Desert Riparian Vegetation Research Articles

Impact of Ecological Water Transfer Project on Vegetation Recovery in Dried-Up Kongque River, Northwest China

The ecological water transfer project (EWTP) plays a pivotal role in reinstating the flow of dried-up rivers in arid regions, promoting river connectivity and vegetation resurgence. An essential facet in ensuring the efficacious execution of the EWTP lies in determining the optimal duration of irrigation to facilitate vegetation recovery. Nevertheless, comprehensive reports concerning the EWTP process in arid river ecosystems are scarce. Here, we leverage remote sensing imagery to assess changes in surface water and vegetation dynamics before and after the implementation of the EWTP in a dried-up river. The results show that before the EWTP (1987–2016), riparian vegetation’s mean normalized difference vegetation index (NDVI) decreased from 0.181 to 0.066. After EWTP (2017–2022), the river’s flow was restored for a distance of 347 km. This restoration resulted in the formation of 81.47 km2 of intermittent water bodies along the river. The mean NDVI increased from 0.065 to 0.093. As irrigation duration increased, the NDVI growth rate exhibited an initial rise followed by a subsequent decline, reaching its peak growth rate by irrigating for 18 days per year. The regions showing increased NDVI values exhibited a pronounced spatial correlation with the areas subjected to water transfer. These improvements in NDVI were predominantly concentrated on both sides of the river within a 550 m range. Interestingly, as moves farther away from the river, the growth rate of NDVI exhibited an initial increase followed by a subsequent decline. The pinnacle of NDVI growth rate materialized at a distance of 40–50 m from the river. These findings reveal the response characteristics of desert riparian vegetation to EWTP, providing valuable insights for selecting appropriate water transfer timing in future EWTP.

Effects of desert riparian vegetation roots on the riverbank retreat process in the Tarim River in China

The desert riparian vegetation zone is an important part of the riparian ecosystem. It is significant for river evolution and to channel beds owing to the reinforcement effect of the vegetation roots on the bank and resistance to erosion. The quantitative effects on toe erosion and bank collapse were first investigated to determine how vegetation roots affect the continuous retreat process of riverbanks under different water levels. The case study is the Tarim river, the degrees of riverbank stability at eight sampling sites during the 2016 and 2017 hydrological years were calculated using the bank stability and toe erosion model (BSTEM). Furthermore, the effects of six vegetation species, Populus euphratica (PE), Tamarix ramosissima (TR), Alhagi sparsifolia (AS), Nitraria tangutorum (NT), Phragmites australis (PA), and Glycyrrhiza uralensis (GU), on riverbank stability were discussed. Taking the right bank of sampling site II (Alar New Bridge) as an example, the bank stability safety factor (Fs) and collapse process under the roots of six vegetation species with different root area ratios (RARs) were calculated. The following conclusions were drawn from the results: (a) Different vegetation roots can reduce toe erosion from 0.4 % to 68.4 %, with peak effects during the dry season and recession stages; (b) The presence of near-bank vegetation roots can strongly influence bank collapse, particularly during the recession stage when water level drops rapidly; and (c) Most of the calculated Fs with the roots of all six plant species studied were higher than those without roots during four stages. Moreover, it was greater than the critical value of 1.3 in most cases. The results also indicate that banks with PE roots had the highest Fs values. By contrast, banks with NT and GU roots had the smallest Fs. As the RAR increased, the Fs values in a hydrological year increased linearly for each vegetation specie. The rate of increase order was as follows: PE > TR > NT > AS > PA > GU under a particular range of RAR (0.9 % < RAR < 1.8 %).

Effect evaluation of ecological water conveyance in Tarim River Basin, China

Ecological water conveyance is an important way to promote the restoration of degraded ecosystems in arid watersheds. However, there are few previous research results on how to quantitatively evaluate the effect of ecological water conveyance on ecological restoration. In this regard, this paper selects the Tarim River Basin as a typical area, analyzing the changes of desert riparian vegetation and hydrological elements, constructing a watershed ecological environment quality evaluation system, and comprehensively evaluating the ecological water conveyance effect of the damaged desert forest ecosystem. The conclusion showed that the proportion of the pixel area with an upward trend of Fractional Vegetation Cover (FVC) from 2000 to 2021 is as high as 84.3%. The plant diversity index in the ecological water conveyance area showed the characteristics of first obvious increase and then stable. The main body of groundwater depth showed an upward trend, and the Temperature Vegetation Dryness Index (TVDI) showed a downward trend of pixel area accounting for 57.0%, which indicated that ecological water conveyance had played a positive role in groundwater recharge and ecological restoration along the Tarim River. The ecological environment quality of the river basin showed a trend of transition from low-grade to high-grade, and the area with excellent ecological quality had increased from 4,635.50 km2 in 2000 to 12,335.0 km2 in 2021. The above research provides important scientific reference for the protection and restoration of vegetation degradation in arid watersheds.

Eco-Hydrological Response of Water Conveyance in the Mainstream of the Tarim River, China

Desert riparian vegetation forms an ecological corridor in extremely arid environments, and ecological water conveyance is an important measure of vegetation restoration and biodiversity conservation in desert riparian zones. Studying the responses of vegetation to ecological water conveyance and changes in this process in arid desert riparian zones and assessing the comprehensive benefits of ecological water conveyance are highly significant for ecological conservation and restoration in addition to the formulation of water transfer policies. Previous studies mainly used a single indicator to evaluate the ecological restoration of the mainstream Tarim River in Northwest China; thus, systematic and comprehensive assessments based on multiple indicators have not been conducted. In the present study, remote sensing data and field surveys were used to analyze the ecological restoration status of the Tarim River during 2015–2021 in terms of hydrological responses, vegetation responses, and ecological water conveyance benefits. The results showed that groundwater levels and soil moisture in the mainstream area of the Tarim River increased significantly from 2015 to 2021. The amount of groundwater storage also increased. Ecological water conveyance has created good hydrological conditions for groundwater recharge and ecological restoration on both sides of the mainstream area of the Tarim River. Desert forest ecosystems, mainly comprising Populus euphratica and Tamarix ramosissima, have been saved and rejuvenated in water conveyance areas. After ecological water conveyance, the Simpson and Shannon–Wiener indices increased significantly, but the diversity level began to decline and then stabilize with the increase in water conveyance frequency. The overall habitat status improved and the quality of the ecological environment below the Wusiman section of the middle reaches of the Tarim River improved significantly.

Assessment of the effect of water-table depth on riparian vegetation along the middle and lower reaches of the Manasi River, Northwest China

The Manasi riparian zone in northwestern China has become a survival habitat for numerous native plant species and requires urgent protection owning to rapid expansion of farmland. The critical factor affecting the growth of desert riparian vegetation in arid regions is recognized to be groundwater, but in this region the threshold of water-table depth for riparian species has been rarely studied. To determine the association between species and their major growth factors and to ascertain the water-table depth threshold, this study employed field investigation, a logarithm distribution model and canonical correspondence analysis. According to the findings, (1) the water-table depth largely regulates the species distribution; (2) from the results of the logarithm distribution model, the water-table depth appropriate for herbs is 1–1.5 m and for shrubs it is 2–4 m, and a water-table depth of less than 6 m could satisfy the growth requirement of major species; (3) species diversity peaks at the water-table depths of 2–3, 3–5, and 2–4 m for herbs, shrubs and all species, respectively; (4) the frequency of appearance of Phragmites communis (grass herb) and Tamarix chinensis (deciduous shrub) was not as sensitive to depth to water table. To reconstruct a riparian zone, Phragmites communis and Tamarix chinensis could be planted in areas with water-table depth of less than 3 m and 2–5 m, respectively. These results may contribute to suitable policy regarding vegetation restoration.

Spatial variation in leaf nutrient traits of dominant desert riparian plant species in an arid inland river basin of China.

Understanding how patterns of leaf nutrient traits respond to groundwater depth is crucial for modeling the nutrient cycling of desert riparian ecosystems and forecasting the responses of ecosystems to global changes. In this study, we measured leaf nutrients along a transect across a groundwater depth gradient in the downstream Heihe River to explore the response of leaf nutrient traits to groundwater depth and soil properties. We found that leaf nutrient traits of dominant species showed different responses to groundwater depth gradient. Leaf C, leaf N, leaf P, and leaf K decreased significantly with groundwater depth, whereas patterns of leaf C/N and leaf N/P followed quadratic relationships with groundwater depth. Meanwhile, leaf C/P did not vary significantly along the groundwater depth gradient. Variations in leaf nutrient traits were associated with soil properties (e.g., soil bulk density, soil pH). Groundwater depth and soil pH jointly regulated the variation of leaf nutrient traits; however, groundwater depth explained the variation of leaf nutrient traits better than did soil pH. At the local scale in the typical desert riparian ecosystem, the dominant species was characterized by low leaf C, leaf N, and leaf P, but high leaf N/P and leaf C/P, indicating that desert riparian plants might be more limited by P than N in the growing season. Our observations will help to reveal specific adaptation patterns in relation to the groundwater depth gradient for dominant desert riparian species, provide insights into adaptive trends of leaf nutrient traits, and add information relevant to understanding the adaptive strategies of desert riparian forest vegetation to moisture gradients.

Water use strategies of Populus euphratica seedlings under groundwater fluctuation in the Tarim River Basin of Central Asia

Riparian vegetation in arid regions is subject to frequent water environmental fluctuations and plays an important role in maintaining the riparian ecosystem. In recent decades, Populus euphratica (P. euphratica) has been threatened by decreasing groundwater levels, and large areas of P. euphratica seedlings are withered. To better understand the adaptive mechanism underlying hydrological process alterations, we designed an experiment to identify the water sources of P. euphratica seedlings using δ18O values (plant stem xylem, soil water and groundwater) under different groundwater scenarios by simulating the wild habitats conditions in the Tarim River bank of Central Asia. Seedlings were grown in lysimeters (2.0 m tall × 0.4 m inner diameter) under varying hydrological conditions. P. euphratica seedlings generally took up water from the shallow soil layer (0–80 cm). Under the T. ramosissima disturbance, the water source depths of the P. euphratica seedlings moved down as the groundwater depth increased. With increasing groundwater depth, the proportions of groundwater used by P. euphratica seedling monocultures (only P. euphratica seedlings) and P. euphratica seedling mixtures (P. euphratica and T. ramosissima seedlings) were reduced. Under shallow groundwater treatment (W1: 25 cm), the plant height growth of the P. euphratica seedlings slowed, and their biomass accumulation decreased. The aboveground and total biomasses of the non-coexistence seedlings under W2 (75 cm) treatment were maximized, while the maximum value for coexistence seedlings occurred under W3 (125 cm) treatment. Thus, we suggest that shallow groundwater depth is not beneficial to P. euphratica seedling growth, and appropriately decreasing the groundwater depth may promote their growth. Such information will be valuable to provide the configuration of water resources for a typical river basin in an arid region of Central Asia based on the ecological water requirements of desert riparian vegetation.

Determining spatio-temporal variations of ecological water consumption by natural oases for sustainable water resources allocation in a hyper-arid endorheic basin

Abstract Natural oases, including desert riparian vegetation and terminal lakes, play an important role in maintaining the stability and development of endorheic river systems in arid areas. Determining the ecological water consumption (EWC) of natural oases is a critical task for achieving sustainable ecosystem restoration and services in endorheic basins. In this study, the evapotranspiration of natural vegetation, evaporation of water area, and seepage in lakes and rivers were considered in the EWC. The Surface Energy Balance System (SEBS) model was combined with Landsat-8 images and eddy covariance flux measurements to calculate the daily EWC of natural oases in the lower Heihe River Basin (northwest China) during 2014 and 2015. The spatio-temporal variations of EWC were investigated in detail. The SEBS model could satisfactorily simulate the evapotranspiration (ET) of forest, shrub, mixed forest and water body with R2 value greater than 0.84 and RMSE value less than 0.9 mm/day. The total EWC in 2014 and 2015 was 6.59 × 108 m3 and 7.65 × 108 m3, respectively, and the EWC of the Ejina Oasis accounted for approximately 50% of total value in the study area. The cumulative EWC increased gradually with the distance away from the riverbank as an exponential function and reached the plateau at certain distance. The monthly variation of EWC followed a unimodal curve with the peak in June and July, accounting for 42.16% and 36.28% of annual EWC in 2014 and 2015, respectively. The water balance analysis indicated that the river flow was not sufficient to provide the EWC of natural oases and water requirements of agriculture and social economy, and additional 1.23 × 108 m3 and 2.09 × 108 m3 of groundwater needed to be exploited in 2014 and 2015, respectively. Several strategies for sustainable water resources allocation and ecosystem management in the study area were provided. Overall, this study provides an integrated approach with combinations of multiple techniques and high-resolution data for EWC calculation, which is useful for water footprint analysis and water resources management on the basin scale in arid areas.

Groundwater Depth and Soil Properties Are Associated with Variation in Vegetation of a Desert Riparian Ecosystem in an Arid Area of China

Groundwater is a major driving force for plant community distribution in arid areas worldwide. Although it is well known that groundwater has a significant impact on soil and vegetation, there is little information on how groundwater depth affects soil and vegetation in an arid inland basin desert riparian ecosystem. Therefore, quantitative analysis of the relationships among groundwater depth, soil properties and plant community distribution is necessary. A desert riparian ecosystem in the lower reaches of the Heihe River in an arid area of Northwest China was used to determine quantitative relationships among groundwater depth, soil and vegetation. Groundwater depth significantly increased with increased distance from the river. Soil and vegetation characteristics showed a significant trend with increasing groundwater depth. With increasing groundwater depth, soil water content, soil total nitrogen, soil total carbon, soil available phosphorus and soil available potassium decreased, while the soil bulk density and soil carbon:nitrogen (C:N) ratio increased. Soil pH and soil electrical conductivity followed quadratic function relationships with groundwater depth. Species richness, aboveground biomass, community coverage, community height, foliage projective cover and leaf area index all significantly decreased with increased groundwater depth. Groundwater depth and soil were associated with vegetation variance, explaining 85.8% of the vegetation variance. Groundwater depth was more important in explaining vegetation variance than soil properties (soil bulk density) and soil pH. Our observations indicate that changes in groundwater depth would have a significant influence on desert riparian forest vegetation, and that maintaining appropriate groundwater depth is necessary to preserve the riparian ecosystem.

Experimental study on water transport observations of desert riparian forests in the lower reaches of the Tarim River in China.

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.

Determining the ecological water allocation in a hyper-arid catchment with increasing competition for water resources

Meeting ecological water requirement adequately plays a significant role in guaranteeing the stability of river basin ecosystems in arid lands. The amounts of water leaked through the riverbed and drawn by ecological brakes in Tarim River were ascertained. Based on data related to soil, hydrology, and vegetation of the river basin, the aims of this paper are to 1) analyse the variation in the size of soil particles and in hydraulic conductivity of four sections of the river; 2) calculate the amount of water that leaked from the riverbed and the percentage of ecological water requirement that can be guaranteed at different frequencies of water inflow; and 3) recommend – using a combination of particle size analysis, Darcy's Law, and GIS – the amount of water to be drawn from ecological brakes along both banks of the river for meeting ecological water requirement adequately. The results showed that 1) the size of soil particles in the riverbed ranged from 1.6μm to 98.9μm; 2) hydraulic conductivity followed the normal distribution from year to year but varied in spatial terms, that is across different section of the river; 3) riverbed leakage varied with water frequencies, being 11.36×108m3, 10.62×108m3, 9.84×108m3, 9.32×108m3, and 8.87×108m3 at the frequencies of 10%, 25%, 50%, 75%, and 90%, respectively; 4) the distance over which the leakage contributed to meeting ecological water requirement in the south bank was greater than or equal to the distance in the north bank; and 5) water drawn from ecological brakes on the north bank exceeded that drawn from the brakes on the south bank by 10.89×108m3–11.28×108m3. Ecological water requirement of the desert riparian vegetation was met mainly from riverbed leakage in the south and by drawing from ecological brakes in the north. The present research not only offers a scientific method that could be used for developing suitable schemes for meeting ecological water requirement but also provides a technical guide on running ecological brakes and achieving the optimal allocation of water resources in Tarim River Basin.

Changes in riparian plant communities due to a canal barrier traversing ephemeral stream channels in the Sonoran Desert

The Central Arizona Project (CAP) is a large canal system that traverses hundreds of ephemeral stream channels in the Sonoran Desert. This longitudinal barrier alters flow during runoff events, causing water to pond behind the canal's wall. We asked: How has riparian vegetation of the ephemeral streams changed over the course of 35 years in response to canal construction? We compared field data (vegetation volume, woody plant stem density and size, and herbaceous cover) collected in distinct zones located upstream and downstream of the canal to unaltered controls.By ponding water and inducing sedimentation of fine particles, the canal has created areas that support dense vegetation. The wettest zone has the greatest vegetation volume and plant height, and supports densities of Prosopis velutina that are six times greater than in the control zone. Larrea tridentata and other desert shrubs are displaced to the border of the wettest zone, but have greater height and stem diameter than typically occur in the desert owing to increased frequency of soil wetting in the canal-associated anthropogenic-floodplain. This research aids in understanding the impacts of canal barriers on desert riparian vegetation, and can be used to predict future outcomes of proposed canals in desert environments.

Configuration of water resources for a typical river basin in an arid region of China based on the ecological water requirements (EWRs) of desert riparian vegetation

Desert riparian vegetation is a natural cover promoting the stability and development of inland river ecosystems in arid regions. Calculating the ecological water requirements (EWRs) of desert riparian vegetation is an important step in achieving reasonable water utilization. Therefore, this study examined the Tarim River, located in an extremely arid region of China, and collected relevant data on hydrology, weather and vegetation using remote sensing. Subsequently, we analyzed the spatial distribution of the desert riparian vegetation in four sections of the Tarim River and calculated the EWR of the desert riparian vegetation using the phreatic evaporation model; additionally, we determined the required runoffs at five hydrologic stations based on the water balance principle. Ultimately, the necessary protection ranges and goals for desert riparian vegetation were established according to the water resource variations in the Tarim River. Our research showed that the total area of desert riparian vegetation along the Tarim River is 16,285.3km2; this distribution area gradually decreased as the distance from the river increased, and areas varied in the different river sections. The EWRs of desert riparian vegetation from Sections 1 to 5 are 5.698×108, 7.585×108, 4.900×108, 4.101×108m3 and 1.078×108m3, respectively. Therefore, the total EWR of the study region is 23.362×108m3. In terms of the transpiration law of the “unimodal type”, the peak value of EWR of natural vegetation occurs in July, and the decreasing trend appears in the other months. Based on the water balance principle, the required runoffs in Alar, Xinquman, Yingbaza, Wusiman and Qiala were determined to be 47.105×108, 35.174×108, 22.734×108, 15.775×108 and 7.707×108m3, respectively. According to the water resource frequency and the EWR of the desert riparian vegetation along the Tarim River, we divided the region into three protection ranges: key protection (8.9–11.8km from the river), basic protection (15.8–21.8km from the river) and influence protection (43.0km from the river). This research not only provides a reasonable calculation method for EWR on the scale of a river basin but also supports the healthy development of the desert riparian vegetation ecosystem and helps to achieve the optimal water allocation for this river.

Impacts of ecological water conveyance on groundwater dynamics and vegetation recovery in the lower reaches of the Tarim River in northwest China.

The ecological water conveyance project (EWCP) in the lower reaches of the Tarim River provided a valuable opportunity to study hydro-ecological processes of desert riparian vegetation. Ecological effects of the EWCP were assessed at large spatial and temporal scales based on 13years of monitoring data. This study analyzed the trends in hydrological processes and the ecological effects of the EWCP. The EWCP resulted in increased groundwater storage-expressed as a general rise in the groundwater table-and improved soil moisture conditions. The change of water conditions also directly affected vegetative cover and the phenology of herbs, trees, and shrubs. Vegetative cover of herbs was most closely correlated to groundwater depth at the last year-end (R = 0.81), and trees and shrubs were most closely correlated to annual average groundwater depth (R = 0.79 and 0.66, respectively). The Normalized Difference Vegetation Index (NDVI) responded to groundwater depth on a 1-year time lag. Although the EWCP improved the NDVI, the study area is still sparsely vegetated. The main limitation of the EWCP is that it can only preserve the survival of existing vegetation, but it does not effectively promote the reproduction and regeneration of natural vegetation.

Desert riparian vegetation and groundwater in the lower reaches of the Tarim River basin

The Green Corridor in the lower reaches of Tarim River in northwestern China has an extreme hot and dry climate. Vegetation here, consisting of arbor, shrub and grass, relies on groundwater exceedingly. However, the increasing anthropogenic activities of large-scale agricultural reclamation and unreasonable water utilization in the upper and middle reaches caused the 321-km riverway in the lower reaches to dry up completely in 1972 and resulted in the sharp decline of groundwater, followed by the ruin of desert riparian vegetation on a large scale. The Green Corridor is on the verge of shrinking. Water has a key role in maintaining ecological balance and socioeconomic development. This paper, focused on the relationship between vegetation and groundwater, discusses (1) the change of groundwater table caused by the ecological water delivery carried out in the lower reaches of Tarim River; (2) the appropriate groundwater depth meeting the vegetation’s survival; (3) the minimum ecological flux and ecological water requirement for the growth of natural vegetation. It was shown that (1) based on the analysis of the monitoring data from the groundwater level of ten times water delivery, such an extensive artificial watering takes positive effect on raising the groundwater level along the two sides of the river; (2) a groundwater table depth of 2–4 m is probably the appropriate ecological water table level for the lower reaches of the Tarim River, and 6 m is the threshold for the local vegetation; (3) at the lower Tarim River, 1.157 × 108 m3 of water flow is needed for itself. The longer the duration of water releases, the greater would be the groundwater rise and the larger the range of vegetation influenced. It was found that the duration and volume of water delivery was closely related to restoration of vegetation in the lower reaches of the Tarim River. The goal of this paper is to offer scientific evidences for water delivery in the rigorous areas to maintain an ecological balance.

Adaptation strategies of desert riparian forest vegetation in response to drought stress

ABSTRACTIn this paper, we discuss the adaptation mechanisms of desert riparian forest vegetation's responses to drought stress. Ecological and physiological parameters are presented, including coverage, density and species diversity. Also investigated are the photosynthesis, fluorescent parameters, leaf water potential, leaf proline (PRO), soluble sugar, malondialdehyde, superoxide dismutase (SOD), peroxidase (POD) and abscisic acid (ABA) of the main constructive species at various buried groundwater depths of the Tarim River's lower reaches. The results reveal a close relationship between vegetation and groundwater depth and show that the most appropriate groundwater depths for the optimal composition and structure of plant communities is 2–6 m. For the main constructive species, Populus euphratica and Tamarix spp., the most appropriate groundwater depths and the critical depths are 4 and 8 m for P. euphratica and 6 and 10 m for Tamarix spp., respectively. However, when groundwater depths exceed 4 m for P. euphratica or 6 m for Tamarix spp., the plants suffer drought stress. During moderate or extreme drought stress, P. euphratica and Tamarix spp. rapidly accumulate PRO, soluble sugar, SOD and ABA in their leaves, thereby reducing leaf water potential and stomatal conductance, enhancing osmoregulation and alleviating the esterification of cell membranes. Such a response decreases the yield as well as the photosynthetic and electron transport rate, thus improving the efficiency of water utilization. Complementary actions in resisting drought stress likewise occurred between SOD activity and POD activity, and between the contents of PRO and soluble sugar in P. euphratica and Tamarix spp. leaf cells. In the main, P. euphratica accumulated a large amount of soluble sugar for osmoregulation to counter drought stress, whereas Tamarix spp. accumulated a large volume of PRO. When suffering extreme drought stress, Tamarix spp. enhanced its POD activity. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Hydraulic lift in Populus euphratica Oliv. from the desert riparian vegetation of the Tarim River Basin

In the Tarim River Basin, the desert riparian forest vegetation is under high-temperature and aridity stress. However, the vegetation can grow continuously because of deep rooting that can reach groundwater, which can thus redistribute water into the upper soil profile. This paper describes patterns of hydraulic lift by Populus euphratica Oliv. and discusses its ecological effects. Our results show that the tap root sap velocity of P. euphratica Oliv. is positive during the day and night. However, a reverse sap flow was observed in the lateral roots during the night. The soil water content of the subsoil was higher than that of the topsoil at depths of 0–120 cm. When the sap flow of the lateral roots was reversed at night, the soil water content clearly increased. In particular, at depths of 60–120 cm, the soil water content at 4:00 was 28–38% greater than that at 16:00. The vapor pressure deficit was a factor that predominantly affected the root sap velocity, and the smaller vapor pressure deficit often facilitated a reverse sap flow in the lateral roots. Our findings demonstrate the hydraulic lift characteristics and ecological effects that occur in the desert riparian forest in extremely arid regions of middle Asia

Assessment of the groundwater threshold of desert riparian forest vegetation along the middle and lower reaches of the Tarim River, China

AbstractSurvival of desert riparian vegetation along the middle and lower reaches of the Tarim River in China depends almost entirely on groundwater. The aim of this project was to study the effect of various water table levels on desert riparian forest vegetation and to determine the groundwater threshold under which desert riparian forest vegetation can grow normally. Using data collected from 2 years (2006–2007) of monitoring (groundwater, vegetation plots, and soil profiles) of the middle and lower reaches of the Tarim River, the relationship between vegetation and environmental factors was analysed using detrended canonical correspondence analysis (DCCA). The average depth of the groundwater table at the study sites was divided into six ranges (0–2, 2–4, 4–6, 6–8, 8–10, and &gt;10 m) to compare the influence of groundwater level on species diversity. Using the ecological suitability theory and data from previous studies of several major plant species in the study area, a logarithm normal distribution relation model was built between the growth of plants and the depth of groundwater. The results of DCCA ordination indicated that groundwater depth (GWD) and soil moisture are the crucial environmental factors affecting the distribution of plants. Species diversity was the highest at the 2–4 m level, followed by the 4–6 m level, and then the 0–2 m level. When the groundwater level dropped to below 6 m, species diversity decreased greatly, and the slope of Hill's index (diversity ordering) tended to level off. Thus, the optimum groundwater level for major plant growth is 2–4 m and the threshold GWD is about 6 m. To restore the vegetation and ecosystem of the lower reaches of the Tarim River, the water table must be kept at a minimum of 6 m in depth. It should also be maintained at 2–4 m in the vicinity of the water way and at 4–6 m for the rest of this arid area. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The ecological characteristics of the riparian vegetation affected by river overflowing disturbance in the lower Tarim River

Based on data obtained from field investigations, this paper aims to analyze the influence of the river overflow on the desert riparian vegetation and discuss the function of the river overflow on the vegetation restoration at the lower Tarim River. The results show that (1) there are only 17 species, 13 genera and 9 families in the study areas before river overflow, while there are 34 species, 26 genera and 12 families after the overflowing in which 18 species emerged newly; (2) judging by the biodiversity indices, the species diversity and species richness in the river overflowed area increase more significantly than those in the un-overflow area; (3) judging by the importance of different species after years of river overflowing, the annual herbs germinate quickly at first, while the perennial herbs with deep roots or root clones become dominant in the plant community; (4) after several times of river overflowing, some arbors and shrubs such as Populus euphratica and Tamarix ramosissima germinate easily and can dominate gradually in the plant community. The results indicate that the river overflowing restores the severely degraded ecosystem in the lower Tarim River and the function is connected with restoration of eco-hydrological processes in the study areas. The results suggest that experimental overflowing has initiated a process of restoring ecosystem function within the riparian forest.