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Abstract Litterfall load is crucial in maintaining ecosystem health, controlling wildfires, and estimating carbon stock in arid regions. However, there is a lack of spatiotemporal analysis of litterfall in arid riparian forests. This study aims to estimate Litterfall load using a BP neural network based on vegetation indices from Landsat 5 and 8 satellite images, litterfall inventory data, slope, and distance to major river tributaries. It also aims to analyze the spatiotemporal distribution pattern of litter in the research area by estimating and analyzing the spatiotemporal pattern of litterfall along the desert riparian forests of the lower Qarqan and Tarim Rivers from 2001 to 2021. The results show that the initiation of the ecological water transfer project has facilitated the decomposition of litterfall, leading to an initial decline. Subsequently, the vegetation gradually recovered, leading to an increase in leaf litter input. Since 2001, litterfall initially decreased until reaching its lowest value of 4.39 × 109 kg in 2005, followed by a subsequent increase, reaching its highest value of 12.5 × 109 kg in 2021. The study concludes that ecological water conveyance promotes both the decomposition and increase of litterfall. Initially, it accelerates litterfall decomposition, while later stages foster an increase in Litterfall load. Meanwhile, due to the ecological water transfer project and the higher vegetation cover along the Tarim River compared to the Qarqan River, the Tarim River basin experiences higher average Litterfall load and variation.

The distance from the river is a crucial factor that affects the structure and function of desert riparian forests, impeding their regeneration and biodiversity due to water conditions. However, few studies have confirmed the long-term variation in structure and function of this azonal riparian forest type caused by water stress. We hypothesize that a complex and diverse stand structure is associated with the distance from the river, and tree size plays a crucial role in establishing random frameworks for stability in forest stands. Our investigation was conducted in the lower Trim River. Based on long-term observation from 2005 to 2023, both stand structure parameters and diversity index were used. The variation in stand structure was analyzed using the least significant difference, and stand stability was assessed using Gaussian distribution and bivariate regression methods. Our study indicated that there were no significant differences in the response of size differentiation and crowding to distance from the river. However, a significant divergence in spatial pattern was observed at greater distances from the river, which became more pronounced over time. Regardless of the distance from the river or time-scale, there were significant differences in DBH, crown diameter and length. Furthermore, structural diversity exhibited varying trends with distance from the river and time-scale, indicating a diverse and complex pattern in stand structure due to water stress. The proportion of random frameworks for stability is influenced by the distance from the river, and tree size, especially crown diameter and length, plays an important role. Our research examines the multiple relationships among water conditions, forest structure, and function in an arid region, highlighting the significance of water conditions in the natural restoration of desert riparian forest ecosystems. The findings provide new insights for further exploration of the relationship between stand structure and stability, enhancing our understanding of the theory of random frameworks-stability. Overall, the study provides scientific guidance for sustainable forest management and conservation in the context of a changing climate, particularly regarding water stress. [ABSTRACT FROM AUTHOR]

In recent years, due to the shortage of water resources and the fragile ecological environment in arid areas, the relationship between vegetation and water resources has been relatively close. The unreasonable allocation of water resources and the excessive demand for ecological water use have led to ecological and environmental problems such as river interruption, land desertification, and the extensive withering of vegetation in arid areas; therefore, rapid, accurate estimation of the vegetation ecological water demand has become a hot research topic in related fields. In this study, we classified the land use types in the lower reaches of the Kokyar River Basin based on Sentinel-2A data and calculated the water requirements of each type of vegetation using a combination of the area quota method and improved Penman–Monteith (PM) based on different vegetation coverage levels. The results revealed that in 2020, the water demand of planted woodlands within 0–2 km of the watershed will be the highest, and the water demand of naturally growing arboreal woodlands will be the lowest, and the water demand of the surrounding desert riparian vegetation forests will be very small in relation to the ecological base flow and will not affect the downstream water use for agriculture, industry, and domestic use for the time being. The ecological water demand of the vegetation in the study area can be accurately estimated using Sentinel-2A data, and the research results provide technical support and a theoretical basis for rapid estimation of the ecological water demand of vegetation in typical riparian forests in arid areas and for the allocation of water resources. [ABSTRACT FROM AUTHOR]

Vegetation in arid desert areas plays an extremely important role in maintaining biodiversity and inhibiting desertification. Quantitative understanding of vegetation response to groundwater is essential to priming drought prevention and mitigation, which is a topic that is attracting considerable attention. However, previous studies have rarely assessed the response of vegetation to groundwater from probabilistic or stoichiometric chemical perspective. To address this question, a highly sensitive copula joint distribution of the response of enhanced vegetation index (EVI) loss to groundwater was constructed and a stoichiometric chemistry indicator of the Nitrogen (N) and Phosphorus (P) concentrations in tree rings was innovatively put forward to verify its sensitivity to groundwater. The results obtained revealed that the correlation of EVI with groundwater was better than that with SPEI and TVDI; Furthermore, the critical groundwater thresholds of the EVI loss rate, described as “beginning”, “reaching the maximum”, and “the slowest” were 3.85 m, 6.18–6.75 m, and 7.89 m, respectively. At the inter-annual scale, by measuring the N and P contents of Populus euphratica tree rings, and constructing response model of element contents to groundwater/rainfall, the key thresholds were determined at 5.20–5.31 m and 60 mm, which challenges the traditional understanding that rainfall does not bring ecological benefits to desert riparian forests. To further promote vegetation restoration, based on the response rule of EVI loss and stoichiometric chemical elements of tree rings to groundwater and the intermediate overflow rotation irrigation method, a more advanced desert riparian forest irrigation mode, named “branch-infiltration-rotation-irrigation”, is here proposed.

The water use of Tamarix ramosissima Ledeb. seedlings after flooding were analyzed both to explore the maintenance mechanism and pattern of natural regeneration of riparian forest, which will provide a scientific basis for restoration of desert riparian forest and ecosystem stability in the lower reaches of inland rivers of arid regions. This study area was located in the hinterland of the Taklimakan Desert in Xinjiang, China. Tamarix ramosissima seedlings growing on different groundwater depths at the river floodplain were used as the study system. The rooting depths of T. ramosissima seedlings with different basal stem diameters were ascertained by the root excavation method. The water source for the T. ramosissima seedlings was clarified using hydrogen and oxygen stable isotope methods, and the water use efficiency of T. ramosissima seedlings was investigated by stable carbon isotope (δ 13C) analysis. As the basal stem diameter classes of the T. ramosissima seedlings increased, their root depths increased. As the groundwater depth increased, the seedlings changed from primarily utilizing deep soil water to utilizing shallow soil water. In the three sample sites, the average depth of water uptake of the seedlings with basal stem diameters of 0–5 mm was 110.5, 44.1 and 39.1 cm, respectively, and that of seedlings with basal stem diameters of 5–11 mm was 83.8, 73.6 and 37.7 cm, respectively. As groundwater depth increased, the average water uptake depth of the seedlings gradually became shallower. There was no significant difference in the δ 13C values of leaves under different groundwater depths, indicating that the seedlings were not subjected to water stress. Thus, surface water played a greater role than groundwater in T. ramosissima seedling water utilization. Therefore, when analyzing ecological water conveyance patterns, attention should be paid to T. ramosissima located in areas with deep groundwater. Shallow-rooted seedlings with small basal stem diameters face an increased risk of wilting if they do not receive timely recharges of surface water. All data generated or analyzed during this study are included in this published article. • The root depth and patterns of water utilization of the Tamarix spp. seedlings at floodplains varied in their performance. • With the increase in groundwater depth, the average water uptake depth of Tamarix spp. seedlings gradually became shallow. • Surface water recharge has a strong influence on the water uptake depth of Tamarix spp. seedlings. • When determining plant water uptake depth, it is recommended that an average water uptake depth model be used. [ABSTRACT FROM AUTHOR]

Analysis of eco-environmental water requirements (EEWRs) and water resource allocation strategies for arid, inland river basins can provide the theoretical basis for sustainable water utilization and management. In this paper, an optimal water resource allocation strategy is proposed for Yarkand River Basin in Xinjiang, China, on the basis of a comprehensive analysis of runoff data collected between 1970 and 2016, three ecological environmental protection goals, basic eco-environmental water requirement (BEEWR) aimed at sustaining aquatic ecosystems within the river, and target eco-environmental water requirements (TEEWR) aimed at protecting various types of riparian vegetation along the river. The results showed that: (1) after the runoff in Kaqun reach subtracting the BEEWR, the annual average river loss (recharge), and the amount of water diversion for irrigation (51.43 × 108 m3) from flows along the Kaqun reach, the remaining water volume during wet years was able to meet all three TEEWRs; (2) during moderately wet years, the remaining water was capable of meeting the second and third TEEWRs; and (3) during dry and extremely dry years, there was little or no residual water available to meet TEEWRs. The proposed optimal water resource allocation strategy, based on the above findings, states that the water diversion requirement for irrigation and domestic use allocated from the total amount of runoff should not exceed the National Water Policy (Three Red Lines) standard first. Then, the BEEWR allocated from the runoff should be met second, and the annual average river loss, third. Depending on the amount of remaining water, the second and third TEEWRs can be fulfilled during wet years, but during moderately wet years, only the third TEEWR can be met. During dry and extremely dry years, only the BEEWR of the river can be met and only during the flood season.

Succession of plant community under different surface-groundwater joint disturbance mode. • The time interval between two overflows was determined. • An intermediate disturbance promoted the formation of a plant community that is both diverse and stable. • Adjustment process adapted to drought stress was revealed in P. euphratica and T. chinensis. • Appropriate disturbance modes are proposed for the restoration and conservation of desert riparian forest ecosystems. Intermediate water disturbance (in the form of overflow of surface water and changes in the depth of groundwater) may help to accelerate restoration of degraded riparian forest ecosystems in extremely arid areas and enhance the resistance of those ecosystems to drought. To test this possibility, we selected the lower reaches of the Tarim River in China to analyse monitoring data on vegetation and hydrology spanning a 16-year period, and a 100-year data record of tree rings in Populus euphratica. The results suggested that ecological water conveyance can restore degraded desert riparian forest ecosystems. In the early stages of restoration, the interval between two overflows should not be longer than 3 years. Intermediate disturbance (1–2 overflows a year, each lasting 21–30 days) proved conducive to the formation of a plant community that is both diverse and stable, thereby supporting the intermediate disturbance hypothesis. Both P. euphratica and Tamarix chinensis adopt suitable strategies for using groundwater from different depths to adapt to drought. Based on these results, appropriate disturbance modes related to both surface water and groundwater are proposed for the restoration and conservation of desert riparian forest ecosystems under varying intensities of drought. These modes offer scientific guidance on more efficient use of water and on ecosystem management in similar areas around the world. [ABSTRACT FROM AUTHOR]

Field experiments were conducted in the lower reaches of the Heihe River basin in the northwest of China to determine relationships between stream and groundwater and to quantify stream lateral seepage. The water table fluctuation, water balance and statistical analysis methods were used to analyze long-term experimental data. Results showed that the groundwater depth along the experimental section responded strongly to the streamflow of the East River in the basin. The streamflow of the East River in all periods significantly influenced the amplitude of groundwater depth within the range of 300 m from the riverbank; the stream lateral seepage was mainly controlled by the streamflow and its durations. The evapotranspiration of riparian forest had used larger proportions of groundwater storage in summer, close to approximating 60%. This study quantified stream lateral seepage to groundwater for different water conveyance and provides support for regional water resources management in an arid inland river basin. [ABSTRACT FROM AUTHOR]

Hyper-arid regions are expected to undergo climatic change, but only a few research works have so far been conducted on the dynamics of carbon dioxide (CO2) fluxes and their consequent responses to various bioclimatic factors, which is mainly attributable to a limited set of flux observations. In this study, the CO2 fluxes exchanged between the forest and the atmosphere have been measured continuously by the eddy covariance approach from June 2013 to December 2016 in a riparian forest, which is a primary body of natural oases located within the lower reaches of inland rivers in China. The present results revealed that the climatic conditions characterized by relatively high mean air temperatures (Ta) with fluctuating annual precipitation (P) during the prescribed study periods were comparable to the historical mean value. The annual net ecosystem productivity (NEP) ranged from approximately 278 g C m−2 year−1 to 427 g C m−2 year−1, with a mean value of 334 g C m−2 year−1. The mean annual ecosystem respiration (Re) and the gross primary productivity (GPP) were found to be 558 and 892 g C m−2 year−1, respectively. The results also ascertained that the high inter-annual variations in NEP were attributable to Re rather than to GPP, and this result was driven primarily by Ta and the groundwater depth under similar eco-physiological processes. In addition, the CO2 fluxes were also strongly correlated with the soil temperature and photosynthetically active radiation for the present study site. In conclusion, the desert riparian forest is a considerably significant carbon sink, particularly in the hyper-arid regions. [ABSTRACT FROM AUTHOR]

Studying the water use processes of desert riparian vegetation in arid regions and analyzing the response and adaptation strategies of plants to drought stress are of great significance for developing ecological restoration measures. Based on field monitoring and test analyses of physiological ecological indicators of dominant species ( Populus euphratica and Tamarix chinensis) in the desert riparian forest in the lower reaches of the Tarim River, the water relations of P. euphratica and T. chinensis under drought stress are discussed and some water use strategies put forward. The results show that (1) concerning plant water uptake, desert riparian forests depend mainly on groundwater to survive under long-term water stress. (2) Concerning plant water distribution, the survival of P. euphratica and nearby shallow root plants is mainly due to the hydraulic lift and water redistribution of P. euphratica under drought stress. (3) Concerning plant water transport, P. euphratica sustains the survival of competitive and advantageous branches by improving their ability to acquire water while restraining the growth of inferior branches. (4) Concerning plant transpiration, the sap flow curves of daily variations of P. euphratica and T. chinensis were wide-peak sin and narrower-peak respectively. T. chinensis has better environmental adaptability. [ABSTRACT FROM AUTHOR]

Balancing human demands for water with environmental requirements to maintain functioning ecosystems requires the quantification of ecological water requirements. In arid regions, high spatial variability of vegetation cover and different water consumption of plant species make it different to estimate reasonable ecological water requirements. We developed a simple and practical approach that estimates the vegetation water requirements (VWRs) of desert riparian ecosystems. This model is species-specific and spatially-explicit; it considers the water consumption characteristics required by different species and highlights the impacts that high vegetation cover spatial variability has in arid regions on evapotranspiration. The model was parameterized based on the observation of the water consumption of two typical desert riparian species, Populus euphratica and Tamarix spp., in the lower basin of the Tarim River in northwestern China. Comparisons between the modeling results and measured data for two mature Populus and Tamarix stands indicate that the model is reasonable predictive. A case study in the lower basin of the Tarim River demonstrated the model's practicality and transferability. This model could run based on near real-time or forest weather data and spatial vegetation patterns, and provides a continuous estimation of the temporal and spatial variations of the VWR. Particularly, this model forecasts VWRs under different vegetation spatial distribution and coverage scenarios, and evaluates the impacts and consequences of different management actions. This model can serve as a useful tool for management agencies interested in improving their decisions to allocate river water between human activities and natural ecosystems in arid regions. [ABSTRACT FROM AUTHOR]

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ETg) from groundwater level diurnal signals. Because of a lack of direct measurements of ETg, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2-year field observation in a hyper-arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ETs) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (Sy) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available Sy determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant Sy value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2-day moving average of r values further enhance ETg estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ETg estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ETg in desert riparian forests with deep groundwater depth. [ABSTRACT FROM AUTHOR]

The object of this paper is the different plant communities in the Ejina desert riparian forest. Groundwater depth, soil moisture, plant water potential, relative leaf moisture content and water use efficiency was monitored, and the response of soil moisture and plant ecology to the groundwater depth and the water use efficiency of the different plant communities was analyzed. The results showed that: (1) Along with the groundwater depth increasing, predawn and midday water potential of the plants, with the exception of Reaumuria soongorica, did not decrease significantly, indicating that when the groundwater depth is less than 3 m, the plant communities in the range of 4 km from the river way did not suffer or slightly suffer from water stress; (2) The distribution of higher soil moisture content within 0-3 m soil layer is suitable with the plants' root system, as indicated in the communities of coexisting overripe Populus euphratica or Taramrix chinensis, both of which can release excessive water into soil for shallow rooted shrubs or herbaceous plants when there is water shortage; (3) R. soongorica can absorb deep soil moisture through deep roots for their own survival; (4) The community consisting of Sophora alopecuroides, Karelinia caspica, T. chinensis, and overripe P. euphratica has the best species combination for restoring the damaged eco-environment in the lower reaches of Heihe River; (5) The order of plants' relative leaf water contents is K. caspica > S. alopecuroides > young P. euphratica > overripe P. euphratica > mature P. euphratica = T. chinensis coexisting with other species > single R. soongorica > single T. chinensis and the order of WUE is single T. chinensis > single R. soongorica > T. chinensis living in symbiosis with other species > S. alopecuroides = young P. euphratica > mature P. euphratica > overripe P. euphratica > K. caspica. Therefore, with ample soil moisture, the plant community helps rapid growth of T. chinensis, young P. euphratica and R. soongorica plants of less moisture content. Despite this they do not have much water storage capability, but have strong drought resistance, and higher moisture contents of S. alopecuroides and K. caspica, thus leaving them with poor drought resistance. Overall, the desert riparian forest plant community in the lower reaches of Heihe River helps the species of higher WUE live on it. [ABSTRACT FROM AUTHOR]

Carbon isotope discrimination (Δ) has been proposed as an indirect estimation criterion for water use efficiency in C3 plants. Because of the higher cost for Δ analysis, ash content or K concentration has been proposed as an alternative criterion for Δ in many species. In five typical habitats of the extreme arid Ejina desert oasis in northwest of China, the seasonal variations of foliar Δ, ash content, and potassium (K) concentration were researched in two constructive desert riparian plants (Populus euphratica Olivier, Tamarix ramosissima Ledeb). The correlations of foliar Δ with ash content and K concentration in both species were also examined to evaluate the feasibility of the foliar ash content and K concentration as surrogates of Δ in P. euphratica and T. ramosissima. Results showed that there were significant effects of plant species, habitats and growth season on foliar Δ, ash content, and K concentration. Foliar Δ and K concentration in P. euphratica were significantly higher than those in T. ramosissima, whereas, the ash content was reverse. Among habitats, the trends of Δ signatures in both P. euphratica and T. ramosissima were similar, Δ values and ash content in both species were the lowest in the dune. Both in the Gobi and dune sites, K concentration in P. euphratica and T. ramosissima was different. In the whole growth period, foliar Δ values and ash content in both species were gradually increased, but K concentration was decreased. Ash content was significantly and positively related to Δ in both P. euphratica and T. ramosissima. However, significantly negative correlations between foliar Δ and K concentration as well as between ash content and K in P. euphratica were found. In T. ramosissima, the relationship was positive but very weak. [ABSTRACT FROM AUTHOR]

Abstract: This study was conducted on the desert riparian forest along the lower reach of the inland Tarim River, which is located in the arid region of Northwest China. Fifteen plant species in 10 families were collected from five monitoring sections, and examined for the infection of arbuscular mycorrhizal fungi (AMF). The impact of different soil factors on AMF infection rate and intensity was compared using the principal component analysis (PCA) method. The results indicate that 11 species are AM and only 4 are non-AM plants. The estimated capacity of AMF infection depends on families of plants and also the parameters (infection rate, infection intensity, fungal spore density) used. The density of fungal spores was relatively higher in Phragmites communis and Populus euphratica in Graminaceae and Salicaceae families, respectively. The infection rate was above 50% in all the AM plants, except Calligonum junceum. The highest infection rate appeared in Alhagi sparsifolia (97%) and Glycyrrhiza inflata (92%). However, when compared by AMF infection intensity, Tamarix spp. became the top one, followed by Alhagi sparsifolia, and Glycyrrhiza inflata was in the middle range of all the species. The PCA has identified that soil total salt, moisture, organic matter, total nitrogen, total P, available K and pH were closely associated with the AMF infection. [Copyright &y& Elsevier]

In the present study, the heat pulse technique was applied to investigate the stem sap flow of Populus euphratica in a desert riparian forest in an extreme arid region from April to October 2003 and from May to October 2004. The experimental sites were in Qidaoqiao (101°10′ E, 41°59′ N) and Bayantaolai farm (101°14′ E, 42°01′ N) in Ejina county, in the low reaches of the Heihe River, China. The results indicated that the diurnal change in the velocity of sap flow showed minor fluctuations. At night, the rising of sap flow could be observed in the main tree species because of root pressure. During the growing season, the maximum average velocity was observed in July, followed by August, and the same velocity was observed in September and May; the minimum velocity was observed in October. The transpiration from June to August during the growing season accounted for approximately 70% of the annual total transpiration. The sap flow velocity of P. euphratica trees of different ages could be arranged in the order: 15 yr > 25 yr > 50 yr. Sap flow velocity was closely related to changes in micrometeorological factors, with average sap flow velocity showing a significant linear correlation with net radiation, air temperature and relative humidity. [ABSTRACT FROM AUTHOR]

• EWCE increased water discharge from ecological sluices, uplifted groundwater depth, and promoted the vegetation growth. • Tree-ring chronology in P. euphratica could be used to reconstruct the TINDVI in the Tarim River. • Higher correlations and sensibility existed between vegetation growth and hydrologic features. • Differences in correlation coefficients between vegetation growth and hydrologic features were significant. In arid areas, ecological water conservancy engineering influences vegetation growth by altering river flooding dynamics in addition to the depths of groundwater. However, few studies have explored how to develop the optimal floodwater and groundwater regulation strategies that take into account both ecological water supply and vegetation growth. Therefore, the aim of the present study was to propose the suitable regulation strategies by revealing that the plant growth responses to hydrological variation (i.e., the flooding and groundwater depth). Based on two growth indicators (i.e., the time-integrated normalized difference vegetation index (TINDVI) for plant canopy growth and the tree-ring chronology for trunk growth) in the middle reaches of the Tarim River, and water consumption, water discharge from ecological sluices for flooding, and groundwater depth data, after the implementation of an scheme in ecological water conservancy engineering, the ecological water transport efficiency, tree-ring chronology in Populus euphratica , and TINDVI improved by 12.4%, 5.6% and 4.2%, respectively. Notably, the tree-ring chronology increased with a decrease in water consumption. Due to a significant correlation between tree-ring chronology and TINDVI, the tree-ring chronology could be used to reconstruct the canopy growth in the case of the unavailability of monitoring data. There were significant correlations between TINDVI and groundwater depth in June–October, between tree-ring chronology and groundwater depth in October, and between water discharge from ecological sluices and vegetation growth in September. In addition, there were high correlation and sensitivity between trunk growth and water discharge (compared to groundwater depth), and between canopy growth and groundwater depth (compared to water discharge). Based on the above results, potentially sustainable groundwater depths (-2.3 to −3.7 m) and discharge water quantities (3.53–5.90 × 108 m3) and regimes were proposed that could further promote the vegetation growth. [ABSTRACT FROM AUTHOR]