May, February, March, January, and April was 1, 0.9, 0.78, 0.78, and 0.34 l/s/h, respectively. The sub-catchment had maximum critical test values of σ = 12.6, μ = 11.9, and γ = 0, while Sor Metu showed the smallest value of 0.80, 1.75, and −0.03. Across the watershed, the sWUE varies with runoff, with a coefficient of variation of 71%. The overall accuracy of the land cover change was 81%, the Landsat 8 images of the soil-adjusted vegetation index showed a maximum value of 0.87 and a minimum of −1.5. The normalized vegetation index ranged from a maximum of 0.58 to a minimum of −1. By 2050, the sWUE will be 10% lower temporally, but its spatial variability will be 25% higher. Therefore, soil infiltration and water storage improve, which decreases runoff and the water lost by ET and raises sWUE.

Spatiotemporal Variation of Vegetation Water Use Efficiency and Its Response to Extreme Climate in Northwestern Sichuan Plateau

The Northwestern Sichuan Plateau, characterised by complex topography, is located on the northeastern edge of the Tibetan Plateau where ecosystems are rapidly changing in response to climate change. However, the spatiotemporal characteristics of water use efficiency (WUE) and its response to climate change remain unclear, especially different responses to changes in climate mean and extremes. In this study, we found that the seasonal variation of WUE for different types of vegetation demonstrates a “unimodal” pattern, with high values during May to September. The variation of WUE is largely dependent on altitude. The annual WUE showed an overall increasing trend from 2001 to 2021 for most types of vegetation, while a decreasing trend was found for wetland, and grassland had a stronger adaptability to altitude and climate change. Mean climatic variables exerted stronger impacts on WUE than climate extremes, and mean temperature change had a stronger impact on WUE than precipitation. However, the synergistic effects of climate extremes and mean climate exerted significant impacts on WUE, with extreme cold events and mean precipitation events generally being unfavourable for an increase in WUE. The WUE of cropland and forest land demonstrated more sensitive responses to climate extremes than other types. The response of the same type of vegetation to climate extremes has obvious regional heterogeneity. However, the diurnal range of temperature, the length of the growing season, and the persistent humidity index had significant influences on the change of WUE for the entire region.

Plant size directly correlates with water use efficiency in Arabidopsis.

Plant transpiration is a fundamental process that determines plant water use efficiency (WUE), thermoregulation, nutrition, and growth. How transpiration impacts on such essential physiological aspects and how the environment modulates these effects are fundamental questions about which little is known. We investigated the genetic and environmental factors underlying natural variation in plant transpiration and water use efficiency in a population of natural Arabidopsis thaliana accessions grown under homogeneous conditions. As expected, we observed large variation of total transpiration capacity, transpiration per surface unit, and WUE among A. thaliana accessions. Despite the variation of stomatal density and ABA content in the population, WUE did not correlate with any of these parameters. On the contrary, a surprising direct correlation was found between WUE and projected leaf area, with bigger plants displaying a more efficient use of water. Importantly, genome-wide association studies further supported our observations through the identification of several loci involved in WUE variation, mutations in which caused a simultaneous reduction in plant size and a decrease in WUE. Altogether, our results strongly suggest that, although WUE depends on many parameters, plant size is an adaptive trait with respect to water use in A. thaliana.

Leaf physiological and morphological constraints of water-use efficiency in C3 plants.

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Water use efficiency at multi-time scales and its response to episodic drought and wet periods in a typical subtropical evergreen forest of Southeast China

Water use efficiency (WUE) quantifies the trade-off between carbon gain and water loss. A deep understanding of its relationships with the controlling factors is essential for predicting ecosystem responses to climate change. Our study utilized the eddy covariance measurements to analyze the variability of WUE and its influencing climate factors at the seasonal, monthly, 8-day, daily, and half-hourly scales and during the episodic drought and wet periods at a typical subtropical evergreen forest of Southeast China. The results show that ET is the dominant factor influencing the WUE variability with a relative contribution of 57.56 ± 2.26% for different seasons in this study area. Secondly, WUE responds differently to climatic drivers from monthly to half-hourly scales. Our results show that WUE has a remarkably high sensitivity to soil moisture at multi-time scales (from monthly to half-hourly), and the connection strength increases with increasing time scale. Thirdly, there is no significant relationship between VPD and WUE as time scale expands. In addition, WUEs during the episodic drought period have higher values than those during the episodic wet period on the monthly scales. The half-hourly WUE during the episodic drought period is close to the WUE during the episodic wet period from 8:30 to 19:00 but is higher from 6:00 to 8:00. These results indicate that soil moisture plays an important role on influencing WUE variation at multi-time scales and under extreme climatic conditions.

Revisiting the role of transpiration in the variation of ecosystem water use efficiency in China

Efforts to develop effective climate strategies necessitate a better understanding of the relationship between terrestrial water and carbon cycles. Water use efficiency (WUE) has been often used to characterize this relationship, while the role of transpiration (T) in the variation of ecosystem WUE has been less investigated. Here, we partitioned WUEET (the ratio of gross primary productivity (GPP) to evapotranspiration (ET)) into a two-component process, i.e., the ratio of gross primary productivity to plant transpiration, GPP/T, that is WUET, and the ratio of plant transpiration to evapotranspiration, T/ET. Based on two GPP datasets (i.e., GPP based on the light use efficiency model or the vegetation index- NIRv) and the GLEAM ET dataset, this study investigated the role of T in the variation of WUE in the ecosystem level and how the role is affected by drought. We found that drought can lead to the change of ET partitioning, thus affecting the variability of WUE. The variability of WUEET was dominated by WUET. In general, the proportion of T increased gradually from humid to arid areas. To adapt to drought conditions, vegetation in arid areas tend to have a high stress resistance by increasing their WUE. We further found that WUET has stronger seasonal stability than WUEET. GPP dominated WUEET variability in humid/sub humid areas, while ET and GPP jointly dominated WUEET variability in semi-arid/arid areas. GPP dataset based on light use efficiency (LUE) could better reflect the impact of drought on vegetation. This study contributes to a better understanding of the change mechanism of ecosystem WUE and emphasizes the critical role of physiological process components in water-carbon cycling.