Variation In Water Use Efficiency Research Articles

Assessing Variations in Water Use Efficiency and Linkages with Land-Use Changes Using Three Different Data Sources: A Case Study of the Yellow River, China

The dependence of water use efficiency (WUE) on changes in land cover types is crucial for understanding of long-term water availability and assessment of water-saving strategies. Investigating the impact of land cover types on ecosystem WUE has important implications when revealing water dynamics and land management. However, the determination of WUE and its dominant factors have always been subject to high data dependency and large calculation consumption within large basins. This paper proposes a framework for processing actual evapotranspiration (AET) and WUE calculation by coupling the Maximum Entropy Production (MEP) method with the Google Earth Engine (GEE). By employing the proposed framework and three data sources available in the GEE platform, results for actual ET and WUE from 2001 to 2020 were obtained in the Yellow River Basin (YRB). The results show that the proposed framework provides an acceptable estimation of actual ET via validation with Eddy Covariance flux sites in the YRB. The calculated WUE values varied greatly in different sub-basins within the YRB, indicating a cumulative growth rate of about 56% during the past 20 years. The dominant factor that led to these changes was the transition from Grasslands into other land-use types. Our results suggest that the use of the GEE platform coupled with the MEP method offers new possibilities for advancing understanding of water exchange and water resource management.

Remote Sensing of Ecosystem Water Use Efficiency in Different Ecozones of the North China Plain

Water use efficiency (WUE), as an environmental factor of metabolism in different ecosystem functional areas, is a key indicator of the ecosystem carbon-water cycle. WUE is defined as the ratio of carbon absorbed by ecosystems to water evaporated. Exploring the spatiotemporal variation in carbon and water cycles in different ecological zones of the North China Plain and their driving factors is important for the ecological management and sustainable development of the different ecological zones in the North China Plain. Based on remote sensing data products, this paper studies the spatiotemporal variations of WUE and their driving factors in different ecological functional areas of the North China Plain from 2001 to 2017. This study found that: (1) The spatial distribution of WUE and gross primary production (GPP) in the North China Plain is similar, with the multiyear average of WUE at 0.74 g C m−2 y−1. The variation trend of WUE is mainly affected by the variation trend of GPP (44.38% of the area of the North China Plain). (2) The change trend of WUE mainly showed a mild decrease and a mild increase, accounting for 73.22% of the area of the North China Plain; the area with medium-low fluctuation of WUE accounted for the largest proportion, accounting for 59.90% of the area of the North China Plain. In addition, the multiyear average values of WUE in the ecological functional area are Qin Ling Mountains deciduous forests > Central China loess plateau mixed forests > Mongolian-Manchurian grassland > Ordos Plateau steppe > Changjiang Plain evergreen forests > Huang He Plain mixed forests > Bohai Sea saline meadow, in the order from high to low. (3) The influence of precipitation on WUE was higher than that of temperature. The area of WUE that increased with the increase of precipitation accounted for 23.74% of the area of the North China Plain and was mainly distributed in the Qin Ling Mountains deciduous forests, Changjiang Plain evergreen forests, and Huang He Plain mixed forests’ ecological functional areas. The results of the study can provide a reference and theoretical basis for the conservation and management of carbon and water cycles in the functional areas of North China’s ecosystems.

Quantifying the variability in water use efficiency from the canopy to ecosystem scale across main croplands

Current, how to use limited water resources efficiently and improve agricultural water use efficiency, has become one of the greatest challenges for global food security. In this study, multiple site-years of carbon and water flux data across the major crops including maize, winter wheat and soybean, were used to quantify the variability in canopy-scale transpiration (T), ecosystem-scale evapotranspiration (ET) as well as the associated water use efficiencies (WUET and WUEET). On the basis of ET partitioning, the results indicated that the transpiration ratio–T/ET as well as T and ET exhibited an obvious single-peak seasonal pattern across the typical croplands. However, at the early and late growing stages, there existed large discrepancies in T and ET owing to low vegetation coverage, while T and ET were very close during the peak period. Among them, maize exhibited the largest T/ET by 0.50 ± 0.12, followed by soybean of 0.43 ± 0.08 and winter wheat of 0.38 ± 0.09, respectively. Furthermore, the coupling relationships between gross primary productivity (GPP) and water fluxes including T and ET changed from linear to nonlinear. The study also found that the variability in WUET and WUEET were not consistent. Specifically, WUEET showed distinct seasonal characteristic whereas WUET kept constant as a plateau almost throughout the growth period, which reflected the inherent physiological property controlled by plant stomata at the canopy scale. Among these crops, maize exhibited the largest WUET and WUEET (5.30 ± 0.89 and 2.48 ± 1.14 g C kg−1 H2O), followed by winter wheat (4.97 ± 1.52 and 2.35 ± 0.64 g C kg−1 H2O) and soybean (4.88 ± 1.59 and 1.89 ± 0.99 g C kg−1 H2O), respectively.

The role of groundwater in the spatio-temporal variations of vegetation water use efficiency in the Ordos Plateau, China

Groundwater influences the water and carbon cycle by supplying moisture to plants in the semi-arid and arid zones. However, little is known about the response of ecosystem water use efficiency (WUE) to climate change in different groundwater depth (GD) sections. Recent research has shown that plant photosynthesis and growth are closely related to GD via field experiments but the wider recognition of GD effect on regional-scale ecosystems has not been yet established. In this study, we test whether the GD has an impact on ecosystem WUE and its variability to climate change at the regional scale. Based on the observed data of nearly 3000 wells, meteorological data (precipitation and pan evaporation), and the 0.01°-resolution remote sensing datasets including gross primary production (GPP), evapotranspiration (ET), and normalized difference vegetation index (NDVI), we explored the spatio-temporal variations of WUE and its composites (i.e., GPP and ET), and their characteristics depending on GD under different aridity conditions and biomes across the Ordos Plateau, a semi-arid to arid area in northern China. Results show that WUE increases with decreasing GD due to water availability in the semi-arid lands where WUE variability is mainly regulated by biological processes (i.e., GPP), while WUE is insensitive to the changes in GD across the arid zone where the physical processes (i.e., ET) control WUE change. However, when drought happens the groundwater-independent vegetation in the arid zone can also utilize groundwater, characterized by lower reductions of GPP with the decrease in GD. A dense vegetation condition (i.e., large NDVI) is more vulnerable to climatic disturbance over the semi-arid zone because it tends to decrease GPP and WUE, especially in the large GD regions. These findings have important implications for reasonable land use and groundwater management over the semi-arid and arid regions.

Enhanced coupling of light use efficiency and water use efficiency in arid and semi‐arid environments

AbstractBoth light use efficiency (LUE) and water use efficiency (WUE) play essential roles in ecosystem production. The extent to which ecosystem production is affected by the coupling between LUE and WUE remains unclear. In this study, we used data from flux measurements and weather stations in the Heihe River Basin, China, along a strong climatic gradient to quantify the relationship between LUE and WUE. Variations of LUE explained up to 85% of the variations of WUE. The contribution of LUE to WUE increased with increasing water stress. Pearson's correlation coefficient between LUE and WUE increased from −0.12 to +0.63 with decreasing LUE. The coordination between LUE and WUE was essential to explaining the insensitive of WUE and GPP with increasing water stress. These results suggest that ecosystems enhance the coordination between light and water use when water stress is high. They enhance our understanding of the drought resilience of ecosystems and reduce uncertainties associated with the carbon cycle in drylands.

Provenance Variation in Early Survival, Growth, and Carbon Isotope Discrimination of Southwestern Ponderosa Pine Growing in Three Common Gardens across an Elevational Gradient

We investigated early survival, growth, and carbon isotope discrimination of ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. scopulorum Engelm.) seedlings from different provenances using common gardens across an elevational gradient in order to examine the potential for adaptation to extreme environments and constraints to artificial regeneration. Twenty-one provenances from a range of elevations across Arizona and New Mexico were planted in three common gardens: a high-elevation meadow in aspen-mixed conifer forest, a mid-elevation ponderosa pine forest, and a low-elevation pinyon juniper woodland. Two years after planting in 2018, survival was highest at the mid-elevation site (54%), low at the high-elevation site (1.5%), and 0% at the low-elevation site. At the hot and dry low-elevation site, provenances from low-elevations survived longer than provenances from mid- and high-elevations, which suggests greater drought tolerance of low-elevation provenances. Mortality agents changed from abiotic (drought) to biotic (herbivory) with an increase in elevation across sites. High mortality of seedlings planted at high-elevation sites from biotic agents, such as rodents, may challenge efforts to establish ponderosa pine in assisted migration projects. Seedlings had significantly higher growth rate and carbon isotope discrimination (∆13C) at the mid-elevation site than the high-elevation site. Provenances differed significantly in diameter, and ∆13C, but not in height growth rate for the first year after planting. Provenance variation in ∆13C suggests genetic variation in water use efficiency that may be useful for future evaluation of southwestern ponderosa pine seed sources for reforestation.

Spatio-temporal variability of water use efficiency and its drivers in major forest formations in India

Forests play a significant role in mitigating the effects of climate change and regulating the biogeochemical cycles. Water use efficiency (WUE) is an important indicator that links the carbon and water cycles in terrestrial ecosystems. In the present study, WUE of major forest formations of India was calculated from 2003 to 2018 as the ratio of Moderate Resolution Spectroradiometer (MODIS) Gross Primary Productivity (GPP, MOD17A2H) to evapotranspiration (ET, MOD16A) using the Google Earth Engine platform. The spatial distribution of WUE was mapped and the inter-annual and monthly variations were analysed. The mean annual WUE of forests ranged from 1.78–2.02 gC kgH2O−1. It was observed that tropical thorn forests had the highest WUE (3.52 ± 1.08 gC kgH2O−1) and moist alpine scrub had the lowest WUE (1.05 ± 0.17gC kgH2O−1) among the forest type groups of India. WUE of forests showed an increasing trend with latitude and decreasing trend with elevation. To understand the influence of various bio-meteorological drivers on WUE and their importance in governing these influences, Random Forest (RF) algorithm was used. The bio-meteorological drivers were able to explain 65% of the variability in WUE. Temperature was identified as the most important driver in influencing the WUE of forests. Based on the findings of the study, it can be expected that the global increase in temperature would negatively affect the WUE of the major forest formations of India.

MORE CROP PER DROP: THE MAGIC OF SWEET POTATO (Ipomoea batatas L.)

<p><strong>Background:</strong> Water is the most valuable resource in agriculture. Agricultural development in semi-arid eastern Kenya is essentially dependent on intermittent rainfall. An accurate estimate of sweet potato water usage and productivity is considered a significant feature of conservation agriculture under such climatic conditions. <strong>Methodology:</strong> The research was conducted for two seasons, from December 2018 to April 2019 and October 2019 to March 2020, with the aim of quantifying the water use efficiencies of two sweet potato varieties, Kabode and Bungoma. Assuming that there were no variations in water-use efficiency between the two varieties. The experiment was established as RCBD for the two seasons. Treatments comprised of sole sweet potato varieties of Kabode and Bungoma, together with their intercrops with common beans. <strong>Results:</strong> Seasonal effective rainfall values were 302 and 639.2 mm, whereas the mean saturation pressure deficit recorded was 2.5 and 2.4 Kpa for S (I) and S (II), respectively. This presented S(I) with 815.1 mm ET<sub>crop</sub> demanding 622.1 mm irrigation water, whereas S(II) envisaged a 3.3% decrease in ET<sub>crop</sub> values; 788 mm and requiring 427.6 mm of supplemental irrigation water to attain optimal sweet potato yields. Similarly, the test crops paraded WUE values of 39.8 and 30.0 kg ha<sup>-1 </sup>mm<sup>-1 </sup>and water productivity indices based on rain water were 1.11 and 0.95 kg m<sup>−3</sup> for Kabode and Bungoma varieties, respectively. HI for S (I) were 40.8 and 35.4% whilst 54.2 and 46.8 % for Kabode and Bungoma varieties, respectively. <strong>Implications:</strong> WUE values of the sweet potato crop increased from warm-dry season S (I) to the warm-wet season S (II), as Kabode portrayed a higher adaptability. These results provide an acumen for decision making in the setting of climate change.<strong> Conclusions:</strong> Kabode variety differed significantly with Bungoma variety in their abilities to efficiently use water, thus portraying its adaptability in such a peculiar environment. The climatic environment: dew point temperatures and saturation pressure deficit had no significant impact on sweet potato water use efficiency.</p>

Determining the Contributions of Vegetation and Climate Change to Ecosystem WUE Variation over the Last Two Decades on the Loess Plateau, China

Exploring the variations in the water use efficiency (WUE) is helpful in gaining an in-depth understanding of the regional carbon and water cycles on the Chinese Loess Plateau (CLP). Here, we employed the spatial variations in the WUE and the quantitative contributions of the influencing factors, including the precipitation (P), temperature (Temp), vapor pressure deficit (VPD), sunshine duration (SD), and leaf area index (LAI), with the drought index varying over the last two decades. Results showed that the multiyear average WUE decreased significantly as the drought index increased for all of the vegetation types. Per-pixel interannual variability of WUE trend was 0.024 gC·m−2·mm−1·year−1. As the drought index increased, the WUE initially increased and then decreased for the forests, grassland, and shrubland, and their peaks occurred at drought index values of 2.60–3.10. Among the influencing factors, the WUE was predominantly controlled by the LAI, with an impact and relative contribution of 0.014 gC·m−2·mm−1·year−1 and 58.3%, respectively. The P and SD contributed the least to the trend in WUE, and impact and relative contribution of both were 0.001 gC·m−2·mm−1·year−1 and 4.17%. Our study also demonstrated that the LAI was the dominant factor affecting the WUE trends for grassland and the Yan River due to the structural parameters and geographical location. In addition, the impact and relative contribution of the residual factors on the WUE trend were 0.004 gC·m−2·mm−1·year−1 and 16.7%. Our findings suggested that comprehensive effects such as micro-geomorphic changes and nitrogen deposition could not be ignored except for vegetation and climate change. This study will clarify the spatial and temporal evolution of WUE and its influence mechanism.

Quantifying Soil Moisture Impacts on Water Use Efficiency in Terrestrial Ecosystems of China

Soil moisture (SM) significantly affects the exchange of land surface energy and the stability of terrestrial ecosystems. Although some conclusions have been drawn about the effects of SM on the ecosystem water use efficiency (WUE), the influence mechanism and the quantitative assessment framework of SM on WUE are still unclear. This study provides an analysis framework for the feedback relationship between SM and WUE based on the dependence of the evaporation fraction on SM and output datasets from remote sensing and the Global Land Data Assimilation System. The results show that the range of WUE of terrestrial ecosystems of China was 0.02–19.26 g C/kg H2O in the growing season with an average value of 1.05 g C/kg H2O. They also show a downward trend in 43.99% of the total area. In the evapotranspiration (ET) pathway, SM negatively affected WUE, and the sensitivity coefficient ranged from −18.49 to −0.04. In the net primary production (NPP) pathway, the sensitivity coefficient ranged from −68.66 to 43.19. Under the dual effects of the ET and NPP pathways, the influence of SM on WUE was negative in 84.62% of the area. Variation in SM led to significant WUE variability. Generally, the percentage change in WUE (ΔWUE) ranged from 0% to 190.86%, with an average value of 28.02%. The maximum ΔWUE ranged from 0% to 758.78%, with an average value of 109.29%. The WUE of forest ecosystems showed strong resistance to SM variation, whereas that of non-forest vegetation was more sensitive to SM variation. This analytical framework provides a new perspective on the feedback relationship between WUE and SM in terrestrial ecosystems.

Intracultivar genetic diversity in grapevine: Water use efficiency variability within cv. Grenache.

The selection of genotypes best adapted to environmental conditions has traditionally focused on agronomic and grape composition parameters. However, to classify the genotypes most adapted to climate change conditions, the aim must be to focus on the ecophysiological responses that will ultimately determine their performance. The variability in water use efficiency of 13 Grenache genotypes over three-seasons was assessed under field conditions at leaf, grape and plant level. Results showed a significant effect of genotype at all three levels, and despite the large interannual variability there was a remarkable consistency among levels. Furthermore, using genotype-specific regressions it was possible to identify significant differences in the intrinsic water use efficiency response of each genotype as a function of the vine water status. The relationship between net photosynthesis and stomatal conductance, as well as carbon isotope discrimination in grapes, were also confirmed as reliable physiological indicators for selecting grapevine genotypes to future environmental conditions. Therefore, the proposed multi-level methodology was useful to quantify the intracultivar variability and the identification of more and less efficient genotypes within Grenache.

Drought occurrence and time‐dominated variations in water use efficiency in an alpine meadow on the Tibetan Plateau

AbstractUnder global climate change, more frequent and widespread droughts have dramatic impacts on the water and carbon cycles. Water use efficiency (WUE) is a commonly used indicator reflecting the trade‐off relationship between carbon sequestration and water consumption. Therefore, this study aims to clarify the variations in WUE and the effects of drought on WUE in sensitive alpine meadows on the Tibetan Plateau. Based on 6‐year (2013–2018) continuous carbon and water flux data observed by the eddy covariance system, WUE was calculated by gross ecosystem productivity (GEP)/evapotranspiration (ET). WUE followed a unimodal variation with time during growing seasons in the years without droughts. However, drought disrupted this pattern by its effects on the coupled relationship between carbon and water fluxes. Specifically, droughts showed divergent effects on WUE variations in different periods of the growing seasons. In the early‐growing seasons, droughts induced an increase in WUE. During this period, WUE was mainly regulated by temperature and leaf area index (LAI), and the GEP was low due to the smaller LAI. Therefore, the drought‐induced reduction in GEP was less than that in ET; thus, WUE increased. In the mid‐growing seasons, droughts depressed WUE. WUE was dominated by water conditions and the LAI during this period. When drought occurred, GEP decreased faster than ET, and hence, WUE decreased. In the late‐growing season, WUE was mainly driven by temperature and the LAI, but short‐term drought slightly enhanced WUE. These results are helpful in understanding the responses of fragile alpine ecosystems to future climate change.

Lengthening single-stem rotation improves biomass yield and water use efficiency in black poplar genotype multi-stem rotation coppice plantations

Poplar short rotation coppice (SRC) plantations have great potential for supplying environmentally friendly bio-based industries. However, little research has focussed on the linkages between SRC management regimes and the consumption of water for biomass production in the Mediterranean environment. Therefore, we compared six hybrid clones and four native black poplar genotypes with an aim to examine how two different lengthening periods (3 vs. 5 years) of single-stem rotation affected growth performance in the following three years of multi-stem rotation coppice. To achieve this goal, we assessed the aboveground dry biomass production and variation in water use efficiency (WUE) of the genotypes annually. A longer single-stem rotation increased biomass productivity and WUE in the multi-stem rotation of the native black poplar, rather than that of the hybrid genotypes. In contrast, biomass and WUE performances did not diverge between the native and hybrid genotypes under the shorter single-stem phase. These findings underline the importance of lengthening the rotation of single-stem SRC plantations in hot and dry Mediterranean climates. Native black poplar genotypes managed in SRC should be strongly considered as environmentally compatible genetic resources both in protected areas and in areas where water supply constrains biomass production.

Spatiotemporal variability of ecosystem water use efficiency in northern Ethiopia during 1982–2014

Water use efficiency (WUE) measures the trade-off between carbon gain and water loss and is an important link between the carbon and water cycles. A better understanding of spatiotemporal variation in WUE and its controlling factors will help to improve ecosystem management for adapting to and mitigating the impacts of climatic change. We examined the spatiotemporal variations of WUE and its controlling factors for five bioclimatic zones and four land covers for 1982–2014 in the Tekeze River basin in northern Ethiopia. We quantified mean annual and seasonal net primary productivity (NPP), actual evapotranspiration (ETa), and WUE (NPP/ETa) during this period and found that each increased significantly. NPP, ETa, and therefore WUE varied spatially amongst the bioclimatic zones and land covers, but with different patterns. WUE was higher in semi-arid than humid zones. Spatiotemporal changes in temperature and precipitation could be important controls on NPP and ETa variations and therefore WUE. Mild and severe droughts in the southern regions of the basin decreased annual WUE, and moderate and extreme droughts slightly increased it, indicating that the measure of environmental rehabilitation implemented since the 1980s have increased NPP in these regions, despite the water stress caused by frequent droughts. The variability of WUE was primarily controlled by climatic change and human activity, the latter dominant since the 2000s. Ecological restoration and measures for conserving soil and water could substantially increase the vegetation productivity of this region, and be a viable management strategy to both adapt to and mitigate the impacts of climate change.

On the Treatment of Soil Water Stress in GCM Simulations of Vegetation Physiology

Current land surface schemes in weather and climate models make use of the so-called coupled photosynthesis–stomatal conductance (A–gs) models of plant function to determine the surface fluxes that govern the terrestrial energy, water and carbon budgets. Plant physiology is controlled by many environmental factors, and a number of complex feedbacks are involved, but soil moisture control on root water uptake is primary, particularly in sub-tropical to temperate ecosystems. Land surface models represent plant water stress in different ways, but most implement a water stress factor, β, which ranges linearly (more recently also curvilinearly) between β = 1 for unstressed vegetation and β = 0 at the wilting point, expressed in terms of volumetric water content (θ). β is most commonly used to either limit A or gs, and hence carbon and water fluxes, and a pertinent research question is whether these treatments are in fact interchangeable. Following Egea et al. (Agricultural and Forest Meteorology, 2011, 151 (10), 1,370–1,384) and Verhoef et al. (Agricultural and Forest Meteorology, 2014, 191, 22–32), we have implemented new β treatments, reflecting higher levels of biophysical complexity in a state-of-the-art LSM, Joint UK Land Environment Simulator, by allowing root zone soil moisture to limit plant function non-linearly and via individual routes (carbon assimilation, stomatal conductance, or mesophyll conductance) as well as any (non-linear) combinations thereof. The treatment of β does matter to the prediction of water and carbon fluxes: this study demonstrates that it represents a key structural uncertainty in contemporary LSMs, in terms of predictions of gross primary productivity, energy fluxes and soil moisture evolution, both in terms of climate means and response to a number of European droughts, including the 2003 heat wave. Treatments allowing ß to act on vegetation fluxes via stomatal and mesophyll routes are able to simulate the spatiotemporal variability in water use efficiency with higher fidelity during the growing season; they also support a broader range of ecosystem responses, e.g., those observed in regions that are radiation limited or water limited. We conclude that current practice in weather and climate modelling is inconsistent, as well as too simplistic, failing to credibly simulate vegetation response to soil water stress across the typical range of variability that is encountered for current European weather and climate conditions, including extremes of land surface temperature and soil moisture drought. A generalized approach performs better in current climate conditions and promises to be, based on responses to recently observed extremes, more trustworthy for predicting the impacts of climate change.

Understanding the peak growing season ecosystem water‐use efficiency at four boreal fens in the Athabasca oil sands region

AbstractEcosystem water‐use efficiency (WUE), a ratio between gross ecosystem production (GEP) and water loss through evapotranspiration (ET) can be helpful for the assessment of coupled peatland carbon and water cycles under anthropogenic changes in the Athabasca oil sands region (AOSR) where extensive oil sands development has been occurring since the 1960's. As such, this study assessed multiyear peak growing season variability of WUE at four fens (poor treed, poor open, treed moderate‐rich, open saline) near Fort McMurray using the eddy covariance technique combined with a set of environmental variables. Freshwater fens were characterized by WUE values within the range reported from other boreal wetlands while a saline fen had significantly lower values of WUE. Negative correlation (Rs < −0.55, p < 0.05) between WUE and net radiation was observed. Moisture conditions were responsible for interannual differences in WUE, whereby increasing WUE under wetter conditions was observed. However, such a pattern was offset by decreased air temperature (Tair) resulting in moisture oversupply. This study also revealed a negative effect of wildfire on WUE due to a prominent decline in GEP and a moderate decrease in ET. WUE can be useful for monitoring the functioning of natural and constructed fens, but a better understanding of WUE variability under a wide range of climatic conditions with respect to differences in vegetation is required.

Drought Affected Ecosystem Water Use Efficiency of a Natural Oak Forest in Central China

Global climate models project more frequent drought events in Central China. However, the effect of seasonal drought on ecosystem water use efficiency (WUE) and water regulation strategy in Central China’s natural forests is poorly understood. This study investigated variations in WUE associated with drought in a natural oak (Quercus aliena) forest in Central China from 2017 to 2020 at several timescales based on continuous CO2 and water vapor flux measurements. Results showed that the 4-year mean gross ecosystem production (GEP), evapotranspiration (ET) and WUE of the natural oak forest was 1613.2 ± 116 g Cm−2, 637.8 ± 163.3 mm and 2.6 ± 0.68 g Ckg−1 H2O, with a coefficient of variation (CV) of 7.2%, 25.6% and 26.4%, respectively. The inter-annual variation in WUE was large, primarily due to the variation in ET caused by seasonal drought. Drought increased WUE distinctly in summer and decreased it slightly in autumn. During summer drought, surface conductance (gs) usually decreased with an increase in VPD, but the ratios of stomatal sensitivity (m) and reference conductance (gsref) were 0.21 and 0.3 molm−2s−1ln(kPa)−1 in the summer of 2019 and 2020. Strong drought can also affect ecosystem WUE and water regulation strategy in the next year. Decrease in precipitation in spring increased annual WUE. These results suggested that drought in different seasons had different effects on ecosystem WUE. Overall, our findings suggest that the natural oak forest did not reduce GEP by increasing WUE (i.e., reducing ET) under spring and summer drought, which could be due to its typical anisohydric characteristics, although it can also reduce stomatal opening during long-term drought.

Transition in multi-dimensional leaf traits and their controls on water use strategies of co-occurring species along a soil limiting-resource gradient

Water use efficiency is among the most important eco-physiological strategies for species adaptation to water-limiting habitats. However, understanding of how co-occurring species adopt diverse water use strategies to cope with water- and resource-limited soil habitats via the adaptation of leaf functional traits remains limited. Here, leaf δ13C and δ18O were used as proxies for water use efficiency (WUE) and stomatal conductance (gs) across a soil limiting-resource gradient from a non-karst primary forest to karst communities including primary forest, secondary forest, shrubland, and grassland. Leaf apparent morphology, stomatal morphology, macro- and micro-nutrients and stoichiometry were measured. We found that there were significant differences in soil physical and chemical properties across five communities, forming a soil limiting-resource gradient. Co-occurring species in each community segregated along a continuous and wide range of WUE and gs, revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies. Multiple leaf functional traits separated into two principal components: leaf economics spectrum (LES) and leaf limiting-resource spectrum (LLRS). As the stress intensity of soil limiting-resource increases, the number of composition traits of LLRS increased and the dominant traits of LLRS shifted from leaf-thickness to macro-nutrients. The major driver of gs variability in each community was LES. Variability in WUE in forests was mainly determined by LES, in shrubland by LLRS, and in grassland by neither LES nor LLRS. Our study illustrated for the first time how soil limiting-resource induced changes in multi-dimensional leaf traits with effects on water use strategies of co-occurring species: the variability in WUE was mainly controlled by the leaf economic spectrum in mild soil limiting-resource habitats; however, that effect was replaced or offset by leaf limiting-resource spectrum in moderate or severe soil limiting-resource habitats.

Fifteen-year Variations of Water Use Efficiency over a Wheat-Maize Rotation Cropland in the North China Plain

Based on fifteen-year observations, we explored the interannual variations of five water use efficiency (WUE) indicators (i.e., standard ecosystem WUE (EWUE), plant-canopy WUE (PWUE), inherent WUE (IWUE), underlying WUE (uWUE) and stomatal slope (G1)) over a typical winter wheat-summer maize rotation cropland in the North China Plain. The results showed that none of WUE indicators had a significant interannual trend. PWUE showed the lowest interannual variability and could be regarded as stationary for these two crops. G1 was stationary for wheat which was under an essentially constant soil water condition, whereas exhibited a strong fluctuation for maize experiencing more variable soil water content (SWC) than wheat. IWUE and uWUE were non-stationary for both crops. SWC and leaf area index (LAI) dominated EWUE for maize and wheat, respectively. PWUE was dominated by the shortwave solar radiation (Rg) in maize seasons and not significantly affected by any meteorological or biophysical factors in wheat seasons. For IWUE and uWUE, air temperature (Ta) and LAI were the most important drivers in maize and wheat seasons, respectively. Atmospheric CO2 concentration (Ca) and vapor pressure deficit (VPD) were not revealed as important controllers. These findings reflected that the meteorological and biophysical regulatory mechanisms of WUE indicators at the ecosystem level were quite different from that at the leaf level. But G1 was an exception due to that for both maize and wheat, G1 was only significantly affected by SWC. Moreover, we found that none of the remote sensing (RS)-based global products performed satisfactorily on capturing interannual variations of EWUE for both crops. These results cautioned that using these global RS-based products to explore the interannual variations of EWUE for the cropland ecosystem may lead to substantial bias.

Drivers of Natural Variation in Water-Use Efficiency Under Fluctuating Light Are Promising Targets for Improvement in Sorghum.

Improving leaf intrinsic water-use efficiency (iWUE), the ratio of photosynthetic CO2 assimilation to stomatal conductance, could decrease crop freshwater consumption. iWUE has primarily been studied under steady-state light, but light in crop stands rapidly fluctuates. Leaf responses to these fluctuations substantially affect overall plant performance. Notably, photosynthesis responds faster than stomata to decreases in light intensity: this desynchronization results in substantial loss of iWUE. Traits that could improve iWUE under fluctuating light, such as faster stomatal movement to better synchronize stomata with photosynthesis, show significant natural diversity in C3 species. However, C4 crops have been less closely investigated. Additionally, while modification of photosynthetic or stomatal traits independent of one another will theoretically have a proportionate effect on iWUE, in reality these traits are inter-dependent. It is unclear how interactions between photosynthesis and stomata affect natural diversity in iWUE, and whether some traits are more tractable drivers to improve iWUE. Here, measurements of photosynthesis, stomatal conductance and iWUE under steady-state and fluctuating light, along with stomatal patterning, were obtained in 18 field-grown accessions of the C4 crop sorghum. These traits showed significant natural diversity but were highly correlated, with important implications for improvement of iWUE. Some features, such as gradual responses of photosynthesis to decreases in light, appeared promising for improvement of iWUE. Other traits showed tradeoffs that negated benefits to iWUE, e.g., accessions with faster stomatal responses to decreases in light, expected to benefit iWUE, also displayed more abrupt losses in photosynthesis, resulting in overall lower iWUE. Genetic engineering might be needed to break these natural tradeoffs and achieve optimal trait combinations, e.g., leaves with fewer, smaller stomata, more sensitive to changes in photosynthesis. Traits describing iWUE at steady-state, and the change in iWUE following decreases in light, were important contributors to overall iWUE under fluctuating light.