Microhabitat heterogeneity plays a crucial role in shaping plant water use strategies and ecophysiological processes in arid ecosystems, yet little is known about the coupled response of leaf water isotopes and photosynthetic physiology to such fine-scale environmental variation. This study employed stable isotope tracing (δ2H and δ18O) and photosynthetic gas exchange measurements to investigate water sources, leaf water isotope dynamics, and photosynthetic traits of the dominant desert plant Haloxylon ammodendron across different microhabitats (flats and dunes) within the oasis-desert transition zone of the Hexi Corridor. In both microhabitats, H. ammodendron relies on groundwater as its primary stable water source, but the depth of soil moisture uptake shows significant differentiation. This differentiation is tightly coupled with seasonal trajectories of leaf water isotope enrichment (Δ2H, Δ18O): the flat habitat exhibits a “high in spring, low in summer” pattern, while the dune habitat shows a “low in spring, high in summer” pattern, accompanied by opposed photosynthetic physiological responses. Notably, the rate of leaf water isotope enrichment peaks at noon when stomatal conductance is at its lowest, confirming the nonlinear regulation of transpiration fractionation by stomatal behavior. Air temperature is the dominant meteorological driver of Δ18O variation, yet habitat specificity is pronounced—a positive correlation exists in flat habitats, while a negative correlation prevails in dune habitats. Hydrogen isotope enrichment, however, is not directly regulated by meteorological factors. Structural equation modeling quantified that xylem water isotopes directly govern leaf water isotopes (path coefficients 0.35–0.58), while soil water isotopes exert indirect regulation. Environmental factors primarily influence leaf isotopes by modulating soil moisture. This study elucidates the coupled regulatory mechanism by which microhabitat heterogeneity drives plant water uptake, leaf isotope enrichment, and photosynthetic physiology, providing important insights into how plants adapt to heterogeneous arid environments.

Numerous woody plants regenerate shoots from roots and stumps after aboveground damage. Frequent disturbances (herbivory, cutting) in desert ecosystems make resprouting critical for plant survival and ecosystem sustainability, yet the adaptive mechanism of new shoots remains unclear. We measured leaf water potential, photosynthesis, and quantified water sources of four desert shrubs. The results showed that resprouting plants did not shift the root water uptake sources. Calligonum mongolicum, Hedysarum scoparium, and Tamarix chinensis mainly relied on deep soil water and groundwater, with a conservative water-use strategy. While Zygophyllum xanthoxylum displayed remarkable plasticity, switching between shallow and deep sources in response to seasonal changes and precipitation events. Specifically, following precipitation, its shallow use proportion increased sharply from 12%∼14% to approximately 50%. Compared with control plants, resprouting plants exhibited significantly higher leaf water potential (p < 0.05), photosynthetic rate, and transpiration rate. These findings suggest that resprouting shrubs adopt a prioritized recovery strategy in water transport and utilization, which confers a competitive advantage and promotes rapid regeneration. The change in water sources regulated stomatal conductance to coordinate the balance between carbon assimilation and water consumption, thereby affecting the water use efficiency of plants and overall water adaptability. Finally, the findings revealed that desert plants can rapidly regrow by competing for limited resources such as water and nutrients following external disturbances. This effectively enhanced the self-repairing potential of the damaged desert communities, maintained the stability of species diversity, and provided a scientific basis for the vegetation restoration and ecological restoration in arid areas.

Hydrological niche segregation (HNS) is widely recognized as a key factor in species coexistence, but experimental evidence on water resource allocation among coexisting shrubs in desert ecosystems remains limited. We investigated the dynamics of hydrological niche segregation between two dominant desert shrubs (Nitraria sphaerocarpa and Reaumuria songarica) by analyzing stable isotope compositions (delta 2H, delta 18O) and soil water availability. The results showed that coexisting species exhibit distinct and flexible water use strategies. N. sphaerocarpa primarily utilizes shallow soil moisture during average precipitation periods but shifts to deeper water sources under arid conditions, with its distribution range spatially overlapping with that of R. songarica. Both coexisting species responded only to larger precipitation pulses, but N. sphaerocarpa exhibited higher uptake intensity and longer reliance duration on shallow water sources post-precipitation. As interannual precipitation decreases, the niche overlap between coexisting species significantly increases, while the niche width of R. songarica expands by 1.11 % and that of N. sphaerocarpa contracts by 3.24 %. These results highlight HNS as a dynamic mechanism that promotes coexistence through flexible resource partitioning. Under climate change scenarios, the plasticity of HNS is crucial for maintaining biodiversity in arid regions.

In desert regions, water availability for plants is extremely limited, making certain dominant species highly dependent on relatively abundant groundwater. To investigate the water distribution and relationships among coexisting xerophytic shrubs in arid habitats, we analyze the water consumption patterns of various species through stable isotope analysis (delta 18O and delta 2H). This analysis examines various water sources within the ecosystem, including soil water, rainfall, and groundwater, collected from six drought-tolerant shrub species. Our investigation revealed that evaporation in the upper layer of soil, which has low moisture content, occurs while deeper soil water, primarily recharged by groundwater, remains relatively abundant. Additionally, insufficient rainfall complicates the full recovery from prolonged soil water deficits. The vertical differentiation of soil moisture leads to varying water usage patterns among plants along the soil water gradient. The small shrubs, such as Reaumuria soongorica and A. sphaerocephala, obtained roughly 50 % of their water from shallow and mid soil layers, about 25 % from deep soil layers and about 25 % from groundwater, showing variable water source preferences according to soil water availability. As for large shrubs like Haloxylon ammodendron, Caragana microphylla, and Calligonum mongolicum, over 60 % of water was taken up from groundwater to meet the canopy water usage. Consequently, the differing water use patterns of coexisting plants stem from the vertical spatial variation of soil moisture, which allows for differentiated utilization of water resources and is essential for species coexistence in desert regions.

In desert regions, precipitation is one of the significant water sources and a key driver of ecohydrological processes over a range of spatiotemporal scales. When precipitation infiltration was combined with the original soil water, the soil water and plant water use will experience significant dynamic changes, which play an important role in the stability and sustainability of the artificial vegetation. Therefore, soil water dynamics and water use strategies of Haloxylon ammodendron were studied by the hydrogen and oxygen stable isotope technique in this study. The results showed that precipitation in the desert soil was mainly in the form of piston flow, and the precipitation that led to the significant increase of infiltration and recharge was different in different precipitation events. The precipitation of 12.8 and 19.6 mm can make the surface soil water sufficient and continue to penetrate into the deep soil, and the duration of soil water response and the recharge depth were much longer and deeper than those for 4.4 and 7.8 mm events. In addition, the H. ammodendron mainly relied on stable and abundant deep soil water and groundwater. The water use source of H. ammodendron showed no significant response to 4.4 and 7.8 mm precipitation. However, a significant difference in water source proportion occurred before and after 19.6 mm of precipitation. The use proportion of shallow soil water increased from 10.7% to 24.2%, while that of groundwater decreased from 48.8% to 23.2%. Therefore, we concluded that precipitation levels have a major impact on soil water at various depths; in particular, heavy precipitation has a significant impact on deep soil water that deeply controlled the survival of the H. ammodendron plantation. These results offer an essential theoretical foundation for vegetation restoration and sustainability in arid regions.

Study region: At the southern edge of Badain Jaran Desert, China. Study focus: The mixed shrubs of Calligonum mongolicum and Nitraria sphaerocarpa are the common species in extreme arid areas. There are few studies on the water use and coexistence mechanisms of the two species. So we examined the water use strategies of these two species based on hydrogen and oxygen stable isotope technology, and the coexistence mechanism of the two desert plants was explored from the perspective of hydrological niche separation (HNS). New hydrological insights for the region: The N. sphaerocarpa had more root biomass in shallow soil than C. mongolicum, whereas the opposite was true in deep soil, indicating a root niche separation. Additionally, the C. mongolicum exhibited a significant dependence on deep soil water (about 26 %) and groundwater (over 30 %); while N. sphaerocarpa has a significant response to precipitation, its proportion of shallow soil water is more than 40 % after heavy precipitation events, in seasons with less precipitation, it relied on groundwater for survival. Both species had a dynamic HNS, the hydrological niche overlap index exhibited a decreasing and then increasing trend; but HNS was low during the dry season, indicating that HNS was affected by precipitation. We conclude that the main reason why the two species can coexist in arid regions is their HNS, which avoid competition for limited water resources.

Leaves constitute a vital bottleneck in whole-plant water transport, and their water strategies are key determinants of plant competition and productivity. Nonetheless, our knowledge of leaf water strategies predominantly stems from single perspectives (i.e., hydraulic, stomatal or economic traits), severely limiting our capacity to comprehensively predict plant vulnerability and sustainability, especially under drought-stress conditions. Here, we examined the leaf hydraulic, stomatal and economic traits of three coexisting shrub species (i.e., Haloxylon ammodendron (C.A. Mey.) Bunge., Calligonum mongolicum Turcz. and Nitraria sphaerocarpa Maxim.) in the Badain Jaran desert-oasis ecotone to comprehensively evaluate their water strategies and drought adaptation mechanisms. The results demonstrated that these three shrubs exhibited significant differences in leaf hydraulic vulnerability, osmoregulatory capacity, stomatal behavior and economic traits. Nonetheless, these traits remain tightly related to guarantee their survival. Interestingly, two distinct interaction mechanisms between stomatal and hydraulic regulation were identified among the three shrubs with varying stomatal sensitivity. Specifically, N. sphaerocarpa and H. ammodendron employed relatively lower isohydric stomatal behavior, characterized by a synergistic decrease in vapor-phase water loss as liquid-phase water transport decreased during severe atmospheric drought. Conversely, C. mongolicum adopted higher isohydric stomatal behavior, rapidly reducing vapor-phase water loss during initial drought stress to compensate for its more vulnerable liquid-phase water transport system. Notably, all three shrubs presented risky leaf water strategies with negative hydraulic safety margins. Among them, the hydraulic dysfunction risk was lowest for C. mongolicum, followed by N. sphaerocarpa and H. ammodendron. Overall, our findings are anticipated to offer valuable insights for afforestation initiatives and ecological conservation efforts in desert-oasis ecotones that function as critical shelterbelts.

Alpine grassland ecosystems on the Qinghai–Tibet Plateau (QTP) provide critical services but face threats from human activity and climate change. Ensuring ecosystem health is vital for sustainability and preserving ecosystem services and processes, especially in delicate ecosystems such as the Gannan alpine grasslands. However, there is currently a lack of a comprehensive model that integrates ecosystem structure, function, processes, and socioeconomic factors. This study proposes a comprehensive ecosystem health assessment approach that combines the revised driver–pressure–state–impact–response (DPSIR) framework with ecological security patterns (ESPs), overcoming the limitations of previous models that focused primarily on ecosystem structure without sufficiently addressing dynamic ecosystem processes. This method aims to diagnose the health of the Gannan alpine grasslands on the QTP from 2000 to 2020. We found that in the context of global climate change, the ecological health was maintained at a relatively high level (covering 75.41 % of the area) in most areas of Gannan, whereas lower levels (12.09 %), were found in the northern areas of Gannan and southwestern areas of Maqu likely resulting from higher livestock density, increased population density, and weaker landscape connectivity. The results of the driver analysis showed that livestock inventory (with an influence Q-value of 0.70) significantly affected the health of the Gannan alpine grassland ecosystem, suggesting that sustainable livestock management is essential for maintaining ecological corridor connectivity, protecting core zones and promoting regional sustainability.

Ecosystem health assessments are crucial to protect the ecological environment and ensure the sustainable ecological functions of alpine ecoregions. At present, few studies evaluating the ecosystem health of the Gannan alpine grassland, China, an ecologically fragile area, based on a remote sensing theoretical framework exist. As such, this study assessed the ecosystem health of the Gannan alpine grassland based on the Remote Sensing-based Ecological Index (RSEI) and provided a comparative analysis of the RSEI and Gross Primary Productivity (GPP), extending the study of their spatiotemporal patterns and influencing factors. The results suggested that RSEI and GPP showed strong comparability in an ecological sense, with the RSEI better reflecting changes in ecosystem health of the Gannan alpine grassland than the GPP. Overall, the health of the Gannan alpine grassland ecosystem was good (RSEI of 0.61-0.76) and a slow, fluctuating upward trend was seen from 2000 (RSEI = 0.66) to 2020 (RSEI = 0.72). Notably, the RSEI was high in the south and low in the north of the region. Over the past 21 years, 43.92% of the ecologically healthy grassland in the southwest of Gannan has been degrading, while the poor ecological health of 39.04% of the grasslands in the southeast and northeast improved. The model test results show that RSEI could reasonably evaluate the ecosystem health of Gannan alpine grassland. Our assessment results provide important scientific data and information on health monitoring and targeted ecological restoration efforts in the Gannan region.

The Bowen ratio (beta), which is the ratio of sensible heat (H) to latent heat (LE), reflects the energy balance and partitioning processes among soil, vegetation, and the atmosphere. Although the spatial patterns of beta have been clearly delineated, the importance of vegetation in the spatial variation of beta is frequently underestimated. Revealing the spatial patterns of beta would improve the understanding of the variation in energy partitioning in terrestrial ecosystems and its reciprocal relationship with environmental change. Here, we calculated beta by integrating H and LE flux values from 80 flux observation sites based on the eddy -covariance method in ChinaFLUX to analyze the spatial pattern and mechanism of beta in China. Terrestrial ecosystems in China had an average beta of 0.64 +/- 0.47. beta varied significantly among ecosystem types. Deserts had the highest beta (2.08 +/- 0.17), while wetlands had the lowest beta (0.37 +/- 0.11). The beta values of terrestrial ecosystems exhibited a significant latitudinal pattern, increasing linearly with latitude. This pattern also existed in forest and cropland ecosystems. The spatial pattern of beta was dominated by climate -shaped vegetation factors, including leaf area index (LAI) and fractional vegetation cover (FVC). Nevertheless, as water and thermal conditions decline, the contribution of vegetation factors gradually wanes. These findings demonstrated the spatial variations and driving mechanisms of terrestrial ecosystem beta and provided insights into the mitigation of future climate change by vegetation.

Functional traits are critical indicators for assessing and predicting plant environmental adaptations and survival strategies. However, less attention has been paid to root functional traits due to the costly and destructive nature of field excavations. This has resulted in a poor understanding of organ trait associations and vegetation survival strategies, particularly for plants in arid environments. In this study, we investigated 11 classical plant functional traits (leaf, stem, and root) and the intact root systems of three dominant coexisting shrubs, Calligonum mongolicum, Nitraria sphaerocarpa, and Haloxylon ammodendron, in a typical oasis-desert ecotone in northwestern China. These three coexisting shrubs generally converge on conservative resource strategies with dimorphic root systems and small leaf mass fractions to cope with strong habitat filtering and survive in arid environments. However, we found significant interspecific divergences in functional traits. Specifically, C. mongolicum had the most conserved traits, the medium root depth (370 cm), and the highest root-shoot ratio (1.72). H. ammodendron had relatively conserved traits, with the most extensive root depth (420 cm, access to groundwater) and the lowest root-shoot ratio (0.45). N. sphaerocarpa had the least conservative traits, the shallowest root depth (200 cm), and the medium root-shoot ratio (1.14). These divergences promote ecological niche segregation and ensure the stable coexistence of shrubs in this resource-limited environment. In contrast to the whole-plant economics spectrum, there was limited coordination between aboveground and belowground functional traits across the three species. Therefore, it is speculated that the different organs of these three species may operate independently to manage different constraints. The deep-rooted H. ammodendron is highly dependent on groundwater; therefore, planting them extensively in the ecotone may increase local groundwater consumption, resulting in the severe degradation of these species, particularly in the context of consecutive oasis expansion and intensified climate change. These results are expected to contribute to the development of effective ecosystem restoration and afforestation practices in such oasis-desert ecotones.

Rainfall partitioning by the vegetation canopy represents a significant component of the local hydrological cycle by reshaping the amount and spatial distribution of rainfall. Measuring the components of rainfall partitioning, however, has been a challenging task due to laborious- and time-consuming field experiments. In this study, to probe the influences of long-term afforestation on dynamic patterns of rainfall partitioning, the dominant sand-stabilizing shrub Haloxylon ammodendron at three different ages was selected for field measurements during the 2020-2021 growing season. The throughfall percentage for young H. ammodendron (YH, 75.9 %) was significantly higher than that for middle-aged H. ammodendron (MAH, 63.4 %) and mature H. ammodendron (MH, 62.4 %) (p < 0.05 for all cases). However, the interception loss percentage of YH (22.3 %) was significantly lower than that for MAH (35.0 %) and MH (36.5 %) (p < 0.05 for all cases). No significant difference was found for stemflow percentage among YH (1.8 %), MAH (1.5 %) and MH (1.1 %). Smaller rainfall events contributed to a higher interception loss percentage and a lower net rainfall percentage for all ages. Both throughfall and stemflow percentage first showed increasing trends and then tended to be stable with increasing rainfall amount and duration, whereas interception loss percentage showed the opposite patterns. Rainfall partitioning was significantly correlated with the plant area index, stem basal area and canopy height (p < 0.05 for all cases), which may account for significant differences in rainfall partitioning patterns, as all shrubs experienced the same weather conditions. The average funneling ratio was 56.6, 26.7 and 17.9 for YH, MAH and MH, respectively. These results suggested that H. ammodendron afforestation can have a significant impact on rainfall partitioning by reducing net rainfall reaching the soil and may have some implications for local water budget and ecosystem management in oasis-desert ecotones.

Canopy interception loss affects the local water budget by removing a non-negligible proportion of rainfall from the terrestrial surface. Thus, quantifying interception loss is essential for thoroughly understanding the role of vegetation in the local hydrological cycle, especially in dryland ecosystems. However, sparse shrubs in dryland ecosystems have not been sufficiently studied, owing to time- and labor-intensive field experiments and challenging model parameterization. In this work, 4-year growing season field experiments on rainfall partitioning were conducted for three dominant shrub species (Haloxylon ammodendron, Nitraria sphaerocarpa, and Calligonum mongolicum) in an oasis-desert ecotone in northwestern China. The revised Gash analytical model was well parameterized, which reliably simulated the cumulative interception loss for sparse shrubs, and the validated model performed better for H. ammodendron, followed by C. mongolicum and N. sphaerocarpa, with relative errors of 8.4%, 15.4%, and 23.9%, respectively. The mean individual interception loss percentage for H. ammodendron (28.4%) was significantly higher than that for C. mongolicum (11.0%) and N. sphaerocarpa (10.9%) (p < 0.05), which could be ascribed to the higher canopy storage capacity and wet-canopy evaporation rate of H. ammodendron. For all shrub species, the majority proportion of interception loss occurred during canopy saturation and drying-out periods, accounting for approximately 79-85% of the cumulative interception loss. Overall, the mean local interception loss of three dominant shrub species in the ecotone removed nearly 17% of the corresponding cumulative rainfall during the growing season. These results not only provide methodological references for estimating the interception loss of sparse vegetation in dryland ecosystems, but also provide scientific insights for water resource management and ecosystem restoration in water-limited regions similar to the experimental site.

The exchange of water, heat, and carbon dioxide between the terrestrial ecosystems and the atmosphere is a fundamental process that underlies mass and energy transfer at the Earth’s surface. The eddy covariance technique has become one of the preferred and state-of-the-art approaches for measuring and calculating the exchange of water, heat, and carbon between the land surface and the atmosphere. This dataset covers the fluxes of water, heat, and carbon dioxide in an oasis agroecosystem, as well as the auxiliary micrometeorological variables in the middle areas of the Hexi Corridor during 2012–2015. The experimental observation campaign was carried out by the Linze Inland River Basin Research Station, a member station of the China Flux Observation and Research Network (ChinaFLUX). The dataset serves as a fundamental source of data, enabling in-depth comprehension water transfer, energy exchange, carbon cycle processes in the oasis agroecosystem in arid regions, and even their environmental and vegetation controlling mechanisms. It offers valuable insights into better understanding hydrological processes, ecosystem-hydrology reaction and discovering the coupling between carbon, water and heat in the context of climate change. Furthermore, the dataset holds significant scientific value in addressing chronic practical challenges related to fragile environment and shortage of water resource, such as water and land resources management, the preservation of oasis stability and sustainable development, the conservation of oasis ecosystems, etc.

定量分析植被冠层对降雨再分配过程的影响,是认识陆地生态系统水文循环的重要环节.然而,由于干旱区天然植被分布稀疏、形态结构特殊,其降雨再分配过程的测算较为困难,相关研究较少,特别是关于荒漠低矮灌丛的降雨再分配研究鲜有报道.本文以河西走廊中段临泽绿洲—荒漠过渡带的天然建群种泡泡刺灌丛(Nitraria sphaerocarpa)为研究对象,基于3年逐个单次降雨事件的观测数据分析了生长季泡泡刺灌丛的降雨再分配特征及主要影响因素,量化了泡泡刺灌丛覆盖下实际进入土壤的有效降雨量及其空间分布特征.结果 表明:(1)生长季泡泡刺灌丛的平均穿透率、树干茎流率和冠层截留损失率分别为87.89％、1.61％和10.50％;(2)降雨量是影响泡泡刺灌丛降雨再分配特征的关键气象因素,其与穿透雨量、树干茎流量、冠层截留损失量之间具有显著的统计关系(P＜0.001);(3)与干旱区其他稀疏植被相比,泡泡刺灌丛的穿透率和集流率较高,冠层截留损失率较低,与其特殊的植被形态特征有关,相关分析的结果表明,泡泡刺灌丛的穿透雨量与植被面积指数和株高呈显著的负相关关系(P＜0.001),树干茎流量与树干倾角呈显著的正相关关系(P＜0.01).这些研究结果增进了我们对于绿洲—荒漠过渡带植被对局地水文过程影响的认识,为合理估算干旱区稀疏植被覆盖下的冠层截留损失提供了方法参考.

蒸散是地球水分循环与能量转换的关键环节. 陆域蒸散的精准测算是地球物理、生物乃至环境过程研究的共同科学难题. 以英国Dalton和Penman等人为代表的开创性工作, 以及大气边界层湍流交换理论与实验的不断发展, 奠基了现代主流的蒸散测算方法. 20世纪90年代以来技术趋于成熟的涡动相关系统和卫星遥感等观测手段, 跨越寒带到热带、干旱区到湿润地区, 涵盖水体、湿地、森林、农田、草地、裸地、城市等不同下垫面类型, 极大地拓展了对蒸散过程认知的深度和广度; 捕捉了诸如地表夜间蒸散、蒸散迟滞现象、非均匀下垫面的湍流间歇、平流关联的岛屿效应、下垫面转捩效应等新的现象和事实, 对经典相似性理论和蒸散测算理论等提出了新的挑战; 最大熵增蒸散模型和非参数化蒸散模型等新方法和新理论的萌芽已经出现. 与此同时, 点位高频观测和航空航天遥感技术构筑了从植物气孔到叶片、植株、冠层、景观、流域等跨尺度测算手段, 在地表蒸散过程观测及机理解析上, 形成纵向深化及横向综合的新进展. 陆域蒸散新理念、新模型、新技术的综合性发展, 成为突破蒸散精准测算难题的基础条件, 这将为揭示地球系统的水-热-碳循环机理等全球性重大基础理论问题, 为满足农业灌溉与粮食安全、水资源精细化管理与生态环境保护、城市热环境调控与全球升温适应对策等国家重大需求, 提供更为严谨坚实的科学理论依据和实验支撑.

Accurate estimation of regional and global patterns of ecosystem respiration (ER) is crucial to improve the understanding of terrestrial carbon cycles and the predictive ability of the global carbon budget. However, large uncertainties still exist in regional and global ER estimation due to the drawbacks of modeling methods. Based on eddy covariance ER data from 132 sites in China from 2002 to 2020, we established Intelligent Random Forest (IRF) models that integrated ecological understanding with machine learning techniques to estimate ER. The results showed that the IRF models performed better than semiempirical models and machine learning algorithms. The observed data revealed that gross primary productivity (GPP), living plant biomass, and soil organic carbon (SOC) were of great importance in controlling the spatiotemporal variability of ER across China. An optimal model governed by annual GPP, living plant biomass, SOC, and air temperature (IRF‐04 model) matched 93% of the spatiotemporal variation in site‐level ER, and was adopted to evaluate the spatiotemporal pattern of ER in China. Using the optimal model, we obtained that the annual value of ER in China ranged from 5.05 to 5.84 Pg C yr−1 between 2000 and 2020, with an average value of 5.53 ± 0.22 Pg C yr−1. In this study, we suggest that future models should integrate process‐based and data‐driven approaches for understanding and evaluating regional and global carbon budgets.

A knowledge of the exchanges of energy and water over the terrestrial surface is the first step to understand the ecohydrological mechanisms, particularly in water-limited ecosystems in the dryland environments. However, patterns of energy exchange and evapotranspiration (ET) are not well understood in the oasis-desert ecotone, which plays an important role in protecting oasis against the threat of desertification in northwestern China’s arid regions. Here the continuous measurements of surface energy fluxes were made using eddy covariance in conjunction with auxiliary measurements for two years (2014-2015) at a shrubland within an oasis-desert ecotone in the arid regions, northwestern China. Statistical analysis on 30-min time scale indicates that about 50% of daytime net radiation (Rn) over the shrubland is dissipated as H on average, which peaks in spring; one third Rn is consumed by soil heat flux (G). Only 9% of Rn was consumed for latent heat flux (λE), which peaks in summer (21% in 2014 and 16% in 2015), corresponding to the season with highest rainfall among all seasons. Daily mean ET is about 1 mm·d−1 during growing season of the shrub species. The rapid and transient increase in ET occurs following a rainfall event. A switch in surface soil moisture from 0.04 to 0.11 m3·m−3 causes an increase in Rn by about 11% and λE by 151% at the shrubland, respectively. Accumulated annual ET were 195 and 181 mm in 2014 and 2015, respectively, exceeding the corresponding P by about 87 and 77 mm, indicating that groundwater may be another important source of water for ET over the shrubland aside from P. These results provide valuable insight into the mechanisms of sustaining energy and water balance at the ecotone, and then produce some management guidelines for allocating water resources and protecting vegetation.

利用LI-6400XT便携式光合作用测定系统和Model 1505植物压力室对河西走廊中部荒漠-绿洲过渡带3种优势种C4植物梭梭(Haloxylon ammodendron)、沙拐枣(Calligonum mongolicum)和C3植物泡泡刺(Nitraria sphaerocarpa)的水分交换过程和叶片水势(Ψ)变化进行了观测试验,对比了荒漠植物生长季降水前后水分传输因子的变化;模拟了气孔导度对主要环境因子和叶片水势的响应;从饱和水汽压差(VPD)对气孔导度的制约作用研究了荒漠植物蒸腾的调控机制.结果表明:影响3种灌木气孔导度的主要因子依次为VPD、气温(T)和Ψ,气孔导度随着VPD和T的升高而降低,随着Ψ的降低逐渐减小;不同荒漠植物气孔导度对环境因子和叶片水势的综合响应模拟研究表明,模型能够很好地模拟气孔导度日内变化,C3植物泡泡刺对这些因子变化的响应比C4植物梭梭和沙拐枣更敏感;通过类比于欧姆定律,表明可用气孔导度和VPD的乘积来对蒸腾速率进行线性模拟,相关性很强.

Terrestrial evapotranspiration (ET) is a crucial link between Earth's water cycle and the surface energy budget. Accurate measurement and estimation remain a major challenge in geophysical, biological, and environmental studies. Pioneering work, represented by Dalton and Penman, and the development of theories and experiments on turbulent exchange in the atmospheric boundary layer (ABL), laid the foundation for mainstream methodologies in ET estimation. Since the 1990s, eddy covariance (EC) systems and satellite remote sensing have been widely applied from cold to tropical and from arid to humid regions. They cover water surfaces, wetlands, forests, croplands, grasslands, barelands, and urban areas, offering an exceptional number of reports on diverse ET processes. Surface nocturnal ET, hysteresis between ET and environmental forces, turbulence intermittency, island effects on heterogeneous surfaces, and phase transition between underlying surfaces are examples of reported new phenomena, posing theoretical and practical challenges to mainstream ET methodologies. Additionally, based on non-conventional theories, new methods have emerged, such as maximum entropy production and nonparametric approaches. Furthermore, high-frequency on-site observation and aerospace remote sensing technology in combination form multi-scale observations across plant stomata, leaves, plants, canopies, landscapes, and basins. This promotes an insightful understanding of diverse ET processes and synthesizes the common mechanisms of the processes between and across spatial and temporal scales. All the recent achievements in conception, model, and technology serve as the basis for breaking through the known difficulties in ET estimation. We expect that they will provide a rigorous, reliable scientific basis and experimental support to address theoretical arguments of global significance, such as the water-heat-carbon cycle, and solve practical needs of national importance, including agricultural irrigation and food security, precise management of water resources and eco-environmental protection, and regulation of the urban thermal environment and climate change adaptation.