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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.

Understanding the reasons of the disappearance of oasis civilizations along the ancient Silk Road will provide useful references for human’s adaptation to environmental changes in the extreme arid regions in the nowadays and future. Although some studies have associated the demise of complex societies with deteriorating climate in the world, the demise of the civilizations along the ancient Silk Road has remained unresolved. Here, this paper used the nearly 2000 years of climate characteristics revealed by Guliya ice cores, combined with the reconstruction of temperature from tree rings located in the west Kunlun Mountains around the Tarim Basin to examine the climate variations in the Northwestern China in the historical periods. Then this paper compared the demise time of the ancient oases civilizations along the ancient Silk Road from the relevant annals of states and counties. The results showed that climate change may be responsible for the rise and demise of past oasis civilizations in the ancient Silk Road. The periods of fourth to fifth centuries and the seventh to eighth centuries were characterized by long-term drought accompanied by cold climate; five ancient oases and seven ancient oases were demised, respectively, during these periods. Cold–dry climate could cause a deficiency in water resources for irrigation; thus, agricultural production fell and the society was destabilized. Recently, creation of a new “Silk Road economic belt” is realized. Modern oases will face more serious threat under the climate change. The region’s irrigation area increased 67.2 % over the past 30 years. The agricultural sector consumes 93 % of regional renewable water resources. Once the drought occurred, many modern oases—like their ancient counterparts—may well trigger more civil uprising and violent conflict in the already water-stressed regions.

The Tarim Basin is the heart of China’s Silk Road Economic Belt. The contradiction between economic growth and environmental protection is particularly evident in the basin region. For the past 20 years, the groundwater has been increasingly overused, causing water tables to decline precipitously. In the past decade in particular, the total water storage (TWS) in the basin has undergone a significant decreasing trend, losing around −13 × 108 m3/year from 2002 to 2013. Prior to 1998, the normalized difference vegetation index (NDVI) of natural vegetation exhibited an increasing trend at a rate of 0.012 per decade, but from 1999 onwards, the NDVI started decreasing at a rate of 0.005 per decade. In the 2000s, the bare soil areas of the Taklamakan Desert boundaries expanded by 7.8 %. At the same time, the effects of global warming on water and ecological systems are likewise intensifying in the Tarim Basin. The increases in warming have outpaced increases in precipitation, and the negative effects of climate change on the region’s vulnerable ecology have intensified. Potential changes in the usage of limited water resources may further increase the risk of desertification and deepen the area’s ecological poverty. Excessive water use and over allocation, including overpumping of groundwater, are the main factors causing the loss of groundwater, the depletion of total water storage, and the overall ecological degradation, but these are being exacerbated by global warming. To address all of these issues in a holistic way, a unified management mechanism that encompasses both surface water and groundwater must be developed, and the implementation of the government plan to return farmland to natural vegetation should be speed up.

The drought and salt tolerance of two pioneer species, Populus euphratica and Tamarix ramosissima, were studied by monitoring the stem and leaf water potentials under various soil moisture and salinity at depths of 0–180 cm. The plants are naturally distributed in arid environment in the lower reaches of the Tarim River in Xinjiang, China. The results showed that P. euphratica can withhold water by prolonging dewatering and adapt to the dry desert weather by reducing water consumption. The lowest soil moisture that would unfavorably influence its growth was found to be 7% soil moisture. T. ramosissima was found to have low water potential and high transpiration efficiency. It is capable of absorbing water from the soil by keeping a low water potential, so there is no critical limit of soil moisture for T. ramosissima. In terms of salt resistance, P. euphratica was found to secrete salt from its body by discharging salty water through portals in its trunks and leaves. A soil salinity of 20% was the minimum concentration at which the salt secretion mechanism of P. euphratica was activated. T. ramosissima secreted salt by storing the accumulated salt in the vacuoles of its salt secretion glands for separation. Thus, it has no minimum soil salinity limit. T. ramosissima was found to have better resistance to drought and salt stress than P. euphratica.

Sap flows of four typical species, Populus. russkii Jabl., Populus euphratica Olive., Ulmus pumila L., and Elaeagnus angustifolia L., of artificial shelter forest in a desert area were monitored in all-weather conditions using SF300 Sap flow Meter based on the theory of thermal compensation. Meanwhile, air temperature (T a), leaf temperature (T l), soil temperature (T s), relative humidity (RH), and wind velocity (V w) were simultaneously recorded by an automatic weather station. The results indicate that (1) the diurnal processes of stem sap flows of P. russkii Jabl., P. euphratica Olive., and U. pumila L., but not E. angustifolia L., show an obvious circadian rhythm. Significant differences of stem sap flow rates were found among species, but not genus. The average sap flow rate of P. russkii Jabl. is 13.8-fold of that of E. angustifolia L. The order of sap flux density (SFD) from the largest to the smallest is P. russkii Jabl., P. euphratica Olive, U. pumila L. and E. angustifolia L.; (2) compared with 373 mm, 747 mm irrigation can induce microenvironmental changes that result in the suppression of photosynthesis and transpiration and the decline of stem sap flow rates of the above four species, indicating 373 mm irrigation meets the growth needs of the above species during experiment; (3) sap flow rates are different at different stem positions: the flow rates of P. russkii Jabl., U. pumila L. and E. angustifolia L., but not P. euphratica Olive, decline gradually from cambium to pith; (4) the correlation analysis indicates that stem sap flow is negatively correlated with RH and T s and positively correlated with T a, T l and saturation vapor pressure deficit (VPD). The sap flow rate of P. russkii Jabl. is significantly affected by V w due to its large size and height. In addition, a model was established by stepwise regression analysis to estimate the relationship between the environmental factors and stem sap flows of the above four typical species of shelter forest in the desert area.

In this case study, the diurnal courses of gas exchange in Populus euphratica at different groundwater depths in the lower reaches of Tarim River were investigated to understand the effects of soil hydrology on photosynthesis and water use efficiency (WUE) of vegetation in arid desert area. It was found that the photosynthetic rate (PN) was not sensitive to the change of groundwater depth (GD) within the range of 4.2–6.8 m. Compared to stomatal conductance (gs) and transpiration rate (E) of P. euphratica grown at GD 4.2, 5.6 and 5.8 m, gs and E at GD 6.8 m both markedly declined in June, suggesting that P. euphratica at deeper GD can avoid overall water loss by stomatal adjustment. The intrinsic water use efficiency of P. euphratica first decreases with the increasing GD, but when GD increased to 6.8 m, intrinsic WUE increased by 1.2–2.2 fold, compared with the WUE of P. euphratica at GD 4.2–5.8 m, indicating that intrinsic WUE of P. euphratica will increase when the plant suffers from moderate drought stress.

This study applied a comprehensive quantitative approach including statistical, principal component and gray relation analyses to assess the groundwater chemistry based on monitored data from 840 samples collected from the lower reaches of Tarim River from 2000 to 2009. The main findings were: (1) there were six types of groundwater chemistry in the lower reaches of Tarim River where Cl·SO4–Na·Mg was the dominant type accounting for 73.57% in all samples. There were linear relationships among chemical parameters, where TDS had significant multiple correlations with Na+, K+, Mg2+, Ca2+ and Cl−, respectively. (2) Three principal components (PC1, PC2 and PC3) were extracted. They included comprehensive measurements for salinization, alkalinity and pH, respectively. Most parameters showed decreasing trends during the period of 2000–2009, as well as the scores on PC1, because the concentrations of various chemical substances were diluted due to the uplift of the groundwater table in the lower reaches and the implementation of the ecological water delivery project in 2000. (3) HCO3− was the most sensitive chemical parameter affected by the groundwater table followed by TA, Mg2+, TH, SO42−, K+, TDS and TS. PC2 was the most sensitive principal component to the change of the groundwater table followed by PC1 and PC3.

Changes of stomatal conductance (g s), net photosynthetic rates (P N) and water use efficiency (WUE) were investigated in Populus euphratica grown on sites with different groundwater depths (GDs) under two CO2 concentrations in the lower reaches of Tarim River, Xinjiang, northwestern China. P N in P. euphratica only slightly decreased when the groundwater depth increased from 4.12 to 7.74 m below the ground surface. P N values significantly increased in response to an elevated CO2 concentration at all GDs except at GD of 4.12 m for its good availability of groundwater. WUE values decreased with an initial increase in the groundwater depth, but increased when the groundwater depth reached 7.74 m especially under the elevated CO2 concentration. The g s values measured at 4.12 and 4.74 m both decreased indistinctively by only 3% due to CO2 enrichment; however, when GDs increased to 5.54 and 7.74 m, the g s values decreased significantly by about 10%. It shows that the response of g s in P. euphratica to elevated CO2 is weaker under lower groundwater depth (mild drought stress) but stronger under deeper groundwater depth (moderate drought stress). Results from this study suggest that groundwater depth could determine the response of photosynthesis to future CO2 enrichment in P. euphratica in arid desert areas.