Effectiveness and challenges of ecological engineering for desert riparian forest restoration along China’s largest inland river

Ecological Engineering 127 (2019) 11–22

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Effectiveness and challenges of ecological engineering for desert riparian forest restoration along China’s largest inland river

T

Ümüt Halika,c, , Tayierjiang Aishana,b, , Florian Betzd, Alishir Kurbane, Aihemaitijiang Rouzia ⁎

⁎

a

Ministry of Education Key Laboratory of Oasis Ecology, Xinjiang University, Shengli Road 666, Urumqi, Xinjiang 830046, China Institute of Arid Ecology and Environment, Xinjiang University, Shengli Road 666, Urumqi, Xinjiang 830046, China c College of Resources & Environmental Sciences, Xinjiang University, Shengli Road 666, Urumqi, Xinjiang 830046, China d Faculty of Mathematics and Geography, University of Eichstaett-Ingolstadt, Ostenstraße 14, Eichstaett 85071, Germany e Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, South Beijing Road 818, Urumqi, Xinjiang 830011, China b

ARTICLE INFO

ABSTRACT

Keywords: Tarim River Ecological engineering Environmental flows Ecohydrological response Desert riparian Populus euphratica forest

Environmental flow, one of the main restoration measures for regulated rivers, aims to restore and sustain degraded riparian ecosystems and the human wellbeing that depends on these ecosystems with a reasonable magnitude, frequency, duration and timing of water flow. However, efficacies of environmental flows are often restrained by other demands along the river and inappropriate water management, leading to only spatially limited habitats being suitable for ecosystem functioning. This paper presents current restoration measures in the lower reaches of the Tarim riparian ecosystem in Northwest China and analyzes ecohydrological dynamics as a function of environmental flows. The groundwater level oscillation before/after and during the environmental flows (i.e., manmade water diversion) was examined, and the Mann-Kendall test was applied to identify the overall trend of change in groundwater depth from 2003 to 2011. The growth, distribution, health status (referred to as “vitality”) and renewal potentials of the dominant tree species Populus euphratica were quantified by using individual-based repeated measurements of P. euphratica morphology in two sampling sites (a 100-ha permanent plot and five random sampling plots with 50-m radii) at Arghan village in the lower reaches of the Tarim River. Based on these measurements, the effects of restoration measures on the groundwater recharge and the revitalization of degraded trees were assessed. The main contribution and achievements of the water diversion project on rehabilitation of the degraded riparian forests were evaluated; further, the limitations and challenges faced by of the restoration project were examined. The results of this research could help establish a reference for monitoring changes in riparian forests in Central Asia and possibly recommend restoration measures for riparian forest ecosystems in arid regions worldwide.

1. Introduction In the Central Asian drylands, the inland rivers and their floodplains have substantial ecological importance (Karthe, 2017). Over the past decades, excessive water use and inappropriate water management combined with the impacts of global climate change have posed a serious threat to natural ecosystems of inland river basins and brought irreversible ecological consequences (Micklin and Aladin, 2008; Cheng et al., 2014; Aishan et al., 2015a,b; Guo et al., 2017). For example, desiccation of the Aral Sea and Lop Nur Lake are typical environmental disasters at an unprecedented scale. Tugai forests are the dominant component of riparian ecosystems along the inland rivers in the continental desert regions of Central Asia. In the Tarim Basin, the desert riparian forest is mainly dominated by Populus euphratica Oliv. (syn.

⁎

Populus diversifolia Schrenk). This type of forest is one of the most important Tugai ecosystems in the world and maintains a large oasis within an extremely arid environment (UNESCO, 2010). Currently, in the lower reaches of the Tarim River, the natural flow closely coordinated with riparian vegetation survival, growth and reproduction has been altered or disconnected by reservoir construction, land use conversion and climate change impacts. Large areas that were originally P. euphratica riparian forests have been deforested, with the remaining forest areas in critical condition (Xu et al., 2008, 2009; Chen et al., 2006, 2013; Ling et al., 2015). According to Giese et al. (2006), the total area of P. euphratica riparian forests in the Tarim Basin decreased from 45.98 × 104 ha in 1958 to 24.59 × 104 ha in the early 1990 s. More seriously, large areas of P. euphratica in the lower reaches of the Tarim River decreased in size from approximately 5.4 × 104 ha

Corresponding authors at: Ministry of Education Key Laboratory of Oasis Ecology, Xinjiang University, Shengli Road 666, Urumqi, Xinjiang 830046, China. E-mail addresses: [email protected] (Ü. Halik), [email protected] (T. Aishan).

https://doi.org/10.1016/j.ecoleng.2018.11.004 Received 26 October 2017; Received in revised form 22 October 2018; Accepted 2 November 2018 0925-8574/ © 2018 Elsevier B.V. All rights reserved.

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in 1958 to 0.67 × 104 ha in the early 1990 s; almost 87.6% of P. euphratica forest area had been lost. As a consequence of these deteriorations, ecosystem services provided by the floodplain area have been severely impacted, and desertification has been aggravated and has expanded in the region (Mamat et al., 2018). A major challenge for water management along the Tarim River is to meet water demands for economic development without degrading the extant Tugai riparian forests. To meet this challenge, decision makers must understand the effects of flow regulation on the ecohydrological processes of affected areas in the lower reaches of the Tarim River. P. euphratica has a very strong ability to rehabilitate itself with two different regeneration strategies, i.e., i) sexual reproduction (new seedlings can be produced if seeds encounter favorable habitat conditions); and ii) clonal or vegetative reproduction (trees tend to produce various types of shoots from a bud at their base and from their roots). The frequency of occurrence of young seedlings is an indicator of ecological restoration output because it is tightly connected to soil physicochemical characteristics (particularly moisture and salt content); therefore, an effort was made to explore the effect of environmental flows on the establishment of young juveniles of P. euphratica trees in the lower reaches of the Tarim River based on data obtained from several extensive surveys of the permanent monitoring plots in our research area. In response to continuing deterioration and challenges facing this ecosystem, an ecological engineering project with the diversion of “emergency environmental flows” has been implemented in the affected areas since 2000 (Zhang et al., 2013). A total of 10.7 billion RMB (approx. 1.8 billion US dollars) was invested. By the end of 2015, environmental flows were diverted seventeen times with an aggregated volume of 51.10 × 108 m3. Environmental flows, also referred to as ecological water allocations, are a set of ecologically-based stream flow guidelines including the quantity, timing, frequency, duration, and quality of river flows that prevent further riverine degradation, protect extant resources, restore ecological function and provide a sufficient water supply for society (Richter et al., 2006; Poff et al., 2010). Ecological restoration of degraded arid and semiarid ecosystems by sustainable water resource management has received much attention worldwide due to accelerated desiccation, salinization and groundwater degradation. Since the 1990s, environmental flows have been used as a measure for ecological restoration, particularly by recharging alluvial aquifers to support existing degraded vegetation and promoting germination and establishment of targeted species (Nilsson and Berggren, 2000). Such attempts have also been applied to the restoration of degraded Tugai riparian vegetation in the lower reaches of the Tarim River. Initially, environmental flows are designed and expected to restore elements of natural river flooding regimes (Merritt and Bateman, 2012). However, implementing such flows can be challenging in practice due to the unpredictable demands by different water use sectors along the river (Stromberg, 2001; Skinner et al., 2008; Stromberg et al., 2016; Glenn et al., 2016; Jarchow et al., 2017a,b). This unpredictability also occurs along the lower reaches of the Tarim River. In most cases, the amount of water and flood pulse diverted for environmental flows are highly regulated and do not represent natural flow events (Liu et al., 2013). For instance, 3.5 × 108 m3 water was planned for allocation into the Tarim River each year, but in reality, this plan failed to be realized during many environmental flow events. There are many challenges to be overcome in maximizing the effectiveness of environmental flows in the lower reaches of the Tarim River. Therefore, the effectiveness of environmental flows should be maximized by optimizing the timing, magnitude and frequency of flows to meet the demands of Tugai riparian vegetation. Since the implementation of the restoration project, quantifying and assessing the effectiveness of restoration measures for highly degraded riparian ecosystems in the lower reaches of the Tarim River are still significant challenges. The Tarim riparian ecosystem has become a focus of desert riparian ecosystem research for both domestic and

international scholars. As an example, with the support of the SinoGerman joint research projects SuMaRiO (www.sumario.de) and EcoCAR (www.ecocar-centralasia.com) funded by the German Federal Ministry of Education and Research (BMBF) and the VolkswagenStiftung, scholars from both countries have conducted intensive research on sustainable management of river oases along the Tarim River (Aishan et al., 2013, 2015a,b; Kuba et al.,2013; Gärtner et al., 2014; Rumbaur et al., 2015; Keilholz et al., 2015; Betz et al., 2015; Lang et al., 2015, 2016; Thomas et al., 2016; Keyimu et al., 2017a,b; Mamat et al., 2018). The central question is how to balance natural ecosystems with their services and water use by agricultural irrigation under climate change conditions leading to modified water availability (Lu, 2004; Zhu et al., 2016; Bao et al., 2017). A substantial amount of the aforementioned research suggested that the implementation of this project has had positive effects on the ecosystems. However, the challenges faced by this restoration project have hardly been mentioned or reported. Richter et al. (2003) suggested that strategies for environmental flows should be based on determining a reasonable magnitude, frequency, duration, timing, and rate of change inflows to meet specific objectives at specific sites. Shafroth et al. (2010) recommended that threshold levels of volume and duration of flows should be determined in order to produce desired outcomes. Achieving maximum positive recovery of degraded riparian ecosystems by using limited water resources rationally is a main task for water managers in the region. Therefore, the solution to these problems is even more important for watersheds where water resources are scarce such as the Tarim River Basin. However, very little is known about the challenges this project is facing, as the following questions have not been addressed in the existing literature: 1) What is the difference between the present groundwater level achieved by the water diversion project and the favorable groundwater level of desert riparian vegetation? 2) How can we ensure a temporal match between the timing of water diversion and the seed dispersal season of the dominant species P. euphratica? The main goal of this study is to assess the effects and challenges of restoration measures on the groundwater recharge and the revitalization of degraded P. euphratica trees. In this way, the main contribution and achievements of the ecological engineering project for rehabilitation of the degraded riparian forests will be evaluated. Moreover, the limitations and challenges faced by the restoration project will be examined with a special focus on the threshold water table, establishment of young generations and development and timing of environmental flows. The results of this research could serve as a reference to help monitor changes in riparian forests in the Tarim Basin and possibly to recommend restoration measures for riparian ecosystems in arid regions worldwide. Most importantly, the results offer guidance to decision makers for sustainable management of these existing endangered P. euphratica riparian forests. 2. Materials and methods 2.1. Study area The study area is located at the lower reaches of the Tarim River near the Arghan village between Daxihaizi Reservoir (syn. Dashkol) and Tetema Lake (syn.Taitema Hu), Northwest China (Fig. 1). Due to the topographical conditions of the Tarim Basin, the climate of the study area is characterized as hyperarid with an annual precipitation of < 17 mm and a potential evaporation of 3000 mm (Betz et al., 2015). The climate of our research area is even more arid, with precipitation below 15 mm (Aishan, 2016). The main water resource in the Tarim River is melt water from high mountains surrounding the Tarim Basin. Under such harsh environmental conditions, the largest natural area of Tugai riparian forests can be found along the Tarim River at the northern and eastern edge of the Taklamakan Desert. Natural riparian vegetation is sparsely distributed and can establish only near lakes and rivers where 12

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Fig. 1. Map showing the location of Tarim Basin, northwest China. Highlighted in red rectangle is study area in the lower reaches of the Tarim River. Adopted from Halik (2003), Paproth and Pietsch (2011)

soil water conditions are suitable (Halik et al., 2006; Halik et al., 2009; Thomas, 2014). The vegetation is mainly composed of trees (Populus euphratica Oliv. and Elaeagnusan gustifolia L.) and shrubs (Tamarix spp.), while herbaceous plants are rare. Near the riverbanks, the vegetation density is relatively higher; in contrast, it sharply decreases towards the margins of the Taklimakan and Kuruk-Tagh deserts. Therefore, the integrity and resilience of the Tarim riparian ecosystem highly depend on the water supply from the Tarim River.

flood regime by replenishing and purifying the groundwater with manmade environmental flows. By the end of 2015, environmental flows had been diverted seventeen times at irregular frequencies throughout 1714 days in sixteen years with a total volume of 51.10 × 108 m3. 2.3. Data source and processing Hydrological data, meteorological data and terrestrial forest inventory data were used in this study. Data for providing an overview of environmental flows into the lower reaches of the Tarim River by the end of 2015 and data for groundwater depth from different wells located 50 m (G2), 150 m (G3), 300 m (G4), 500 m (G5), 750 m (G6) and 1050 m (G7) from the river where environmental flows cross were provided by the Tarim River Basin Administration Bureau (TRAB). These data were collected at long-term groundwater monitoring wells at 6 monitoring points, 90° to the different distances from the main water-flowing channel at the study area of Arghan village in the lower reaches of the Tarim River. In this study, the available groundwater depth data from these 6 monitoring wells during the period of 2000–2012 were used. The number of young seedlings of P. euphratica was recorded from a permanent sampling site with a total area of 100 ha (100 square plots, each 100 × 100 m in size). Data for tree vitality were collected repeatedly from random sampling sites with five circle plots, each of which comprised a circular area with a radius of 50 m around a central P. euphratica tree (resulting in an area of 7854 m2 per plot),set up as plot 1 (next to the river) to plot 5 (next to the

2.2. Manmade environmental flows Desert riparian ecosystems are extremely susceptible to water shortage resulting from both climate change and human disturbance (Deng et al., 2014; Ding et al., 2017). Due to upper-middle stream diversions for agriculture, particularly with the construction of the Daxihaizi Reservoir (a typical plain reservoir in the sandy desert) in 1972, flows into the lower reaches of the Tarim River were significantly reduced, and the downstream area with a length of 320 km was completely dried up; as a result, the water table dropped to its lowest level. To counter the widespread eco-environmental consequences in the region, manmade restoration measures (i.e., environmental flows into the affected areas) have been implemented in recent years, starting in May 2000. Over the nearly 30 years of water shortage, the existing Tugai vegetation was severely degraded in terms of growth status and extension, and rejuvenation in particular could not take place. The entire ecosystem was in danger of extinction. One of the main goals of these restoration attempts was to reconnect the riparian vegetation to its 13

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Taklamakan Desert). The groundwater depths of the plots ranged from 4 m to more than 10 m. Tree vitality refers to the growth status (including crown loss, leaves, branches and degree of defoliation) and extension of the canopy. According to the degree of vitality, the P. euphratica trees within the study area were grouped into six vitality classes: healthy trees (V0), i.e. leaf loss ≤10%; good trees (V1), i.e. leaf loss 11–25%; medium trees (V2), i.e. leaf loss 26–50%; senesced trees (V3), i.e. leaf loss 51–75%; dying trees (V4), i.e. leaf loss 76–99% ; dead trees (V5), i.e. leaf loss 100%.The criteria for classification and parameterization of this measure in detail can be obtained from Lam et al. (2011) and Aishan et al. (2015a). Buffer analysis and construction of a distribution map of young seedlings of poplar trees were performed using ArcGIS 10.2 software in conjunction with a GIS database established by integrating QuickBird data with terrestrial field survey data. Mann-Kendall tests (Ampitiyawatta and Guo, 2009; Yue and Wang, 2004) were used to analyze the monthly and annual trends of groundwater depths at different distances from the river. Data visualization was carried out with Microsoft Excel 2010, Origin 8.5 (OriginLab Corp. 2011) and R statistical software (R Core Team, 2013). Tests of monthly and annual trends of groundwater depth variation were performed with the combination of packages “xts” and “Kendall” in R.

the well closest to the river is below 4 m, a depth considered to represent a threshold for favorable groundwater depth for floodplain forest growth (Song et al., 2000). At the same time, there was a rise in the water table at the well nearest (50 m) to the river channel. An obvious fluctuation pattern of groundwater depth can be seen during water diverting time and non-water diverting time. At a distance greater than 300 m from the river, however, the groundwater depth of G5/G6/G7 is well below 6 m. These levels are unlikely to support the normal growth of floodplain forests. Floodplain forests growing farthest from the river might still be suffering from drought stress. 3.2. Spatiotemporal changes in groundwater depths during vegetation and nonvegetation seasons Maintaining the dynamic balance of the groundwater level is crucial in sustaining the survival and normal growth of nonzonal herbaceous or xerophytic vegetation in arid regions (Thomas, 2014; Lang et al.,2016). Previous studies have shown that groundwater depths of 3.15–4.12 m and 2.16–3.38 m are suitable for growth and effective restoration of two keystone species (P. euphratica and Tamarix ramosissima) in the lower reaches of the Tarim River. According to Song et al. (2000) and Chen et al. (2006, 2014), a groundwater depth greater than 9 m causes significant drought stress for P. euphratica trees and is thus regarded as an unfavorable groundwater depth. Fig. 3 shows the mean monthly groundwater depth during the vegetation and nonvegetation seasons of P. euphratica riparian forest. The blue and red lines in Fig. 3 indicate suitable and critical groundwater depths for P. euphratica, respectively. During the vegetation season, the groundwater depth at the G2/50 m, G3/150 m and G4/300 m wells exhibited clear changes and a tendency to reach the most suitable groundwater depth of 4 m, indicating that environmental flows had beneficial effects in this zone.

3. Results 3.1. Dynamics of groundwater depths induced by environmental flows After the manmade environmental flows were implemented, groundwater levels in the different buffers around the river responded correspondingly. Temporal and spatial fluctuations in groundwater depth at the study area (Arghan transect) were observed (Fig. 2). When the first water diversion for degraded riparian forest restoration in the lower reaches of the Tarim River was implemented, groundwater levels responded with different degrees of recovery (Hou et al., 2007). Analysis of groundwater oscillation from 2003 to 2011 at the Arghan transect showed that there were significant spatial and temporal differences in groundwater recharge between environmental flows with different impact intensities. As seen in Fig. 2, the groundwater depth at

3.3. Overall trend of changes in groundwater depth To determine the trend in the available time series data of groundwater depth for Arghan, the Mann-Kendall test was applied to explore the tendency for groundwater depth changes at the long-term

Fig. 2. Variations in groundwater depth at different distances from the river channel at the Arghan transect. 14

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Fig. 3. Spatiotemporal changes in mean monthly groundwater depths during vegetation (green boxes) and nonvegetation seasons (gray boxes); G indicates the number of the groundwater monitoring well (from G2 to G7), and the number after the slash (/) indicates the distance of the groundwater well from the river channel. Horizontal dashed lines (blue and red) represent suitable and favorable groundwater depths, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

monitoring plots in the study area at Arghan over the course of 9 years from 2003 to 2011. Table 1 shows that values of Z for the different wells range from −2.07 to 5.99, with the fluctuation of groundwater depth at all wells exhibiting a statistically significant declining trend for the entire period of 2003–2011. In accordance with the data distribution, the entire data set was split into two categories: one for the period of 2003–2009 and another for 2010–2011. The Mann-Kendall test for the first period (2003–2009) revealed that there was a significant declining trend (drop of the groundwater table) in mean monthly and annual groundwater depths, which might be associated with an insufficient volume and duration of the water diversion between the 5th and 10th diversions. This potential association follows from the fact that during this time (from 2004 to 2009, 6 years), only 6.02 × 108 m3 of water was diverted into the lower reaches of the Tarim River, accounting for approximately 20% of the total volume over the 9 years. By contrast, the groundwater depths at all wells for the period from January 2010 to December 2011 showed a significantly increasing trend (rise in the groundwater table).

2004, 2007 and 2011 to assess the health conditions of the trees in those plots (Fig. 5). The dynamics of tree vitality among the investigated years were analyzed (Fig. 5), and the overall results are summarized in Fig. 6. On average, the proportion of trees belonging to the vital, good and medium classes declined with increasing distance from the river to the Taklamakan Desert (i.e., from the P1 plot to the P5 plot). Large portions of the trees in the P1, P2, and P3 plots were mainly composed of trees from the V0, V1 and V2 vitality classes. For example, in the P1 plot, trees belonging to the V0-V2 classes accounted for approximately 61% of all trees in 2004, 65% in 2007 and 67% in 2011 (Fig. 6). In the P2 plot, such trees accounted for 57%, 63% and 69% for the years 2004, 2007 and 2011, respectively. Considerable numbers of trees in the P4 and P5 plots, however, were dying, dead or fallen. The P5 plot had the largest number of dead trees. Specifically, 6 fallen trees were observed in 2007, and 13 were observed in 2011, with only a single high-vitality tree observed in 2011. This result indicates that trees in this plot are still suffering from drought stress and are in the process of decline. Over the seven years from 2004 to 2011, the number of trees in the different vitality classes in the five plots exhibited different patterns of change, with fluctuations mainly taking place between trees in the V0, V1, V2 and V4 classes. In the plots closer to the river, the number of trees (V0 and V1) exhibited an increasing trend. Most of the trees belonging to the V3 class in 2004 shifted to the V2 and V1 classes as an effect of the favorable groundwater availability induced by environmental flows, suggesting that restoration measures took effect in halting further degradation of these trees. However, the proportion of trees in the V4, V5

3.4. Spatiotemporal changes in tree vitality The measure “tree vitality” used in this study can be a key indicator in assessing the overall situation of a forest, e.g., its health conditions, integrity and resilience. Based on the visual classification criteria for the vitality of P. euphratica trees, the tree vitalities at the five random plots along the water stress gradients were grouped into six classes as shown in Fig. 4. Each of the classes was then analyzed for the years 15

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Table 1 Mann-Kendall test for the mean monthly and annual groundwater depths at different distances from the river at the Arghan transect. Well ID/Distance from the river channel

Data set from January 2003 to December 2011 S

G2/50 m G3/150 m G4/300 m G5/500 m G6/750 m G7/1050 m Well ID/Distance from the river channel

−1691 −1865 −1854 −1746 −1440 −647

P value

−5.434 −5.993 −5.957 −5.611 −4.627 −2.077

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 0.037

Z

P value

−9.772 −9.602 −4.677 −8.362 −6.292 −2.529

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 0.011

Z

P value

0.149 0.155 0.155 0.165 0.160 0.152

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001

Data set from January 2003 to December 2009 S

G2/50 m G3/150 m G4/300 m G5/500 m G6/750 m G7/1050 m Well ID/Distance from the river channel

Z

−2011 −1976 −963 −1721 −1295 −521 Data set from January 2010 to December 2011 S

G2/50 m G3/150 m G4/300 m G5/500 m G6/750 m G7/1050 m

179 187 187 199 193 183

and V6 classes did reach high levels. Overall, the trees in all plots demonstrated different degrees of recovery, but examples of maximum recovery seemed to be restricted to the P1 and P2 plots. Trees in the other plots showed a smaller degree of recovery and even some degradation trends.

3.5. Distribution of saplings along a gradient of groundwater depth Fig. 7 shows the distribution of young P. euphratica saplings with a diameter at breast height (DBH) ≤ 4 cm. It is clear that the majority of saplings were observed very close to or 20–50 m from the river. The occurrence of saplings decreased dramatically as the distance from the

40.1359

River 40.1359

Tree status

r

40.1357

code 1

0m =5

code 2

DBH (cm) 0 10 20

40.1355

30 40 40.1353

40.1353 88.3768

88.3770

88.3772

88.3774

88.3776

88.3778

Fig. 4. Map showing the distribution of trees in plot 1. The green circle represents the boundary of the plot, red dots (code 1) represent relatively healthy trees (with vitality ranging from V0 to V3), and blue dots (code 2) represent highly degraded trees without living branches or with dieback (with vitality ranging from V4 to V5). 16

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Fig. 5. Distribution of trees in the different vitality classes in plot 1 for the investigated years. The map section is identical for the investigated years 2004, 2007 and 2011.

Fig. 6. Variation in the number of trees in different vitality classes in plots for the investigated years.

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Fig. 7. Spatial distribution of saplings of P. euphratica at different distances from the river.

main river course increased. At a distance from the river of 200 m or more, saplings of different diameters were sparsely distributed. Densely established saplings were observed in the western part of the investigation plot. During the fieldwork, we confirmed that there was a microrelief with a shorter depth to groundwater, and those saplings experienced suitable site conditions for their establishment. At distances of more than 400 m from the river, almost no saplings were found. In our research area, high densities of P. euphratica saplings mainly occurred near the zones highly affected by environmental flows or around suitable microreliefs. Fig. 8 shows the density of sapling and its relation to the distance from the river. The saplings with a diameter of 2–3 cm at different distances from the river exhibited a higher density than other saplings. The number of saplings of all DBH values was significantly associated with the distance to the river. The number of trees with a diameter of 0–1 or 1–2 cm was significantly negatively correlated with the distance of the trees from the river course (R2 = 0.75 and R2 = 0.7 at a P < 0.01 significance level, respectively). Similar results were also obtained for the correlations between the number of young trees with a diameter of 2–3 or 3–4 cm and the distance to the river; however, the negative correlations were significant only at the P < 0.05 significance level.

groundwater depths (suitable groundwater depth for growth, i.e.,≤ 4 m and groundwater depth resulting in partial or total desiccation (m), i.e., > 9 m), suggesting that current environmental flows are effective in keeping the groundwater depth at a certain level at which P. euphratica riparian forest can grow and survive. However, under this water regime, the majority of shrubs and grasses with a shallow root system still suffer from water scarcity (Chen et al., 2015). As mentioned in previous research, environmental flows have a time-lag effect on groundwater recharge. Since the suitable groundwater depth controlled by current environmental flows is a major water source for riparian vegetation in the lower reaches of the Tarim River, it is essential to assess groundwater depth not only for short time intervals but also for a long time interval. Ecological engineering practices in recent years clearly demonstrated that the timing, volume, duration and intensity of environmental flows in the lower reaches of the Tarim River were random and uncertain. As a result, the response of groundwater depth has fluctuated considerably. For instance, significant increasing trend in groundwater depths in our study area was observed during the years 2010 and 2011 by using Mann-Kendall test. This result is likely because the amount of water (5.49 × 108 m3) diverted to the lower reaches of the Tarim River from 2010 to 2011 was the same as the total amount diverted in the years 2004–2009, thus suggesting that groundwater fluctuation is highly dependent on water volume and the duration of the diversion. Additionally, the magnitude of the decrease in groundwater depth at the G2 well (at a 50 m distance from the river) was higher than that at any other well. Clearly, variations in the groundwater depths are strongly correlated with the volume and duration of the water

4. Discussion 4.1. Assessment of the status quo of groundwater depth Groundwater depth at all wells was between two threshold 18

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Fig. 8. Spatial distribution of saplings and the relationship between sapling size and distance to the river. Density refers to a statistical kernel density estimation which is a measure for the cumulative probability of the occurrence of saplings with certain diameters in a certain distance from the river.

diversions. Therefore, the minimal amount of water sufficient to guarantee favorable groundwater levels for the normal growth of vegetation should be allocated and diverted into the affected region (Aishan,

2016). In addition, wind erosion hazard is a key environmental issue in the region. According to Betz et al. (2015), increasing cover with grass and

Table 2 Timing of environmental flows and the biological life cycle of riparian vegetation in the restoration area (the temporal match between the environmental flows and the seed dispersal season of P. euphratica trees is indicated by the red frame).

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shrub species is crucial for controlling wind erosion-related hazards. However, compared with the root system of P. euphratica, that of grass and shrub species is too shallow to use the groundwater under current water management. Therefore, close attention should be paid to restoring other species during the restoration of the target species (poplar tree), which would also help improve biodiversity and maintain ecological resilience.

dynamics, new poplar sites can be permanently established. Historically, Tugai riparian forest development along the Tarim River was inherently dependent on high fluvial dynamics, which cause channel movement and sediment deposition. Correspondingly, these deposited sediments provide potential regeneration sites. Thevs et al. (2008) investigated the link between germination and establishment of P. euphratica and the river course movement of the middle reaches of the Tarim River during the past decades based on a historical map from Sven Hedin, Landsat satellite images and field survey analyses and suggested that the section of the middle Tarim River with prevailing natural river dynamics was crucial for the continuous natural regeneration and sustainable conservation of the Tugai forests. Li et al. (2017) demonstrated that the middle reaches of the Tarim River have a high mean migration rate of 17.2 m/y. During the short flood season, the investigated parts of the river show minimal resistance to erosion, thereby promoting high rates of bank erosion and lateral migration, since channel margins have a very low clay content and lack riparian vegetation. In recent decades, as the water consumption in the upper reaches of the Tarim River has increased, water diversion has significantly reduced flow discharge along the middle and lower reaches of the Tarim River. However, channel movement does not take place in the lower reaches of the Tarim River, mainly because there is no flood event sufficiently large to create migration under current manmade water allocations. In addition, reservoirs along the river modify not only the discharge but also the sediment flux, which is as crucial as the discharge for the morphodynamics of a river (Aishan, 2016). Yu et al. (2016) and Li et al. (2017) already suggested that significant decreases in discharge downstream have likely decreased the rates of lateral migration. Stella et al. (2011) investigated the effects of abandoned river channels on the colonization of pioneer riparian trees of Populus fremontii within the riparian corridor of a large river, which showed that in rivers where tree recruitment along the active channel is severely limited by hydrological regulation, abandoned channels developed by river migration may provide an even more important habitat for sustaining an ecologically functional riparian corridor. Therefore, it appears that regulated rivers can no longer sustain the natural development of forests, since the stream flow cannot be managed to mimic pre restoration conditions. Based on these findings, it can be assumed that these impacts will likely continue if a holistic water management strategy is not implemented along the entire Tarim River and thus will probably induce an ecological crisis in this area associated with serious channel shrinkage.

4.2. Temporal mismatch between environmental flows and vegetation water demands Against the background of the described ecohydrological dynamics and the scarcity of the water resources in the affected downstream area, different factors have to be considered and coordinated for water and ecosystem management. The most important questions to be taken into account under the given circumstances of environmental flows are when (timing) and how much (volume) water should be diverted to the lower reaches. Table 2 shows the timing of environmental flows and the biological life cycle of the riparian vegetation in the restoration area. Several environmental flows took place outside the scope of the vegetation season. Floods outside the period from June to October may have no effect or less effect with regard to rejuvenation of the stands, since there are no mature, germinating seeds. The months of August and September are especially important during this period. As is apparent from Table 2, the majority of the seeds matured over these two months. There were frequent temporal mismatches between the environmental flow time and the seed dispersal season of P. euphratica trees. From Table 2, it can be shown that a total number of 22 environmental flows took place between 2000 and 2015. Of these flows, only three (the 11th, 12th and 13th environmental flows) meet the criteria of an appropriate time (July and August) as well as a sufficient amount (water reaches the terminal lake). A statement about suitable flood peaks cannot be made in this study (Aishan, 2016) due to the scarcity of discharge data and the fact that the data exist only at a very coarse resolution. According to Xu et al. (2009) and Cao et al. (2012), sufficient soil moisture is a determining factor for activating the soil seed bank and the establishment, survival and growth of young P. euphratica seedlings. P. euphratica stands have limitations in regenerating their stand structure by seeds in zones far away from the river due to the lack of flooding. Therefore, already-established seedlings will not be able to make contact with adequate soil moisture or groundwater because of their shallow root system. In this case, root suckers play the main role in regeneration, as has already been shown in P. euphratica riparian forest regeneration. Root suckers may emerge some distance from the originating plant, are considered a form of vegetative dispersal, and may originate in a habitat patch where that tree is the dominant species (Wang et al., 1996; Wiehle et al., 2009; Eusemann et al., 2013; Kramp et al., 2018). Suckers also may arise from the roots of trees that have been cut down. From this point of view, additional research on determining the effectiveness of environmental flow regime on the occurrence of root suckers of P. euphratica trees in the lower reaches of the Tarim River is needed. Therefore, we suggest that two aspects should be taken into account when considering the amount of water. On the one hand, a sufficient volume of water must be discharged into the entire lower reaches as far as the terminal lake (Tetema) to fulfill both ecological and socio-economic demands. As is apparent from Table 2, the use of such a volume has not always been the case. Above all, the high evaporation, the water consumption of the vegetation and the seepage have to be taken into account, since hardly any anthropogenic water extractions take place in the downstream area. On the other hand, sufficient flood peaks have to be considered. The use of sufficient flood peaks is the only way to ensure that the seeds are deposited in higher areas and that those seedlings that are produced are not washed away, even with small outflows. In addition, the natural morphodynamics of the river can be restored in this way. By means of larger-scale riverbed

5. Conclusions and outlook The research findings indicate that the environmental flows in the lower reaches of the Tarim River have played a significant role in raising groundwater tables and in vitalizing Tugai riparian forest to some degree. The contributions of the environmental flows to the revitalization of the highly degraded P. euphratica trees that are growing relatively far from the main riverbed were not obvious. In this paper, we mainly focused on the rehabilitation of the target species P. euphratica as an example of restoration success; in fact, there are also other species such as Tamarix spp. and other shrubs and herbs located in the area. Therefore, taking the water requirements of those species into account is necessary in future work. The main focus of this research was monitoring and assessing P. euphratica riparian forests over long time periods to determine the effectiveness and challenges of ecological restoration in the lower reaches of the Tarim River. Since water is a crucial determining factor for both natural and anthropogenically modified ecosystems, addressing how limited water resources are allocated and managed effectively is of key importance to sustaining ecological and socio-economic development in the region. Lessons, experience and scientific knowledge from past and current restoration activities regarding the difficulties of restoring degraded natural ecosystems in the region should be considered and 20

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applied to future efforts associated with the objectives of ecological restoration projects. The results will provide a scientific basis for determining how restoration efforts in arid regions can contribute to highly degraded ecosystems. At the same time, the results may also serve as supplementary information for riparian forest managers and other practitioners. As mentioned by many scientists both inside and outside of China, the vitality of the degraded forests along the Tarim River riparian ecosystems depends strongly on how the limited water is managed in the present and the future. Specifically, the following recommendations can be made:

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1. Due to the significant differences among the water use strategies of individual P. euphratica stands, methods for estimating the optimum water requirements of P. euphratica stands for different degradation gradients and hydrological conditions should be developed not only at the stand scale but also at the landscape scale. 2. Generative reproduction is the best way to maintain genetic diversity and achieve long-term restoration of P. euphratica trees (Wang et al., 1996; Kramp et al., 2018). Future studies should also focus on how water availability can affect the mechanisms of vegetative and generative reproduction of P. euphratica, the latter of which is critical to generating genetic diversity of P. euphratica for long-term forest management. 3. The temporal aspect of environmental flows should be adapted to the phenology of the main species P. euphratica and Tamarix spp. Environmental flows outside the vegetation period from June to October may have little effect on the rejuvenation of the stands since there are few mature and germinating seeds. The months of July and August are essential in activating the soil seed bank and establishing new recruitment at a large scale. 4. A potential trade-off between the restoration of the Tugai forest in the lower reaches of the Tarim River and the water diversion source, i.e., the wetland ecosystem in Bosten Lake (syn. Bagrash Lake), should be taken into account in future restoration efforts. Restoration of the Tugai forest along the lower reaches of the Tarim River should be initiated without sacrificing the integrity and sustainability of the wetland ecosystem of Bosten Lake, which is the largest freshwater lake in the region. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant Nos.:U1703102, 31700386,and 31770750), Doctoral Research Project of Xinjiang University (Grant No.: BS160258),and the Thousand Youth Talents Plan of China: Xinjiang Projects. We thank the Tarim River Basin Administration Bureau for providing hydrological data. The authors are grateful to the anonymous reviewers for their constructive comments. References Aishan, T., Halik, Ü., Cyffka, B., Kuba, M., Abliz, A., Baidourela, A., 2013. Monitoring the hydrological and ecological response to water diversion in the lower reaches of the Tarim River, Northwest China. Quat. Int. 311, 155–162. https://doi.org/10.1016/j. quaint.2013.08.006. Aishan, T., Halik, Ü., Kurban, A., Cyffka, B., Kuba, M., Betz, F., Keyimu, M., 2015a. Ecomorphological response of floodplain forests (Populus euphratica Oliv.) to water diversion in the lower Tarim River, northwest China. Environ. Earth Sci. 73 (2), 533–545. https://doi.org/10.1007/s12665-013-3033-4. Aishan, T., Halik, Ü., Betz, F., Tiyip, T., Ding, J.L., 2015b. Stand structure and heightdiameter relationship of a degraded Populus euphratica forest in the lower reaches of the Tarim River, Northwest China. J. Arid. Land. 7 (4), 544–554. https://doi.org/10. 1007/s40333-015-0046-8. Aishan, T., 2016. Degraded Tugai Forests under Rehabilitation in the Tarim Riparian Ecosystem, Northwest China: Monitoring, Assessing and Modelling. Ph.D. Thesis. Katholische Universität Eichstätt‐Ingolstadt, Eichstätt, Germany, pp. 120–125. Ampitiyawatta, A.D., Guo, S.L., 2009. Precipitation trends in the Kalu Ganga basin in Sri Lanka. J. Agric. Sci. 4 (1), 10–18. https://doi.org/10.4038/jas.v4i1.1641. Bao, A.M., Huang, Y., Ma, Y.G., Guo, H., Wang, Y.Q., 2017. Assessing the effect of EWDP

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