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.

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.

The structure of the vegetation has a significant impact on how much runoff and sediment is produced. Choosing the right parameters to evaluate the correlations between vegetation and runoff-sediment yield, however, is fraught with uncertainty. We chose 18 vegetation plots with a range of vegetation types and structural char-acteristics for this study to investigate the applicability of various vegetation structure parameters in identify in the response relationship between vegetation and runoff-sediment yield through field artificial rainfall experi-ments. The vegetation coverage, stratified vegetation cover index (Cs), species diversity and functional diversity were among the criteria of vegetation structure. The findings revealed that: (1) There was a significant difference in the runoff start time and sediment yield differed significantly between vegetation types, with shrub com-munities having a longer start time and the lowest sediment yield; (2) The vegetation coverage of the community had a significant impact on the runoff start time and, when combined with Cs, explained 48% of the runoff start time. (3) Simpson index (D) and functional richness (FRic) significantly influenced the total runoff yield and along with Cs explained 46% of the total runoff yield. Cs, coverage and FRic also positively influenced the runoff end time, with a total explanation of 29%. Functional divergence (FDiv) had a significant negative effect on the total sediment yield. Together with coverage and Cs, FDiv explained 66% of the total sediment yield. These results demonstrated how biodiversity consideration helped to clarify connections between vegetation structure and runoff-sediment yield.

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.

Biodiversity is related to the function of ecosystem, while slope and soil factors are the main factors influencing runoff-sediment yield. This study is based on biodiversity in Hilly area of Loess Plateau, and coupled with runoff and sediment yield by introducing three indicators: slope, soil water content before rainfall and soil bulk density before rainfall. Through artificial simulated rainfall tests in the field, the runoff start time and the runoff-sediment yield under three vegetation communities of the herbaceous communities, shrub communities and forest communities were recorded along with soil and vegetation surveys. The survey results show that compared with herbaceous communities and forest communities, shrub communities performed better in extending the runoff start time and reducing the total sediment yield. The functional dispersion index（FDiv） of vegetation community has the greatest influence on total sediment yield, the Simpson index（D） has the greatest influence on total runoff yield, and slope is still the second most important influencing factor that cannot be ignored. The regression relationship between the total runoff yield and slope, soil bulk density, D and functional richness index（FRic） was superior to that of relationships built using the above-mentioned parameters singly. The regression relationship between total sediment yield and slope and FDiv index was superior to that of relationships built using the above-mentioned parameters singly. Vegetation protection and slope improvement should beset as the focus of water and soil conservation planning in this area.

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.

Plant functional traits (PFTs) can reflect the response of plants to environment, objectively expressing the adaptability of plants to the external environment. In previous studies, various relationships between various abiotic factors and PFTs have been reported. However, how these factors work together to influence PFTs is not clear. This study attempted to quantify the effects of topographic conditions, soil factors and vegetation structure on PFTs. Four categories of variables were represented using 29 variables collected from 171 herb plots of 57 sites (from different topographic and various herb types) in Xindian SWDP. The partial least squares structural equation modeling showed that the topographic conditions and soil properties also have a direct effect on plant functional traits. Among the topographic conditions, slope (SLO) has the biggest weight of 0.629, indicating that SLO contributed the most to plant functional traits and vegetation structure. Among soil properties, maximum water capacity (MWC) contributes the most and is followed by soil water content (SWC), weighted at 0.588 and 0.416, respectively. In a word, the research provides new points into the quantification of the correlation between different drivers that may be important for understanding the mechanisms of resource utilization, competition and adaptation to the environment during plant recovery.

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.

ABSTRACT Evapotranspiration (ET) is a major component of water and energy budgets in terrestrial ecosystems and quantifying ET over a heterogeneous land surface is of extraordinary significance. However, previous studies on the spatial distribution of daily ET in the middle Hexi corridors were mostly temporal discontinuous, and thus hinder the applications in agriculture and meteorology. Based on Land Remote-Sensing Satellite (Landsat) 8 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) data and the Surface Energy Balance System (SEBS) model, surface ET in 2014 were estimated over a desert oasis with heterogeneous land cover in the middle Hexi corridors of the arid regions in northwestern China. Daily ET was derived from the reference crop ET which was based on the relationship between actual ET and potential ET. The main results indicated that (1) the estimated daily ET values agreed well with the Eddy Covariance (EC) measurements for cropland (The coefficient of determination (R 2) = 0.96, p < 0.05, Root Mean Square Error (RMSE) = 1.32 mm day−1, Mean Absolute Error (MAE) = 1.11 mm day−1). Daily ET was more than 2.5 mm day−1 from June to July in 2014. (2) Seasonal ET was the largest in the summer (190.55 mm), followed by spring (100.03 mm), autumn (62.27 mm), and winter (6.83 mm). (3) ET from the waterbody was the highest, followed by ET from cropland, shelterbelt, bare land, and shrubland, suggesting that ET was mainly controlled by the water availability of the underlying surface. Multiple land-cover types showed similar variations in a year, but the seasonal variations were different. This study provides a foundational understanding of the patterns of water and heat exchange over heterogeneous regions, which can be helpful to make local policy in rational water management and land resource utilization.

Knowledge of exchanges of energy and water over terrestrial surfaces is the first step towards understand the ecohydrological mechanisms, particularly in water-limited ecosystems in 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 oases against the threat of desertification in arid regions of northwestern China. Here, the continuous measurements of surface energy fluxes were made using eddy covariance in conjunction with auxiliary measurements for 2 years (2014-2015) in an oasis-desert ecotone mainly covered by phreatophyte shrubs Haloxylon ammodendron, Nitraria tangutorum/sphaerocarpa, and Calligonum mongolicum in arid northwestern China. Based on the collated data for 2 years, statistical analysis on a 30-min time scale indicated that approximately 50% of daytime net radiation (R-n) in the ecotone was dissipated as H on average, and one-third of R-n was consumed by soil heat flux (G). Only 9% of R-n was consumed for latent heat flux (lambda E), which peaked in summer (21% in 2014 and 16% in 2015), corresponding to the highest rainfall season. Daily mean ET was approximately 1 mm days(-1) during the growing season of the shrub species. Accumulated annual ET was 195 and 181 mm in 2014 and 2015, respectively, exceeding the corresponding precipitation (P) by approximately 87 and 77 mm, indicating that groundwater may be another important source of water for ET in the ecotone aside from rainfall. Results within provide valuable insights into the mechanisms responsible for sustaining energy and water balance in the ecotone, a potentially groundwater-dependent ecosystem. These results also offer a foremost ecohydrological implication for water and land resources management and ecotone conservation, such as avoiding heavy groundwater pumping for extensive agricultural irrigation use to sustain groundwater availability for these shrub species in the ecotone.

An imbalanced surface water N:P (nitrogen:phosphorus) stoichiometric ratio degrades aquatic ecological and environmental functions. Although it is known that the N and P balance and hydrological processes in terrestrial ecosystems affect the surface water N:P ratio, how these factors regulate surface water N:P ratios remains unclear. Here, the N:P ratios in the nutrient budgets, soil, overland flow, groundwater, and surface water in two comparable tea and forest ecosystems and catchments were observed during the 2015-2017 period to link the hydrological processes with the surface water N:P ratio. The results suggested that although the N:P ratio of the nutrients released into the soil-water system was 1.84 times greater in the tea ecosystem than in the forest ecosystem, the soil N:P ratio (5 vs. 10 mol mol(-1)) was lower in the tea ecosystem, probably due to soil N and P decoupling processes associated with tea field establishment. The overland flow N:P ratios of the tea and forest ecosystems (30 and 28 mol mol(-1), respectively) were similar and were mainly affected by surface hydrological processes. The groundwater N:P ratios were higher in the tea ecosystem than in the forest ecosystem (245 vs. 86 mol mol(-1)), indicating more intense soil N leaching than P leaching in the tea ecosystem. An ecosystem releases excessive N through the groundwater process, which contributes to the stability of the soil N:P ratio. It is estimated that the groundwater N:P ratio contributed more than the overland flow N:P ratio to the surface water N:P ratio (31.5-58.6% vs. 0.1-0.7%), which highlights the importance of the groundwater N:P ratio and subsurface hydrological processes for regulating the surface water N:P ratio in catchments. Therefore, to reduce the environmental risk posed by an unbalanced surface water N:P ratio in tea-growing regions, NP fertilizer application must be improved, and NP leaching should be minimized.

冠层导度是植被与大气间碳、水、热交换的关键调控因子,可靠合理的冠层导度估计对于量化陆地表面蒸散的物质与能量交换具有重要意义.基于Jarvis模型原理,采用叶片气孔导度对环境因子响应的分时段函数和叶面积指数构建了适用于西北干旱区灌溉绿洲农田生态系统的冠层导度模型,并用Penman-Monteith方程结合环境因子观测数据和涡度相关数据的反推计算结果对模型进行了验证,结果表明冠层导度模型能够提供合理的预测;应用该模型进一步计算了在叶面积指数大于3时的蒸散,模拟值与实测值也具有很好的一致性;此外,叶片气孔导度向冠层导度的尺度提升需要考虑遮荫系数(shelter factor),并拟合得到了其与叶面积指数的对应函数关系.这为干旱区土壤水分条件较好的农田生态系统提供了估算冠层导度和提高蒸散计算准确度的方法,对于理解植物与大气间物质和能量交换机制以及当地的水资源管理具有重要意义.

蒸散是地表水热平衡的重要分量,也是陆地生态过程与水文过程之间的重要纽带,尤其在干旱区地-气相互作用、碳循环、水循环等过程所包含的物质与能量交换中占有极其重要的地位。基于Landsat 8遥感影像和资源三号影像（ZY3）的高分辨率植被信息,利用SEBS模型对西北干旱区河西走廊中段临泽绿洲北部区域地表蒸散量进行了估算,并用绿洲内部和绿洲-荒漠过渡带两个通量塔涡动相关数据对模型进行评估,分析了不同土地覆盖类型对蒸散量空间分布的影响。结果表明:（1）SEBS模型模拟值与实测日蒸散值之间拟合效果较好,且在均一地表时（绿洲农田区）估算精度更高（R~2=0.96,P＜0.001）,RMSE、MAE分别为0.84 mm/d、0.56 mm/d;（2）从季节变化来看蒸散量与作物生长密切相关,夏季灌溉和降雨使得研究区水分充足,植被覆盖度高,蒸散量相应增加,在绿洲地区可达5.95 mm/d,而冬季最小仅为0.52 mm/d;（3）从蒸散量的空间变化来看,水体蒸散值最大,其余依次为农田、防护林、裸地和灌木丛,说明除水体外,随着植被覆盖的增大,蒸散量也逐渐增加。通过ZY3影像的高分辨率植被信息与Landsat 8影像热红外数据融合,提高了SEBS模型对该区域蒸散量的模拟效果,增进了我们对绿洲下垫面与大气间水热交换规律、水文过程、生态-水文相互作用的深入理解。

潜在蒸发量表征局地大气蒸发能力,是研究陆面过程和水文循环的关键参量.基于中国科学院临泽内陆河流域研究站2015-2016年实测气象数据,对比分析了综合法、辐射法和温度法共10种潜在蒸发量计算公式在河西走廊中段干旱气候条件下的差异,并将计算结果与台站内E601型和Φ20型蒸发皿记录的蒸发量数据进行了统计分析.结果表明:(1)影响E601型和Φ20型蒸发量的气象因素主要为饱和水气压差、净辐射和温度;(2)两种蒸发皿折算系数(ETp-E601/ETp-蚴)的算术平均法和一元线性回归法计算值分别为0.65和0.62;(3)总体上综合法最适用,其次是辐射法,基于温度的各方法适用性最差;(4)综合法中FAO-56法最优,与E601型蒸发皿值拟合值为1.02(R2 =0.70);其次是基于辐射的Doorenbos-Pruitt法,与Φ20型蒸发皿值拟合值为0.78(R2=0.85).以上研究结果为估算我国西北干旱区及类似环境下潜在蒸发量提供了方法上的借鉴.

Aims Root architecture is a crucial determinant in the water use of desert shrubs. However, lack of integrated research on the root functional type and water uptake dynamic hinders our current understanding of the water-use strategies of desert species. Methods A field experiment was conducted to investigate the root functional type of three dominant desert species, Haloxylon ammodendron, Nitraria tangutorum and Calligonum mongolicum, and the dynamics of their root water uptake. The stem sap flow and microclimate were monitored, and the intact root systems of these shrubs were excavated in their native habitats on the oasis-desert ecotone of northwestern China during the summer of 2014. Important Findings Based on root functional type, H. ammodendron is phreatophytic, while N. tangutorum and C. mongolicum are non-phreatophytic species, which means H. ammodendron can utilize multiple potential water sources, N. tangutorum and C. mongolicum mainly utilize shallow and middle soil water. The average root water uptake rates (RWU) of H. ammodendron, N. tangutorum and C. mongolicum were 0.56 (+/- 0.12), 1.18 (+/- 0.19) and 1.31 (+/- 0.30) kg m(-2) h(-1), respectively, during the experimental period; the contributions of night-time RWU to total water uptake amount for the corresponding species were 12.7, 2.9 and 10.6%, respectively. The diurnal and seasonal dynamics of RWU in the three species were significantly different (P < 0.05), and closely related to environmental variables, especially to photosynthetically active radiation and vapor pressure deficit. Our results suggested that the three species have distinct water-use patterns in combination with the patterns of root distribution, which may alleviate water competition during long-term water shortages. H. ammodendron appears to be more drought tolerant than the other species due to its use of multiple water sources and stable water uptake rates during growing season.

Conservation management for the water dependent desert-oasis ecotone in arid northwest China requires information on the water use of the dominant species. However, no studies have quantified their combined water use or linked species composition to ecotone transpiration. Here, the water use of three dominant shelterbelt shrubs (Haloxylon ammodendron, Nitraria tangutorum, and Calligonum mongolicum) within an ecotone was measured throughout the full leaf-out period for three shrub species from 30 May to 16 October 2014, with sap flow gauges using the stem heat balance approach. Species-specific transpiration was estimated by scaling up sap flow velocities measured in individual stems, to stand area level, using the frequency distribution of stem diameter and assuming a constant proportionality between sap flow velocity and basal crosssectional area for all stems. The mean peak sap flux densities (J(sn)) for H. ammodendron, N. tangutorum, and C. mongolicum, were 40.12 g cm(-2) h(-1), 71.33 g cm(-2) h(-1), and 60.34 g cm(-2) h(-1), respectively, and the mean estimated daily area-averaged transpiration rates (T-daily) for the same species were 0.56 mm day(-1), 0.34 mm day(-1), and 0.11 mm day(-1). The accumulative stand transpiration was approximately 140.8mm throughout the measurement period, exceeding precipitation by as much as 42.1 mm. Furthermore, T-daily of these shrubs appeared to be much less sensitive to soil moisture as compared to atmospheric drivers, and the relationship between J(sn) and atmospheric drivers was likely uninfluenced by soil moisture regimes in the whole profile (to 1-m depth), especially for H. ammodendron and C. mongolicum. Results indicate that these shrubs may use deep soil water recharged by capillary rise, or may directly access shallow groundwater. This study provides quantitative data offering important implications for ecotone conservation and water and land resource management. Copyright (C) 2016 John Wiley & Sons, Ltd.

Independent measurements of stem sap flow in stems of Calligonum mongolicum and environmental variables using commercial sap flow gauges and a micrometeorological monitoring system, respectively, were made to simulate the variation of sap flow density in the middle range of Hexi Corridor, Northwest China during June to September, 2014. The results showed that the diurnal process of sap flow density in C. mongolicum showed a broad unimodal change, and the maximum sap flow density reached about 30 minutes after the maximum of photosynthetically active radiation (PAR) , while about 120 minutes before the maximum of temperature and vapor pressure deficit (VPD). During the studying period, sap flow density closely related with atmosphere evapor-transpiration demand, and mainly affected by PAR, temperature and VPD. The model was developed which directly linked the sap flow density with climatic variables, and good correlation between measured and simulated sap flow density was observed in different climate conditions. The accuracy of simulation was significantly improved if the time-lag effect was taken into consideration, while this model underestimated low and nighttime sap flow densities, which was probably caused by plant physiological characteristics.