Variation In Water Use Efficiency Research Articles

District-level assessment of the ecohydrological resilience to hydroclimatic disturbances and its controlling factors in India

The carbon and water cycles play an important role in ecosystem functioning and are linked to each other through different physical and biological processes. The hydroclimatic disturbances such as droughts affect both hydrological as well as the ecological processes. Increasing hydroclimatic disturbances under climate change will adversely affect the ecohydrological processes and hence, the assessment of the ecohydrological resilience and its controlling factors is important for the sustainability of the ecosystems. In this study, an assessment of the resilience of terrestrial ecosystems in India to hydroclimatic disturbances was carried out at the district (i.e. administrative division) scale. Ecosystem water use efficiency (WUEe), defined as the ratio of net primary productivity (NPP) to evapotranspiration, was used as an indicator of ecosystem functioning or its response to hydroclimatic disturbances. We found a large spatial variation in WUEe in India at district scale, which was significantly higher in lower Himalayan regions compared to rest of the country. Increasing trend in WUEe was found for central parts of the country. The resilience was measured in terms of the ratio of the WUEe under the dry conditions and the mean WUEe, which indicates the ability to absorb hydroclimatic disturbance. Out of 634 districts considered for this study, only 241 (~38%) districts were found resilient to dry conditions, whereas a significant reduction in WUEe was observed for some of the districts. The resilience at district scale indicates the cross-biomes response of ecosystems. In general, the forest dominated districts had higher resilience compared to districts dominated by other biome types. Also, districts having temperate climate were found having higher resilience. Out of 30 states and union territories (UTs) only 10 states had more than 50% resilient area. The results of this study highlight the need for better ecosystem management policies in the country.

Differences in ecosystem water-use efficiency among the typical croplands

Water use efficiency (WUE) is an important parameter to assess agricultural production and the reasonable utilization of water resources. Especially in the context of changing hydrological environment, more attention need to be paid on how to use limited water resource to improve crop yield for ensuring food security. Based on 33 site-years of flux measurements over 10 cropland sites using the eddy covariance (EC) technique, the study systematically evaluated the large differences in seasonal and interannual variations of gross primary productivity (GPP), evapotranspiration (ET) and ecosystem WUE across the four crops worldwide including soybean, maize, winter wheat and paddy rice. The lengths of the growing seasons across the main crops extracted from time-series MODIS NDVI data, implied that the longest growth period in winter wheat and the shortest growing season in paddy rice field. Further analyses suggest that maize cropland has the strongest ecosystem WUE with 2.48 ± 0.69 g C kg−1 H2O, followed by winter wheat (2.00 ± 0.39 g C kg−1 H2O), soybean (1.92 ± 0.52 g C kg−1 H2O), and paddy rice (1.88 ± 0.63 g C kg−1 H2O). Meanwhile, the variability in ecosystem WUE exhibited apparent seasonality, and peaked together with GPP in the most active summertime. A series of biotic and abiotic factors affected the GPP as well as WUE variability. Given the complicated interactions among these environmental factors, this study revealed the great potential to remotely retrieve the WUE variability using time-series MODIS NDVI data over large areas. Ecosystem WUE of the C4 crop –maize was obviously higher than the other three C3 crops. Engineering C4 feature into C3 crops may be a feasible way to increase photosynthesis and yield.

Ten-year variability and environmental controls of ecosystem water use efficiency in a rainfed maize cropland in Northeast China

As a critical link between carbon and water cycles, water use efficiency (WUE) is an important metric for assessing ecosystem response to climate change. However, the controlling mechanism of WUE is still unclear because a number of environmental factors are usually cross-correlated in natural systems. Using the structural equation modeling (SEM) method, this study investigated the seasonal and inter-annual variations of WUE and their controlling mechanism based on 10 years of eddy covariance data in a rainfed maize cropland. The results showed that annual WUE varied between 2.1 g C kg−1H2O and 3.6 g C kg−1H2O, with a multi-year mean of 2.8 g C kg−1H2O. At a daily timescale, leaf area index (LAI) was the primary controlling factor of WUE, while air temperature (Ta) and vapor pressure deficit (VPD) were both shown to have significant indirect effects on WUE through regulating LAI. Distinct controlling mechanisms of daily WUE were detected for years with different hydro-climatic conditions. The primary controlling factors were generally consistent in the 3 dry years (LAI, Ta and VPD). Moreover, WUE consistently showed negative response to soil water content in dry years, while an opposite response was found in wet years. Mean annual Ta was shown to explain 54% of the inter-annual variation of WUE. Despite the insignificant relationship between WUE and precipitation amount at an annual scale, the precipitation frequency was found to be a good predictor of the annual WUE. These findings provide important implications for the accurate simulation of ecosystem WUE, especially under drought conditions in semi-arid regions.

Spatio-temporal variations in water use efficiency and its drivers in China over the last three decades

Ecosystem water use efficiency (WUE) reflects the intimately coupled relationship between the carbon and water cycles in terrestrial ecosystems. However, the inter-annual variation and its drivers of WUE are poorly understood at regional/global scale, due to either limited data availability or uncertainties in current data streams. In this study, we used process-based models simulated gross primary productivity (GPP) and evapotranspiration (ET) data to estimate the ecosystem WUE (eWUE, GPP/ET) in China for 1979–2012. The eWUE estimates were validated against eddy covariance-based values from 35 flux towers. The inter-annual variation of the eWUE was quantified and its responses to annual precipitation (AP), annual mean temperature (AMT), and annual mean leaf area index (AMLAI) were analyzed. The key findings were as follows. (i) The mean annual eWUE over China was 1.48 ± 1.04 g C kg−1 H2O and had a slightly increasing but not significant trend (7.32 × 10−4 g C kg−1 H2O yr−1, p < 0.05) from 1979 to 2012. (ii) The spatial distribution of the eWUE trend showed large spatial variability. ∼21.4% and ∼19.0% of vegetated land in China had significant increasing and decreasing trends (Mann-Kendall test, p < 0.1), respectively. The increasing eWUE was mainly found in the northeast, southwest, and central areas of China, while the decreasing eWUE was mostly distributed in west China. (iii) The inter-annual variation of the spatially averaged annual eWUE was negatively correlated with that of AP and AMT, and positively correlated with that of AMLAI. In ∼41.4%, ∼9.9%, and ∼3.1% of vegetated land in China the inter-annual variation of eWUE was dominated by the inter-annual variations of AP, AMT, and AMLAI, respectively. In most land of north China and west China the inter-annual variation of eWUE was dominated by the inter-annual variation of AP, while in central, east and south China all the AP, AMT, AMLAI, and other drivers played important roles.

Effects of water stress on water use efficiency of irrigated and rainfed wheat in the Loess Plateau, China

The Loess Plateau, the largest arid and semi-arid zone in China, has been confronted with more severe water resource pressure and a growing demand for food production under global changes. For developing sustainable agriculture in this region, it is critical to learn spatiotemporal variations in water use efficiency (WUE) of main crops (e.g. winter wheat in this region) under various water management practices. In this study, we classified irrigated and rainfed wheat areas based on MODIS data, and calculated the winter wheat yield by using an improved light use efficiency model. The actual evapotranspiration (ETa) of winter wheat and the evapotranspiration drought index (EDI) were also investigated. Then we mainly examined the synergistic relationship between crop yield, ETa, and WUE, and analyzed the variations in WUE of irrigated and rainfed wheat under water stress during the 2010–2011 growing season. The results suggested that winter wheat in the Loess Plateau was primarily dominated by rainfed wheat. The average yield of irrigated wheat was 3928.4 kg/ha, 22.2% more than that of rainfed wheat. High spatial heterogeneities of harvest index (HI) and maximum light use efficiency (εmax) were found in the Loess Plateau. The ETa of irrigated wheat was 10.2% more than that of rainfed wheat. The ratio of irrigated and rainfed wheat under no water stress was 31.55% and 17.16%, respectively. With increasing water stress, the WUE of rainfed wheat decreased more quickly than that of irrigated wheat. The WUE variations in winter wheat under water stress depended strongly on the synergistic effects of two WUE components (crop yield and ETa) and their response to environmental conditions as well as water management practices (irrigated or rainfed). Our findings enhance our current understanding of the variations in WUE as affected by water stress under various water use conditions in arid and semi-arid areas.

Evaluation of leaf carbon isotopes and functional traits in avocado reveals water-use efficient cultivars

Plant water-use efficiency (WUE) describes the ratio of carbon gain to water loss during photosynthesis. It has been shown that WUE varies among crop genotypes, and crops with high WUE can increase agricultural production in the face of finite water supply. We used measures of leaf carbon isotopic composition to compare WUE among 24 cultivars of Persea americana Mill (avocado) to determine genotypic variability in WUE, identify potentially efficient cultivars, and to better understand how breeding for yield and fruit quality has affected WUE. To validate carbon isotope measurements, we also measured leaf photosynthetic gas exchange of water and carbon, and leaf and stem functional traits of cultivars with the highest and lowest carbon isotope composition to quantify actual WUE ranges during photosynthesis. Our results indicate large variation in WUE among cultivars and coordination among functional traits that structure trade-offs in water loss and carbon gain. Identifying cultivars of subtropical tree crops that are efficient in terms of water use is critical for maintaining a high level of food production under limited water supply. Plant functional traits, including carbon isotopes, appear to be an effective tool for identifying species or genotypes with particular carbon and water economies in managed ecosystems.

Characteristics of ecosystem water use efficiency in a desert riparian forest

The Ejin oasis, located on the river sides of a hyper-arid region, represents some of the richest desert ecosystems, populated with riparian forest. Water availability is the main limited factors for the distribution and growth of the forest. Currently, the water–carbon process and mechanisms in the desert riparian forest remain uncertain. The study investigated the seasonal and inter-annual variations in the gross primary productivity (GPP), evapotranspiration (ET) and ecosystem water use efficiency (WUE), and their relationships with the event (e.g., irrigation) and climatic factors. The results showed that the desert riparian forest produced new plant biomass with 0.79 g C kg−1 of water loss during the growing season. In the study period, monthly GPP and ET was tightly coupled, and the mean cumulative GPP and ET from May–October were 676 g C m−2 and 864 mm, respectively. Daily GPP and ET had similar relationships with global radiation (Rg), air temperature (Ta) and vapor pressure deficit at monthly scale. Consequently, the seasonal variations in WUE were not obvious in the growing season, except in September. In September, the value of WUE decreased significantly with an increase in Ta (for Ta > 15 °C) after irrigation, which was attributable to the increased ET and a relatively steady GPP. It is possible that the time of irrigation is able to affect the seasonal course of WUE. The study has implications that enable more rational water management to maintain the the health and activities of the riparian forest in such a hyper-arid region.

Heavy mowing enhances the effects of heat waves on grassland carbon and water fluxes

Heat waves (HWs) are a type of extreme weather event that is of growing concern in the scientific community. Yet field data based on sound experiment on the variation of ecosystem CO2 levels under HWs remain rare. Additionally, ecosystems react to HWs and the coupled human activities (such as grazing in grasslands) are unknown. Thus, a 3-year field experiment was conducted to simulate HWs in conjunction with different mowing intensities that mimicking grazing in a Stipa krylovii steppe on the Mongolian Plateau. HWs significantly decreased ecosystem exchange (NEE) of CO2, ecosystem respiration (Re) and gross ecosystem productivity (GEP) by 31%, 5% and 16%, respectively, over the three years. Continuous HWs over multiple years produced cumulative effects by reducing NEE at 20%, 34% and 40% in the first, second and third HW years, respectively. During three pre-defined three periods of HWs (during HW period, after HW period in the same year, and after HW period in the next year), variations in water use efficiency indicated that the grassland ecosystem exhibited a strategy for adapting to the continuous HWs to a certain extent, by adjusting community structure or increasing litter biomass. Finally, mowing increased the effects of HWs by extending the legacy effect, such that restoration of the grassland required a greater amount of time under the combination of HWs and mowing.

Genome‐Wide Association Mapping of Carbon Isotope and Oxygen Isotope Ratios in Diverse Soybean Genotypes

Water deficit stress is a major factor limiting soybean [Glycine max (L.) Merr.] yield. High water use efficiency (WUE) offers a means to potentially ameliorate drought impact, but increased WUE is often associated with a reduction in transpiration (T) and an accompanied reduction in photosynthesis. This interdependence of T and photosynthesis is a major constraint in selection for high WUE by breeding programs. Measurement of genetic variability in WUE and T through carbon isotope ratio (δ13C) and oxygen isotope ratio (δ18O), respectively, could be important in identifying genotypes with high WUE that also have relatively high T, and thus higher rates of biomass production. This study's objective was to identify genomic regions associated with δ13C and δ18O. A diverse collection of 373 soybean genotypes was grown in four field environments, and whole‐plant samples collected at early reproductive growth were characterized for δ13C and δ18O. After quality assessment, 31,260 polymorphic single‐nucleotide polymorphism (SNP) markers with a minor allele frequency ≥5% were used for association analysis. Genome‐wide association analysis identified 54 environment‐specific SNPs associated with δ13C and 47 SNPs associated with δ18O. These SNP markers tagged 46 putative loci for δ13C and 21 putative loci for δ18O and may represent an important resource for pyramiding favorable alleles for drought tolerance and identifying genotypes with high WUE.

Heritability and genetic variation of plant biomass, transpiration, and water use efficiency for an apple core-collection

To study the heritability and genetic variation of plant biomass, transpiration and water use efficiency (WUE) in apple (Malus × domestica Borkh.), 193 genotypes from an INRA apple core-collection were evaluated in 2014. Four grafted replicates per genotype grown as one-year-old scions were studied in a high-throughput phenotyping platform (PhenoArch). Individual pot weight was recorded twice a day and irrigation was scheduled to maintain constant soil water content (SWC) and avoid any water stress during 46 days. Plant 3D pictures were automatically taken every two days. Analysis of pictures and pot weights differences allowed the estimation of whole-plant biomass and transpiration during the experiment. WUE was calculated as the ratio of accumulated biomass to transpiration. For each trait, we tested nine mixed models to account for the genetic effect and spatial variability inside PhenoArch. The Best Linear Unbiased Predictors (BLUPs) of genetic values were estimated after model selection. For each trait, broad-sense heritability (H2) was calculated from variance estimates. Plant biomass, transpiration and WUE had H2 values of 0.76, 0.54, and 0.73, respectively. The H2 value of WUE is high enough to consider that this trait is under genetic control in apple. The genetic variation of WUE indicated that apple genetic resources such as the INRA core-collection could be extremely useful to improve apple plant material for the use of water.

Spatial-temporal variation of ecosystem water use efficiency in Beijing’s suburban region

Suburban ecosystem has multiple functions such as soil conservation and water regulation, which are critical for the welfare of human beings in the city. Water use efficiency (WUE) is an important indicator of ecosystem function that represents the amount of productivity per unit mass of evapotranspiration (ET). Improving WUE of suburban ecosystem is significant to climate regulation by carbon sequestration and water consumption, especially for cities with severe water shortage like Beijing, the capital of China. Based on remote sensing data, this paper examined the spatial and temporal variations in WUE in Beijing’s suburban region from 2002 to 2010. The results showed that the average annual WUE was 0.868 g C mm-1 m-2. It has large spatial variation with the minimum of 0.500 g C mm-1 m-2 in the Miyun District. During the study periods, the area with significant increasing trend of WUE was 63.2% of the total suburban region. In terms of ecosystem type, the value of WUE was following the sequence, deciduous coniferous forest (0.921g C mm-1 m-2) > mixed forest (0.887g C mm-1 m-2) > deciduous broadleaf forest (0.884 g C mm-1 m-2) > shrubland (0.860 g C mm-1 m-2) > evergreen coniferous forest (0.836 g C mm-1 m-2) > grassland (0.830 g C mm-1 m-2). As ET was similar among the ecosystems, the difference in WUE was mainly due to the discrepancy of NPP. We found that NPP significantly correlated with the diversity of ecosystem type (represented by Shannon-Wiener index). Our results suggest that ecological engineering construction, scientific ecosystem type selection, ecosystem diversity improvement and drought-resistant species cultivation are conductive to improve ecosystem WUE in Beijing’s suburban region.

Variations of water use efficiency and its relationship with leaf nutrients of different altitudes of Wuyi Mountains, China

We determined the water use efficiency and nitrogen and phosphorus concentrations of plants at different altitudes (600, 900, 1300, 1500, 1800, 2000, 2100 m) in Wuyi Mountains to understand the relationship of water use efficiency with foliar nutrients. The results showed that plant water use efficiency increased with altitude, and the leaf δ18O of tree showed no significant variance with altitude. On the whole, leaf nitrogen concentration showed no obvious trend, while leaf phosphorus concentration at high altitude was significantly higher than that at low altitude. No significant relationship between water use efficiency and foliar nitrogen concentration was found in this study, but water use efficiency had a positive correlation with foliar phosphorus concentration. In conclusion, the change of water use efficiency was mainly caused by the difference in photosynthetic rate. The effect of water status on plant water use efficiency was not significant. The variances of leaf phosphorus concentrations along the altitudinal gradient may affect photosynthetic rate and in turn the water use efficiency of plant in this area.

Phenology Plays an Important Role in the Regulation of Terrestrial Ecosystem Water-Use Efficiency in the Northern Hemisphere

Ecosystem-scale water-use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is an important indicator of coupled carbon-water cycles. Relationships between WUE and environmental factors have been widely investigated, but the variations in WUE in response to biotic factors remain little understood. Here, we argue that phenology plays an important role in the regulation of WUE by analyzing seasonal WUE responses to variability of photosynthetic phenological factors in terrestrial ecosystems of the Northern Hemisphere using MODIS satellite observations during 2000–2014. Our results show that WUE, during spring and autumn is widely and significantly correlated to the start (SOS) and end (EOS) of growing season, respectively, after controlling for environmental factors (including temperature, precipitation, radiation and atmospheric carbon dioxide concentration). The main patterns of WUE response to phenology suggest that an increase in spring (or autumn) WUE with an earlier SOS (or later EOS) are mainly because the increase in GPP is relatively large in magnitude compared to that of ET, or due to an increase in GPP accompanied by a decrease in ET, resulting from an advanced SOS (or a delayed EOS). Our results and conclusions are helpful to complement our knowledge of the biological regulatory mechanisms underlying coupled carbon-water cycles.

Remotely Monitoring Ecosystem Water Use Efficiency of Grassland and Cropland in China’s Arid and Semi-Arid Regions with MODIS Data

Scarce water resources are available in the arid and semi-arid areas of Northwest China, where significant water-related challenges will be faced in the coming decades. Quantitative evaluations of the spatio-temporal dynamics in ecosystem water use efficiency (WUE), as well as the underlying environmental controls, are crucial for predicting future climate change impacts on ecosystem carbon-water interactions and agricultural production. However, these questions remain poorly understood in this typical region. By means of continuous eddy covariance (EC) measurements and time-series MODIS data, this study revealed the distinct seasonal cycles in gross primary productivity (GPP), evapotranspiration (ET), and WUE for both grassland and cropland ecosystems, and the dominant climate factors performed jointly by temperature and precipitation. The MODIS WUE estimates from GPP and ET products can capture the broad trend in WUE variability of grassland, but with large biases for maize cropland, which was mainly ascribed to large uncertainties resulting from both GPP and ET algorithms. Given the excellent biophysical performance of the MODIS-derived enhanced vegetation index (EVI), a new greenness model (GR) was proposed to track the eight-day changes in ecosystem WUE. Seasonal variations and the scatterplots between EC-based WUE and the estimates from time-series EVI data (WUEGR) also certified its prediction accuracy with R2 and RMSE of both grassland and cropland ecosystems over 0.90 and less than 0.30 g kg−1, respectively. The application of the GR model to regional scales in the near future will provide accurate WUE information to support water resource management in dry regions around the world.

Evaluation of MODIS-based estimates of water-use efficiency in Amazonia

ABSTRACTThis study aimed to assess the spatial-temporal patterns of water-use efficiency (WUE) obtained through MODIS gross primary productivity (GPP) and evapotranspiration (ET) products (MOD17 for GPP and MOD16 for ET) in the Upper Tapajos and Curua-Una River basins, located in the oriental flank of the Amazon region, and to validate the results with flux tower measurements within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) project. The spatial variation of WUE was primarily related to the larger presence of forested areas in the Upper Tapajos River basin (western part) compared with the Curua-Una River basin (eastern part), which is situated within the so-called arc of deforestation. Temporally, WUE showed a pronounced seasonal pattern, varying with the dry and wet seasons in the region. A decrease of ~3% in WUE was observed during the dry season, which was related to the low water availability and increased vapour pressure deficit during the dry period, which induces stomatal closure, leading to a decline in the photosynthetic rate. Comparison between the WUE estimates obtained by MODIS data and observations from the LBA towers showed an average error of 17%, varying between ~12% and ~28% for the different sites. MODIS WUE depends on the accuracy of both GPP and ET estimation. In this sense, we highlight that improvements in both MODIS GPP and ET products are necessary and should focus on reducing the uncertainties related to the biophysical vegetation parameters and meteorological data that serve as input information in the algorithms.

Intra-population variability in the drought response of a beech (Fagus sylvatica L.) population in the southwest of Europe.

Phenotypic variability within forest species populations is considered of special relevance for local adaptation under new environments, albeit it has been analyzed to a lesser extent than inter-population phenotypic variability. A common garden study was carried out to assess phenotypic variability in response to water stress in half-sibling families from a marginal population of Fagus sylvatica L. at its south-western range edge distribution in Europe. Two irrigation regimes were applied, well-watered (WW) seedlings and those submitted to weekly cycles of drying-rewatering of growth media. Seedling growth and their leaf functional traits were recorded during the last cycle of water stress. Most of the phenotypic changes were explained by phenotypic plasticity in response to water stress, but there was also a significant effect of family in the expression of some of the studied traits. The relationship of carbon isotope fractioning with gas exchange traits across families under WW conditions did not follow the same pattern as the phenotypic trends. The leaf net photosynthesis across families was modified by the nitrogen content on a leaf mass basis that was in turn correlated positively with leaf nitrogen isotope fractionation. The results point to an important role of leaf nitrogen in determining the intrinsic water-use efficiency (WUE) across families. Variation in WUE was ruled mainly by control of stomatal conductance to water vapor under water stress, but by leaf net photosynthesis under wet conditions. Relatively high inter-family phenotypic variability in growth and functional traits were observed. Within-population phenotypic variability, and the plasticity of some of the studied traits, is of fundamental importance to cope with the harsher environments beech will have to endure in the future at different points in its distribution range.

How do leaf and ecosystem measures of water-use efficiency compare?

The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant water-use efficiency (WUE), is a useful indicator of vegetation function. WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques. We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs. Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE.

Variations of water use efficiency and foliar nutrient concentrations in Cunninghamia lanceolata plantations at different ages

We studied water use efficiency (WUEi), nitrogen (N) and phosphorus (P) status of leaves at different leaf ages (current year, 1-, 2-, and 3-year-old foliage) as well as their relationships in a subtropical chronosequence of Chinese fir (Cunninghamia lanceolata) forests (3-, 8-, 14-, 21- and 46-year-old). The results showed that foliar WUEi varied significantly with foliar age in the order of current year foliage > 1-year-old foliage > 2-year-old foliage > 3-year-old foliage, while stand age had no significant impact on foliar WUEi. Foliar N/P ranged from 11.4 to 19.6 and was higher in younger and older stands than in stands at the fast-growing stage. The foliar N and P concentrations tended to display similar trends with foliar ages in the order of current year foliage>1-year-old foliage>2-year-old foliage>3-year-old foliage. WUEi did not change significantly with stand ages, probably because the photosynthetic rates and stomatal conductance decreased simultaneously with stand age. There was no relationship between WUEi and foliar N. WUEi had significant positive correlation with foliar P and significant negative correlation with foliar N/P. It is indicated that foliar P concentration would be a key factor affecting WUEi with increasing atmospheric N deposition in subtropical forests.

Water use efficiency in a primary subtropical evergreen forest in Southwest China

We calculated water use efficiency (WUE) using measures of gross primary production (GPP) and evapotranspiration (ET) from five years of continuous eddy covariance measurements (2009–2013) obtained over a primary subtropical evergreen broadleaved forest in southwestern China. Annual mean WUE exhibited a decreasing trend from 2009 to 2013, varying from ~2.28 to 2.68 g C kg H2O−1. The multiyear average WUE was 2.48 ± 0.17 (mean ± standard deviation) g C kg H2O−1. WUE increased greatly in the driest year (2009), due to a larger decline in ET than in GPP. At the diurnal scale, WUE in the wet season reached 5.1 g C kg H2O−1 in the early morning and 4.6 g C kg H2O−1 in the evening. WUE in the dry season reached 3.1 g C kg H2O−1 in the early morning and 2.7 g C kg H2O−1 in the evening. During the leaf emergence stage, the variation of WUE could be suitably explained by water-related variables (relative humidity (RH), soil water content at 100 cm (SWC_100)), solar radiation and the green index (Sgreen). These results revealed large variation in WUE at different time scales, highlighting the importance of individual site characteristics.

국립수목원의 전나무(Abies holophylla) 조림지의 물 이용 효율의 계절 및 경년 변동

물 이용 효율(water use efficiency, WUE)은 생태계의 에너지-물질-정보의 흐름과 연관된 프로세스-구조 사이의 관계에 대한 정보를 제공하는 중요한 생태학적 지표로 간주된다. 생태계 단위의 WUE 는 총일차생산량(gross primary productivity, GPP)과 증발산(evapotranspiration, ET)의 비로 정의될 수 있다. 이 연구에서는 국립수목원에 위치한 전나무(Abies holophylla) 조림지의 WUE 를 조사하기 위해 KoFlux 에서 장기간(2007-2015) 에디공분산 방법으로 관측된 이산화탄소와 수증기 플럭스를 사용하였다. 연구의 목적은 전나무 조림지의 WUE의 계절 및 경년 변동을 규명하여 탄력(resilience) 평가를 위한 총체적인 생태학적 지표의 개발에 활용하는 것이다. 분석 결과에 따르면, 전나무 조림지의 WUE는 8월에 최소값(<TEX>$1.8-3.3g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX>), 2월에 최대값(<TEX>$5.1-11.4g\;C\;(kg\;H_2O)^{-1}$</TEX>)을 갖는 오목한 형태의 계절 변동을 보였다. 성장기(4 월-10 월)의 WUE 는 평균 <TEX>$3.5{\pm}0.3g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX> 이었고, 휴면기(11 월-3 월)의 WUE는 평균 <TEX>$7.4{\pm}1.0g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX>로서 경년 변동의 폭이 컸다. 이 전나무 조림지의 WUE 는 문헌에 보고된 다른 온대 지역 침엽수림의 WUE 와 비교했을 때, 상대적으로 높은 범위에 속한다. 성장기는 4 월부터 10 월까지의 기간으로 정의하였으나, 실제 성장기의 길이(growing season length, GSL)는 매년 변화하였고, 이러한 GSL의 변화가 성장기 WUE의 경년 변동의 62%를 설명하였다. 이 연구는 생태계 단위 WUE의 장기 변동을 정량화 한 국내 첫 결과로서, 산림생태계 모형, 위성 알고리즘 및 탄력을 시험하는 데 활용할 수 있다. Water use efficiency (WUE) is considered as an important ecological indicator which may provide information on the process-structure relationships associated with energy-matter-information flows in ecosystem. The WUE at ecosystem-level can be defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). In this study, KoFlux's long-term (2007-2015) eddy covariance measurements of <TEX>$CO_2$</TEX> and water vapor fluxes were used to examine the WUE of needle fir plantation in Korea National Arboretum. Our objective is to ascertain the seasonality and inter-annual variability in WUE of this needle fir plantation so that the results may be assimilated into the development of a holistic ecological indicator for resilience assessment. Our results show that the WUE of needle fir plantation is characterized by a concave seasonal pattern with a minimum (<TEX>$1.8-3.3g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX>) in August and a maximum (<TEX>$5.1-11.4g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX>) in February. During the growing season (April to October), WUE was on average <TEX>$3.5{\pm}0.3g\;C\;(kg\;H_2O)^{-1}$</TEX>. During the dormant seasons (November to March), WUE showed more variations with a mean of <TEX>$7.4{\pm}1.0g\;C{\cdot}(kg\;H_2O)^{-1}$</TEX>. These values are in the upper ranges of WUE reported in the literature for coniferous forests in temperate zone. Although the growing season was defined as the period from April to October, the actual length of the growing season (GSL) varied each year and its variation explained 62% of the inter-annual variability of the growing season WUE. This is the first study to quantify long-term changes in ecosystem-level WUE in Korea and the results can be used to test models, remote-sensing algorithms and resilience of forest ecosystem.