Inherent Water Use Efficiency Research Articles

Rising water-use efficiency in European grasslands is driven by increased primary production

Water-use efficiency is the amount of carbon assimilated per water used by an ecosystem and a key indicator of ecosystem functioning, but its variability in response to climate change and droughts is not thoroughly understood. Here, we investigated trends, drought response and drivers of three water-use efficiency indices from 1995–2018 in Europe with remote sensing data that considered long-term environmental effects. We show that inherent water-use efficiency decreased by −4.2% in Central Europe, exhibiting threatened ecosystem functioning. In European grasslands it increased by +24.2%, by regulated transpiration and increased carbon assimilation. Further, we highlight modulation of water-use efficiency drought response by hydro-climate and the importance of adaptive canopy conductance on ecosystem function. Our results imply that decoupling carbon assimilation from canopy conductance and efficient water management strategies could make the difference between threatened and well-coping ecosystems with ongoing climate change, and provide important insights for land surface model development.

Inherent Water-Use Efficiency of Different Forest Ecosystems and Its Relations to Climatic Variables

Inherent water-use efficiency (IWUE) is a vital parameter connecting the carbon and water cycles. However, the factors influencing the IWUE in different forest ecosystems are still a subject of debate. In this work, FLUXNET platform measurements of 67 forest sites were used to detect trends of the IWUE of four forest ecosystems, namely deciduous broadleaf forests (DBF), evergreen broadleaf forests (EBF), needle-leaf forests (ENF), and mixed forests (MF). The IWUE differed significantly among different forest ecosystems and positively correlated with temperature and solar radiation. The IWUE of EBF was the highest at 32.02 g·C·Kg·H2O−1. The values of DBF and MF were similar and higher than that of ENF. With increasing latitude, the IWUE increased first and then decreased, with a maximum of 35° N. The IWUE of EBF was negatively correlated with precipitation and leaf area index. Temperature and solar radiation were the main factors controlling the IWUE of forest ecosystems, whereas precipitation was the major factor controlling the inter-annual variation in the ΔIWUE of forest ecosystems. Our results provide a scientific basis for the study of forest carbon sinks, forest eco-hydrological processes, and forest ecosystem responses to global climatic changes.

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.

Separating the effects of climate change and human activity on water use efficiency over the Beijing-Tianjin Sand Source Region of China

Water use efficiency (WUE) is a central parameter for linking carbon and water exchange processes in terrestrial ecosystems. The Beijing-Tianjin Sand Source Region (BTSSR) in China has undergone tremendous vegetation restoration and climate change. Understanding the WUE responses to climate change and human activity and their relative contributions to the trends and inter-annual variations (IAVs) in WUE is necessary to improve water use efficiency and strengthen water resource management. The evapotranspiration (ET) dataset based on the model tree ensemble (MTE) algorithm which was a machine learning approach using flux-tower ET measurements and the GLASS GPP dataset, as well as the variance decomposition method, were used to analyze the spatiotemporal changes in water use efficiency and inherent water use efficiency (IWUE) and the impacts of climate change and human activities. The results showed that the annual WUE and IWUE exhibited significantly increase in most regions of the BTSSR. The trend of human activity played the most important role in the increases of WUE and IWUE, with relative contributions of 88.2% and 85.9%, respectively, followed by the IAV of human activity for WUE (6.1%) and the trend of climate change (8.7%) for IWUE. The contribution of IAV to climate change was relatively small. Moreover, WUE and IWUE were all positively correlated with precipitation and temperature in most regions. Our results indicated that ecological restoration projects had significantly improved water use efficiency in BTSSR and may decrease the water burden in the BTSSR.

Performance of Solar-Induced Chlorophyll Fluorescence in Estimating Water-Use Efficiency in a Temperate Forest

Water-use efficiency (WUE) is a critical variable describing the interrelationship between carbon uptake and water loss in land ecosystems. Different WUE formulations (WUEs) including intrinsic water use efficiency (WUEi), inherent water use efficiency (IWUE), and underlying water use efficiency (uWUE) have been proposed. Based on continuous measurements of carbon and water fluxes and solar-induced chlorophyll fluorescence (SIF) at a temperate forest, we analyze the correlations between SIF emission and the different WUEs at the canopy level by using linear regression (LR) and Gaussian processes regression (GPR) models. Overall, we find that SIF emission has a good potential to estimate IWUE and uWUE, especially when a combination of different SIF bands and a GPR model is used. At an hourly time step, canopy-level SIF emission can explain as high as 65% and 61% of the variances in IWUE and uWUE. Specifically, we find that (1) a daily time step by averaging hourly values during daytime can enhance the SIF-IWUE correlations, (2) the SIF-IWUE correlations decrease when photosynthetically active radiation and air temperature exceed their optimal biological thresholds, (3) a low Leaf Area Index (LAI) has a negative effect on the SIF-IWUE correlations due to large evaporation fluxes, (4) a high LAI in summer also reduces the SIF-IWUE correlations most likely due to increasing scattering and (re)absorption of the SIF signal, and (5) the observation time during the day has a strong impact on the SIF-IWUE correlations and SIF measurements in the early morning have the lowest power to estimate IWUE due to the large evaporation of dew. This study provides a new way to evaluate the stomatal regulation of plant-gas exchange without complex parameterizations.

Linking plant functional trait plasticity and the large increase in forest water use efficiency

AbstractElevated atmospheric CO2 concentrations are expected to enhance photosynthesis and reduce stomatal conductance, thus increasing plant water use efficiency. A recent study based on eddy covariance flux observations from Northern Hemisphere forests showed a large increase in inherent water use efficiency (IWUE). Here we used an updated version of the same data set and robust uncertainty quantification to revisit these contemporary IWUE trends. We tested the hypothesis that the observed IWUE increase could be attributed to interannual trends in plant functional traits, potentially triggered by environmental change. We found that IWUE increased by ~1.3% yr−1, which is less than previously reported but still larger than theoretical expectations. Numerical simulations with the Tethys‐Chloris ecosystem model using temporally static plant functional traits cannot explain this increase. Simulations with plant functional trait plasticity, i.e., temporal changes in model parameters such as specific leaf area and maximum Rubisco capacity, match the observed trends in IWUE. Our results show that trends in plant functional traits, equal to 1.0% yr−1, can explain the observed IWUE trends. Thus, at decadal or longer time scales, trait plasticity could potentially influence forest water, carbon, and energy fluxes with profound implications for both the monitoring of temporal changes in plant functional traits and their representation in Earth system models.

The influence of hydrological variability on inherent water use efficiency in forests of contrasting composition, age, and precipitation regimes in the Pacific Northwest

The Pacific Northwest (PNW) region of the United States has some of the most productive forests in the world. As precipitation regimes may shift with changing climate in this area, droughts are predicted to increase in both frequency and degree of severity, which will have a significant impact on already drought-prone ecosystems. When modeling ecosystem responses to drought, it is important to consider the physiology of individual tree species since the variations in drought sensitivity among species is easily overlooked when plants are characterized using broad plant functional types. Here we explore the use of inherent water-use efficiency as an index of drought sensitivity in semi-arid young and mature ponderosa pine forests and a mesic mature Douglas-fir forest in the PNW. Summer maximum of an evapotranspiration-based WUE (WUEi) was 2.5 times higher in young and mature pines in semi-arid climate than Douglas-fir in mesic climate (12.2 and 11.3 versus 4.7gCkPa per kg H2O, respectively). In contrast, annually averaged WUEi was similar among the sites (2.8gCkPa per kg H2O for pines and 2.4gCkPa per kg H2O for Douglas-fir). The effect of drought stress on WUEi was most pronounced in young pine, followed by mature pine and Douglas-fir (32, 11, and 6% increase in WUEi per % decline in soil water content, respectively) which reflect differences in age-related ecosystem structure (root system, stem capacitance, and soil water holding capacity). Among sites, the responses of WUEi to climate variability were largely driven by changes in evapotranspiration (ET) compared to gross primary productivity (GPP). However, in areas where evaporation is the primary component of ET, such as the open canopy ponderosa forests of the PNW, the contribution of soil processes to ET can overshadow the reaction of vegetation transpiration (T) to changes in water availability. In these cases, utilizing a transpiration-based WUE (WUEi_T) in vegetation models will yield a more accurate representation of plant activity during drought. These results highlight the importance of incorporating differences in species- and age-related WUEi in models in diverse forest types at regional and global scales to improve predictions in ecosystem responses to climate change.

Development of a coupled carbon and water model for estimating global gross primary productivity and evapotranspiration based on eddy flux and remote sensing data

Terrestrial gross primary productivity (GPP) and evapotranspiration (ET) are two key ecosystem fluxes in the global carbon and water cycles. As carbon and water fluxes are inherently linked, knowing one provides information for the other. However, tightly coupled and easy to use ecosystem models are rare and there are still large uncertainties in global carbon and water flux estimates. In this study, we developed a new monthly coupled carbon and water (CCW) model. GPP was estimated based on the light-use efficiency (LUE) theory that considered the effect of diffuse radiation, while ET was modeled based on GPP and water-use efficiency (WUE). We evaluated the non-linear effect of single (GPPOR) or combined (GPPAND) limitations of temperature and vapor pressure deficit on GPP. We further compared the effects of three types of WUE (i.e., WUE, inherent WUE, and underlying WUE) on ET (i.e., ETWUE, ETIWUE and ETUWUE). CCW was calibrated and validated using global eddy covariance measurement from FLUXNET and remote sensing data from Moderate Resolution Imaging Spectroradiometer (MODIS) from 2000 to 2007. Modeled GPPAND and GPPOR explained 67.3% and 66.8% of variations of tower-derived GPP, respectively, while ETUWUE, ETIWUE and ETWUE explained 65.7%, 59.9% and 58.1% of tower-measured ET, respectively. Consequently, we chose GPPAND and ETUWUE as the best modeling framework for CCW, and estimated global GPP as 134.2PgCyr−1 and ET as 57.0×103km3 for vegetated areas in 2001. Global ET estimated by CCW compared favorably with MODIS ET (60.5×103km3) and ET derived from global precipitation (56.5×103km3). However, global GPP estimated by CCW was about 19% higher than MODIS GPP (109.0PgCyr−1). The mean global WUE value estimated by CCW (2.35gCkg−1H2O) was close to the mean tower-based WUE (2.60gCkg−1H2O), but was much higher than the WUE derived from MODIS products (1.80gCkg−1H2O). We concluded that the new simple CCW model provided improved estimates of GPP and ET. The biome-specific parameters derived in this study allow CCW to be further linked with land use change models to project human impacts on terrestrial ecosystem functions.

Daily underlying water use efficiency for AmeriFlux sites

Water use efficiency (WUE) is a crucial parameter to describe the interrelationship between gross primary production (GPP) and evapotranspiration (ET). Incorporating the nonlinear effect of vapor pressure deficit (VPD), underlying WUE (uWUE = GPP · VPD0.5/ET) is better than inherent WUE (IWUE = GPP · VPD/ET) at the half-hourly time scale. However, appropriateness of uWUE has not yet been evaluated at the daily time scale. To determine whether uWUE is better than IWUE, daily data for seven vegetation types from 34 AmeriFlux sites were used to validate uWUE at the daily time scale. First, daily mean VPD was shown to be a good substitute for the effective VPD that was required to preserve daily GPP totals. Second, an optimal exponent, k*, corresponding to the best linear relationship between GPP · VPDk* and ET, was about 0.55 both at half-hourly and daily time scales. Third, correlation coefficient between GPP · VPDk and ET showed that uWUE (k = 0.5 and r = 0.85) was a better approximation of the optimal WUE (k = k* and r = 0.86) than IWUE (k = 1 and r = 0.81) at the daily scale. Finally, when yearly uWUE was used to predict daily GPP from daily ET and mean VPD, uWUE worked considerably better than IWUE. Comparing observed and predicted daily GPP, the average correlation coefficient and Nash-Sutcliffe coefficient of efficiency were 0.81 and 0.59, respectively, using yearly uWUE, and only 0.59 and −0.83 using yearly IWUE. As a nearly optimal WUE, uWUE consistently outperformed IWUE and could be used to evaluate the effects of global warming and elevated atmosphere CO2 on carbon assimilation and evapotranspiration.

Change in terrestrial ecosystem water-use efficiency over the last three decades.

Defined as the ratio between gross primary productivity (GPP) and evapotranspiration (ET), ecosystem-scale water-use efficiency (EWUE) is an indicator of the adjustment of vegetation photosynthesis to water loss. The processes controlling EWUE are complex and reflect both a slow evolution of plants and plant communities as well as fast adjustments of ecosystem functioning to changes of limiting resources. In this study, we investigated EWUE trends from 1982 to 2008 using data-driven models derived from satellite observations and process-oriented carbon cycle models. Our findings suggest positive EWUE trends of 0.0056, 0.0007 and 0.0001g Cm(-2) mm(-1) yr(-1) under the single effect of rising CO2 ('CO2 '), climate change ('CLIM') and nitrogen deposition ('NDEP'), respectively. Global patterns of EWUE trends under different scenarios suggest that (i) EWUE-CO2 shows global increases, (ii) EWUE-CLIM increases in mainly high latitudes and decreases at middle and low latitudes, (iii) EWUE-NDEP displays slight increasing trends except in west Siberia, eastern Europe, parts of North America and central Amazonia. The data-driven MTE model, however, shows a slight decline of EWUE during the same period (-0.0005g Cm(-2) mm(-1) yr(-1) ), which differs from process-model (0.0064g Cm(-2) mm(-1) yr(-1) ) simulations with all drivers taken into account. We attribute this discrepancy to the fact that the nonmodeled physiological effects of elevated CO2 reducing stomatal conductance and transpiration (TR) in the MTE model. Partial correlation analysis between EWUE and climate drivers shows similar responses to climatic variables with the data-driven model and the process-oriented models across different ecosystems. Change in water-use efficiency defined from transpiration-based WUEt (GPP/TR) and inherent water-use efficiency (IWUEt , GPP×VPD/TR) in response to rising CO2 , climate change, and nitrogen deposition are also discussed. Our analyses will facilitate mechanistic understanding of the carbon-water interactions over terrestrial ecosystems under global change.

Five years of carbon fluxes and inherent water-use efficiency at two semi-arid pine forests with different disturbance histories

ABSTRACTFive years of eddy-covariance and other measurements at a mature ponderosa pine forest and a nearby young plantation are used to contrast the carbon fluxes for long-term averages, seasonal patterns, diel patterns and interannual variability, and to examine the differing responses to water-stress. The mature forest with larger leaf area and wetter and cooler soils has a net uptake of carbon 3.3 times that of the young plantation. In the spring, photosynthesis is larger at the mature site as expected based on the difference in leaf area, however, another important factor is the reduction in springtime respiration at the mature site due to lower soil temperatures because of more shade from the canopy. Patterns of photosynthesis, inherent water-use efficiency (IWUE) and tree transpiration indicate that the young plantation responds to the seasonal drought sooner and to a more severe degree. Lower sensitivity to seasonal drought at the mature site is likely due to higher soil moisture reserves year round and a deeper root system that can access more water. Outside the seasonal drought period, the IWUE is the same at both sites, suggesting a species-specific value. Larger interannual variability at the plantation is associated with water-year drought and aggrading.

The effects of drought and timing of precipitation on the inter-annual variation in ecosystem-atmosphere exchange in a Mediterranean grassland

We studied the seasonal and inter-annual variation in carbon, water and energy fluxes over a Mediterranean grassland ecosystem in Portugal, dominated by annual species, using the eddy-covariance technique. The study period, from 2004 to 2008, was characterised by high intra- and inter-annual precipitation variation: the hydrological years 2004–2005 and 2007–2008 had precipitation below average (‘dry’ years) whereas the other two hydrological years had precipitation above average (‘wet’ years). The variation in energy partitioning into sensible heat ( H) and latent heat ( λE) fluxes was associated with changes in soil water content, coinciding with plant emergence and senescence. The ecosystem was λE-dominated in those periods with soil water content above 15%, and became H-dominated when soil water content fell below 15%. Annually integrated net ecosystem carbon exchange (NEE) was highly sensitive to drought, varying from a carbon sink in ‘wet’ years (e.g., 2005–2006, −190 g C m −2 y −1), to a carbon source in ‘dry’ years (e.g., 2004–2005, +49 g C m −2 y −1). There was a negative correlation between annually integrated NEE and annual precipitation ( r 2 = 0.78). However, timing of precipitation influenced NEE, with individual rain events in the dry season resulting in large pulses of ecosystem respiration ( R eco). Low precipitation at the peak of the growing season, as in the spring of 2007, decreased carbon sequestration. The best descriptor for inter-annual variation in NEE was leaf area duration (LAD), which explained 96% of the variance. Average radiation-use efficiency (RUE) was 1.59 g MJ −1 and inherent water-use efficiency (IWUE) was 24.6 g C hPa kg −1 H 2O. RUE and IWUE had similar trends in inter-annual variation, reflecting differences in productivity and timing of precipitation. Our study showed that carbon and energy fluxes in this Mediterranean environment depended strongly upon water availability.

The effect of atmospheric humidity on stomatal control of gas exchange in two tropical coniferous species

Stomatal response to changes in leaf to air vapor pressure difference (VPD) and its influence on total gas exchange were measured for leaves of Podocarpus oleifolius and Podocarpus rospigliosii in an open gas exchange system. Stomatal conductance (g) of both species declined as VPD increased. Once a critical VPD was attained, the decrease in g was sufficient to decrease transpiration rate in spite of increasing VPD. This response pattern suggested "feedforward" control of stomatal response to humidity rather than negative feedback control based on changes in leaf water status. Coupling between CO2 assimilation and g and inherent water-use efficiency were greater in P. oleifolius. Stomatal response to humidity in P. oleifolius was consistent with a recent hypothesis that stomata optimize CO2 assimilation with respect to a given level of water loss by maintaining constant the ratio of the sensitivities of transpiration rate (E) and assimilation rate (A) to changes in g (∂E/∂g/∂A/∂g). This gain ratio did not remain constant in P. rospigliosii as VPD was varied. The possible ecological basis for these differences in gas exchange behavior is discussed.