Drought Resistance of Desert Riparian Forests: Vegetation Growth Index and Leaf Physiological Index Approach

Abstract

The Hotan River, the sole river traversing the Taklimakan Desert in northwest China, sustains a critical desert riparian ecosystem dominated by Populus euphratica. This riparian habitat is integral to biodiversity maintenance. However, global climate change and anthropogenic disturbances have profoundly impacted the Taklimakan desert landscape, leading to fragmentation and reduced environmental heterogeneity. Consequently, there has been a notable decline in P. euphratica populations. This study aimed to assess the physiological resilience of P. euphratica under harsh conditions and analyze the vegetation distribution patterns along the desert riparian zone. Laboratory tests were employed to determine the physiological indexes including Relative Water Content (RWC), Chlorophyll (Chl), Soluble Sugar (SS), Free Proline (Pro), and Peroxidase Activity (POD) of P. euphratica, providing insights into its capacity to endure challenging environmental conditions. Quadrat surveys were conducted at varying distances from the riverbed to examine vegetation distribution patterns. Plant growth indexes were analyzed to unveil the resistance of the desert riparian forest to drought. The study identified 45 shrubs and herbs belonging to 17 families in the Hotan River understory, with P. euphratica exhibiting the highest abundance. In river flats, annual herbs dominated due to favorable water conditions, while shrub grasslands displayed a relatively complete community structure with trees, crowns, and grasses. As the distance increased from the river channel, more perennial herb and shrub species prevailed, leading to a decline in overall species richness as annual herbs diminished. Physiological assessments revealed that P. euphratica in a medium growth grade (VS3) exhibited the highest physiological indexes, indicating its adaptability to environmental changes. The findings underscore the significance of water conditions in the growth and development of vegetation in desert riparian forests, particularly highlighted by the physiological indexes of P. euphratica. This research contributes valuable insights that can inform the preservation and restoration of desert riparian forests, providing a scientific basis and technical guidance for conservation efforts.

1. Introduction

It is important to understand that deserts play a vital role in the global environment despite their arid and seemingly inhospitable nature. As a result of extreme water scarcity, their vegetation has evolved ingenious mechanisms for survival, such as deep roots that stabilize soil and prevent erosion [1]. These hardy plants not only sequester carbon dioxide from the atmosphere but also function as carbon sinks. Moreover, desert landscapes impact river formation and flow, which makes them integral to river dynamics. Providing a reliable water supply for humans and sustaining downstream ecosystems is crucial to deserts’ ability to store and release water slowly [2]. In the context of today’s changing climate and ecosystems, the intricate relationship between deserts, their vegetation, and river dynamics emphasizes the environmental significance of these seemingly harsh landscapes. We can better understand these arid ecosystems by understanding the theory and paradigm that apply to deserts [3]. There are a few general ecological theories and paradigms that are specific to deserts; the understanding of how organisms adapt to extreme water scarcity is one of the central paradigms of desert ecology [4]. In arid environments, plants and animals have evolved various adaptations to survive, such as succulence and deep roots. Deserts are thought to experience sporadic, yet intense, resource inputs triggered by irregular rainfall events [5]. Adapting to these resource pulses has enabled organisms in desert ecosystems to influence population dynamics and community composition. An island of fertility is a localized area located within a desert, such as an oasis or a rocky outcrop, where the soil moisture and nutrients are higher [6]. For desert plants and animals, these areas are crucial refuges. According to the stress-gradient hypothesis, as environmental stress, such as droughts and high temperatures, decreases, competition between plants and other organisms becomes more intense [7,8]. Facilitative interactions may be more prevalent on less stressful microsites. Microclimate, topography, and rainfall patterns result in considerable spatial and temporal variability in desert ecosystems. Community dynamics and species distribution are affected by this variability [9]. Biodiversity and endemism are common in desert regions, where some species are found nowhere else in the world. It is believed that desert biodiversity develops as a result of its unique adaptations and speciation processes [10]. The desert ecosystem is subjected to a variety of disturbances, including flash floods, wildfires, and sand dune movement. Desert ecology focuses on the evolution of desert organisms and how they have adapted to arid conditions [11]. Desert conservation and management theories are becoming increasingly important as human activities encroach on desert ecosystems.

In arid regions, these theories address issues such as habitat degradation, invasive species, and sustainable land use. Our understanding of these unique and fragile ecosystems continues to evolve as desert ecology evolves. In the face of environmental changes and human impacts, these theories and paradigms aid ecologists and conservationists in understanding and preserving desert environments. In the Taklimakan Desert, there is only one river that flows into the Tarim River: the Hotan River (Figure 1). Throughout the lower reaches of this river, vegetation is symmetrically distributed on both sides, forming a continuous green corridor. In the Taklimakan Desert, the green corridor serves as an important passage to deserts, and its growth level is a key indicator of desertification. With little rain and strong sunshine, sharp changes from cold to hot, and frequent dust storms, the climate conditions in this area are typical of those found in the continental deserts [12,13]. As a result of the arid climate and human disturbance, desert riparian forests are more susceptible to these disturbances because of their low biodiversity and weak resilience [14,15]. It is, therefore, important to comprehensively analyze the growth conditions of desert riparian forests in response to crucial environmental conditions. The Karakash and Yurungkash Rivers have diverted water from the mainstream of the Hotan River in recent years [16,17]. In both floodplains and terraces along the river channel, the groundwater level is reduced, resulting in a decline or death in trees and shrubs that are growing farther from the channel. The ecological environment of the green corridor is deteriorating, the forest and grass along the river bank are degrading, and the threat of the desert approaching the river bed is becoming more serious [18,19]. Populus euphratica, as the dominant species of desert vegetation, has great ecological significance in the improvement and maintenance of the regional environment [20,21].

A decrease in P. euphratica populations in the lower reaches of the Hotan River has been observed since the upper reaches of the river have been overused by consumptive water resources. Thus, the management and protection of P. euphratica in this area are urgently needed [22,23]. In order to protect vegetation resources in the lower reaches of the Hotan River, many scholars have conducted similar research on desert vegetation’s responses to interference by drought. Using remote sensing techniques, Zhou Min et al. obtained information on forest land area, river depth, and river width along the middle and lower reaches of the Hotan River during the rainy season [24]. Previously, a researcher used 3S technology to study the spatial pattern of vegetation in the Hotan River basin and found that the vegetation cover in the whole basin was negatively correlated with the distance from the river bank and the elevation difference of the river surface [25]. The ecological water consumption of the lower reaches of the Hotan River was estimated using the phreatic water evaporation method and the area quota method and it was concluded that the river sub-surface water could meet the ecological water demand of the vegetation near the river channel [26]. In previous research, the historical change process of Hotan River diversion was studied based on historical documents. In these studies, monitoring data were combined with remote sensing data to study Hotan River water volumes over time [27].

The literature reports on the survey of vegetation along the Hotan River are relatively scarce, with the majority based on an analysis of the dynamic changes in natural vegetation along the river bank. In addition to discussing the changes in the ecological environment of the entire river basin, it also reveals a relationship between river runoff loss and vegetation water requirements [28]. With regard to the research on P. euphratica, a previous study analyzed the characteristics of the population structure of P. euphratica in different habitats of Hotan River, discussed its competitive relationships, and found that change in the habitat would cause change in the species composition and structure [29]. As a result of human disturbance in the Basin, the desert landscape is seriously fragmented, reducing environmental heterogeneity and affecting P. euphratica populations. An earlier study evaluated the ecological service value of the natural P. euphratica forest in the Hotan River basin and found that the average service value of the P.euphratica forest ecosystem in the Hotan River basin was 672 million yuan/year, indicating that the ecological value of the Hotan River basin could not be ignored [30]. It was proposed that the regeneration and rejuvenation methods of P. euphratica forest in the Hotan River basin indicate that the site conditions of silty and sandy soil are conducive to the survival of P. euphratica seedlings after a flood [31]. There has been no report on the physiological index of P. euphratica in this area, despite the fact that the growth conditions and ecological service value of the species in this basin have been examined.

This study examines the scientific question, “How have distribution pattern and growth indicators of natural vegetation as well as the physiological indicators of P. euphratica changed under different growth conditions in desert riparian forests?” The primary objective of this study is to comprehensively assess the physiological resilience of P. euphratica, the dominant species in the desert riparian ecosystem along the Hotan River in northwest China. This involves evaluating its adaptive mechanisms under harsh environmental conditions, particularly considering the impacts of global climate change and anthropogenic disturbances. The study aims to provide a nuanced understanding of the ecological dynamics within the Hotan River system, emphasizing the critical role of water conditions in shaping vegetation distribution patterns and overall biodiversity. Additionally, the research seeks to contribute scientifically informed insights to guide the preservation and restoration of the desert riparian forests, addressing the notable decline in P. euphratica populations.

Based on the results of field surveys and monitoring, this study analyzes the survivability of the vegetation in the desert riparian forest based on the variation in the growth index and physiological indexes of P. euphratica. In addition to serving as a scientific basis and technical support for the protection and restoration of desert riparian forests, the results of this study will also be helpful for the implementation of ecological protection in the downstream green corridor of the Hotan River basin. The novelty of this study lies in its holistic approach to understanding the physiological resilience of P. euphratica in a desert riparian ecosystem facing contemporary environmental challenges. Notably, the research integrates laboratory tests, quadrat surveys, and plant growth indexes to offer a comprehensive evaluation of the species’ adaptability. The identification of 45 shrubs and herbs within the Hotan River understory, coupled with a detailed analysis of vegetation distribution patterns, adds a novel layer to the understanding of biodiversity dynamics in this unique habitat.

2. Material and Methods

2.1. Overview of the Study Area

The Hotan River is formed by the confluence of the Karakash and Yurungkash Rivers (Figure 1). Stretching a total length of 319 km, the river traverses the Taklimakan Desert from its confluence to the north, ultimately flowing into the Tarim River. The riverbed of the mainstream is characterized by shallow depths, maintaining an average width of 1–2 km. Notably, the river channel experiences flow only during the flood season, typically occurring from July to September, during which waterside erosion becomes a significant concern. Situated in the heart of the desert, the mainstream of the Hotan River faces challenges such as strong evaporation and substantial leakage, designating it as a runoff loss area. In the context of this study, “runoff loss area” signifies a region where surface water runoff occurs, resulting in the removal of water and associated materials, such as soil particles, nutrients, or pollutants, from their original location within the landscape. The term “loss” emphasizes the displacement of water and materials away from their initial landscape position [32,33].

Due to the relative water conditions in the riverbed and riverbank, the vegetation structure includes trees, shrubs, and herbs, forming a unique desert riparian forest vegetation system. The vegetation in the river bed is relatively sparse, and the two banks are composed of naturally growing plants including Populus pruinosa, Populus euphratica, Tamarix taklamakanensis, Phragmites australis, and Glycyrrhiza uralensisand and constitute the green belt. The annual average values for the primary meteorological elements of the study area are shown in

Table 1. Meteorological characteristics of the study area.

Month	Average Temperature/°C	Relative Humidity/%	Thunderstorm Frequency/d	Daily Sunshine Time/h	Maximum Wind Speed m/s	Wind Direction	Precipitation/mm	Average Evaporation/mm
January	−5.6	53	0	174.3	11	SW	1.7	43.7
February	−0.3	49	0	157.1	12	SW	2.3	84.9
March	9	35	0	192	15.5	SW	1.8	205.7
April	16.5	29	0.2	197.2	15	W	2.9	276
May	20.4	35	1	233	19	W	7.2	341
June	23.9	37	1.2	257.2	17.3	SWW	7.3	359.7
July	25.5	40	0.6	248.7	15.3	W	5.2	345.7
August	24.1	44	0.1	232	16.8	SW	4.1	308.3
September	19.7	43	0	237.6	13	SW	2.6	239.5
October	12.4	40	0	265.3	11	SSW	1.1	168.7
November	3.8	45	0	225.6	11.5	SSW	0.4	103.8
December	−3.2	54	0	190.5	12.5	SW	0.9	46.9
Annual	12.18	42	3.1	2610.5	14.2	SW	37.5	2523.9

Note: data source: National Meteorological Science Data Center, http://data.cma.cn/site/index.html (accessed on 18 December 2023).

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2.2. Field Monitoring Data

A field survey was conducted in the lower reaches of the Hotan River in July 2022. The sample plot survey method was used to conduct surveys in river flats, grasslands, shrub grasslands, and desert forests. The types of sample plots were identified based on the “Classification Standard of Land Use/Cover Change” of China (GB/T21010-2017) [34]. According to this standard, the river flat refers to the area where land is submerged when it is deeper and exposed when it is shallower. Grassland (or desert grassland) refers to land that is dominated by perennial herbaceous plants and contains small shrubs that develop and form under drought conditions. Due to the small amount of river water in desert areas, the river flats are basically in a state of no water and higher vegetation coverage. Shrub grassland refers to grassland with a shrub coverage exceeding 20%. Desert forest refers to a plant community composed mainly of P. euphratica and P. cinerea on the alluvial plains along the inland rivers of the desert, with sparse shrubs and herbaceous plants. Taking into account the representativeness of vegetation types and the typicality and operability of the landscape, the sample plots of trees, shrubs, and grasses were set up, the geographic coordinates and elevation of the observed sample plots were recorded, and the vegetation types and vegetation attributes in the sample plots were investigated and classified. A total of nine random monitoring sections were identified according to the accessibility of the target riparian forest (Figure 1). Based on the topographic conditions, the sample plots measured 50 m × 50 m or 25 m × 25 m. Within each of the large quadrats, 3–5 shrub quadrats (5 m × 5 m) and herbaceous quadrats (1 m × 1 m) were sampled (Figure 2).

The types of trees, shrubs, and grasses, the number of individuals of each species, the growth grade, and other ecological indicators were obtained.

2.3. Calculation of Vegetation Growth Index

The Patrick richness index, Simpson dominance index, Shannon–Wiener diversity index, and Pielou evenness index of the vegetation in different monitoring sections were calculated using the following equations [35,36]:

Patrick richness index:

A = N/ln(S)

(1)

where N is the number of species and S is the quadrat area.

Simpson dominance index:

$$D=1-{\displaystyle \sum _{i=1}^S{P}_i^2 (\mathrm{I}=1,2,3,4\dots \mathrm{S})}$$

(2)

Shannon–Wiener diversity index:

$$H=-{\displaystyle \sum _{i=1}^S(P_i\ln P_i)}$$

(3)

Pielou evenness index:

J = H/lnS

(4)

where D is the dominance index, s is the number of the species in the quadrat, i is the ith vegetation, P_i is the proportion of the number of individuals of species i to the number of individuals of all species, H is the Shannon–Wiener index, J is the evenness index.

The obtained vegetation index was analyzed using one way ANOVA to compare the growth status of vegetation (SPSS 22). The analysis of variance (ANOVA) is a widely known and used statistical technique that allows the interpretation of the experimental results by obtaining the contribution ratio of each parameter. ANOVA is used to examine the significance of each parameter for the problem to be solved using computational fluid dynamics (CFD). CFD enables a comprehensive understanding of water flow patterns, erosion dynamics, and sediment transport, providing crucial insights into the river’s behavior under varying conditions. This technique ensures a robust analysis of the hydrological complexities in the context of the Taklimakan Desert, enhancing the precision of our assessments for effective ecosystem management and conservation planning. The calculation steps used in ANOVA are as follows:

The total sum of squares (SSr) was calculated using Equation (5).

$$\mathrm{S}\mathrm{S}\mathrm{r}={\sum }_{n=0}^i({Yi-Ý)}^2$$

(5)

where n is the number of cases in the orthogonal array, and Yi is the experimental/numerical result for the ith experiment and can be obtained using Equation (6):

$$Ý=1/N{\sum }_{I=1}^nÝi$$

(6)

2.4. Growth Grade Standard of Populus Euphratica

In addition to canopy richness, tree vitality is a comprehensive biological, ecological, and morphological concept that reflects the growth trends and conditions of trees and shrubs. It is one of the important indicators for measuring the overall vitality of forests, including health conditions, structural integrity, and resilience. In this study, according to the Forest Growth Monitoring Standards of Germany (FGMS, 1992), combined with the arid desert environmental conditions of our study area and the unique ecological adaptability and individual attributes of P. euphratica, we have preliminarily formulated the grading standards for the growth condition of P. euphratica, as shown in Figure 3. Further, the growth trend of P. euphratica at different growth conditions was investigated in combination with the arid desert environmental conditions in the lower reaches of the Hotan River and the unique ecological adaptability and individual attributes of P. euphratica. According to the growth grade standard of P. euphratica given in Figure 3, healthy and non-mechanical damaged tree samples of similar ages (DBH about 20~25 cm) were selected from different locations from the riverbed. Each growth grade was represented by two sample trees. Likewise, the tree’s height was estimated, and the vegetation types beneath the tree were described.

Sample trees of P. euphratica with different growth grades are selected. A P. euphratica crown is split into upper, middle, and lower layers. Approximately 50 to 70 leaves from each layer are cut, wrapped in gauze, and cooled in liquid nitrogen before being tested in the lab. The laboratory testing of the P. euphratica crown was conducted randomly, since it is difficult to differentiate between the upper, middle, and lower layers of the crown. The leaves were randomly chopped off to determine growth grades.

2.5. Determination of Physiological Indexes of P. euphratica

According to the five growth grade standards, a total of 10 P. euphratica were selected for leaf sampling. The experiments were conducted in the Central Laboratory of Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences. The physiological indexes of P. euphratica were determined according to Zeng et al. [37] and Chen et al. [38]. Five physiological indicators, namely, the relative water content (RWC), chlorophyll (Chl), soluble sugar (SS), free proline (Pro), and peroxidase activity (POD), were selected. For the assays of the RWC, Chl, SS, Pro, and POD of the leaf, firstly, the shoot dry weight of collected leaves was determined. The relative water content (RWC) of the leaf was calculated based on the leaf’s fresh weight. After hydration, the samples were taken out of the water and were well dried of any surface moisture quickly and lightly with filter/tissue paper and immediately weighed to obtain a fully turgid weight (TW). Samples were then oven-dried at 80 °C for 24 h and weighed (after being cooled down in a desiccator) to determine the dry weight (DW). The RWC was calculated using the following equation:

RWC (%) = [(W − DW)/(TW − DW)] × 100%

(7)

where W is the sample fresh weight, TW is the sample turgid weight, and DW is the sample dry weight.

RWC is an appropriate estimate of plant water status in terms of cellular hydration under the possible effect of the leaf water potential. The method is simple, and this is another advantage. It allows us to estimate the water content of the sampled leaf tissue relative to the maximal water content it can hold at full turgidity. Normal values of RWC range from 98% in fully turgid transpiring leaves to about 30–40% in severely desiccated and dying leaves, depending on the plant species. In most crop species, the typical leaf RWC at around initial wilting is about 60% to 70%, with exceptions.

Leaf chlorophyll (Chl) was extracted by soaking leaves in a dimethyl sulfoxide solution for 48 h. The absorbance of extracts was measured at 635 and 645 nm with a spectrophotometer. For the measurement of the SS content, the absorbance value was determined using distilled water as a blank control at a wavelength of 620 nm, and the soluble sugar content was calculated through a standard curve using the following equation.

SS (%) = C × V × n/(10⁶ × α × W)

(8)

where C is the sugar content obtained from the standard curve (g), V is the extracted liquid volume (mL), α is the volume of extracted sample solution (mL), n is the dilution ratio, and W is the tissue weight (g).

In general, the ratio of the soluble sugar (SS) ranges between 0.1% and 0.5%. For the measurement of Pro content, after washing and drying the fresh leaves with clean water, we weighed 0.1 g of leaves and deposited it into a mortar; using a control tube as a blank control, the absorbance value at a wavelength of 520 nm was determined, and the content of arginine against the plotted proline standard curve was obtained. For the measurement of POD, the oxidation of guanaco was measured using the increase in absorbance at 470 nm for 1 min. The reaction mixture contained 50 µL of 20 mM guanaco, 2.8 mL of 10 mM phosphate buffer (pH 7.0), and 0.1 mL of enzyme extract. The differences and the response mechanism of these indicators within different growth conditions were analyzed. Statistics were performed with two replicates, and SPSS 22 was used for the correlation analysis. The analysis of variance (ANOVA) and least-significant difference (LSD) test were used for the data from physiological parameter measurements; less than 0.05 p-values were considered significant.

3. Results

3.1. Vegetation Types in the Lower Reaches of the Hotan River

Our study analyzed the distribution pattern of vegetation in the nine monitored sections (Figure 1), including the river flat, understory shrub grassland, shrub grassland, and desert forestland. As each section has a different topographic condition, the area and distance from the river channel were not considered. The trees in the green corridor of the Hotan River are mainly P. euphratica individuals. The common shrub species in the forest mainly include Tamarix ramosissima, Tamarix setifolia, Hippophae rhamnoides Diglossa humeralis, Halimodendron halodendron, and Nitraria schoberi. The common perennial plants mainly include Phragmites australis, Calamagrostis epigeios, Glycyrrhiza uralensis, Alhagi sparsifolia Shap, Acroptilon repens, Aeluropus pungens, Glossocardia bidens, Asparagus cochinchinensis, Takhtajaniantha austriaca, and Crypsis aculeata. The annual herbs mainly include Sophora alopecuroides, Salsola pellucida, Halogeton glomeratus, Tetras tigmatonkinense Gagne, and Chenopodium glaucum. The floodplain nearest to the river channel is mainly composed of young poplar/euphratic poplar forests, as well as shrubs, perennial herbs, and annual herb seedlings that have sprouted in the past two years. Because the soil moisture condition in this section is good, it provides an opportunity for seed germination. However, the diversity richness decreases with the increase in the distance from the riverbed. The distribution of different vegetation types is shown in

Table 2. Plant list of green corridors in the lower reaches of Hotan River.

Family	Plant Species	Understory Shrub-Grassland	River Flat	Shrub Grassland	Desert Forestland
Tamaricaceae	Tamarix ramosissima	+	+	+	+
	Tamarix hispida	+	−	+	−
	Tamarix taklamakanensis	+	−	+	+
Gramineae	Phragmites australis	+	+	+	+
	Calamagrostis epigeios	+	+	+	−
	Calamagrostis macrolepis	+	+	+	−
	Achnatherum splendens	+	+	+	−
	Aeluropus pungens	+	+	+	−
	Poa annua	+	+	+	−
	Crypsis aculeata	+	+	+	−
Leguminosae	Alhagi sparsifolia	+	+	+	+
	Glycyrrhiza inflata	+	+	+	+
	Sophora alopecuroides	+	+	−	−
	Halimodendron halodendron	+	+	+	+
Chenopodiaceae	Halostachys caspica	+	−	+	+
	Salicornia europaea	+	−	+	−
	Chenopodium glaucum	+	+	−	−
	Suaeda glauca	+	−	+	−
	Salsola pellucida	+	+	+	−
	Sympegma regelii Bunge	+	+	+	−
	Kalidium foliatum	+	−	+	−
Compositae	Karelinia caspica	+	+	+	+
	Scorzonera divaricata	+	+	+	−
	Taraxacum mongolicum	+	+	−	−
	Cirsium japonicum	+	−	+	−
	Artemisia sp.	+	−	+	−
	Acroptilon repens	+	+	+	−
	Inula salsoloides	+	−	+	+
	Seriphidium sp.	+	+	+	−
Apocynaceae	Apocynum venetum	+	+	+	+
	Poacynum hendersonii	+	+	+	+
Asclepiadaceae	Cynanchum kaschgaricum	+	+	+	−
	Cynanchum auriculatum	+	+	+	−
Polygonaceae	Calligonum roborowskii	−	−	+	+
	Polygonum aviculare	+	+	−	−
Salicaceae	Populus euphratica	+	+	+	+
	Populus pruinosa	+	+	+	−
Liliaceae	Asparagus cochinchinensis	+	+		−
Typhalaceae	Typha orientalis	+	+	−	−
Baldaceae	Hippophae rhamnoides	−	−	+	−
Cyperaceae	Scirpus validus	−	+	−	−
	Scirpus planiculmis	+	+	+	−
Ledange	Cistanche tubulosa	−	−	+	+
	Lycium ruthernicum	+	−	+	+
Tribulus	Nitraria tangutorum	+	+	+	−
Cynomorium	Cynomorium songaricum	−	−	+	+

Note: “+” indicates that there are such plants in the survey area; the “−” indicates that there are no such plants in the survey area.

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3.2. Changes in Vegetation Growth Index in the Vertical Direction of the River

The survey sample plots perpendicular to the river course and from near to far from the river course were the river flat, understory shrub grassland, shrub grassland, and desert forest land. The plant species richness of the river flat and understory shrub grassland is relatively high, whereas the species richness of the shrub grassland and desert forest land is relatively low (Figure 4).

According to the distribution of species in the sample plot, the constructive vegetation and associated vegetation in the sample plot at different distances from the vertical to the river channel mainly have the following distribution characteristics: in the higher flat near the river, Phragmites australis is the constructive species, accompanied by Calamagrostis epigeios, Typha orientalis Presl, Apocynum venetum, Glycyrrhiza inflata Batalin, Neotrinia splendens and Schoenoplectus tabernaemontani. The next is the arbor forest dominated by grey poplar and the shrubbery dominated by tamarisk. The common species of vegetation in the forest are T. ramosissima Ledeb, Phragmites australis, Glycyrrhiza inflata Batalin, Hippophae rhamnoides, and Alhagi sparsifolia Shap. We will now focus on the mixed shrubbery in the arbor forest or the edge of the forest. In this section, Tamarix multiforme is the constructive species, and the associated plants are mainly composed of Phragmites australis, Alhagi sparsifolia Shap, Alhagi camelorum Fisch, Halostachys caspica, Glycyrrhiza inflata Batalin, Nitraria sibirica Pall., Apocynum venetum, Poacynum hendersonii, and Karelinia caspia. The outer side of the riparian forest is sandy land, with desert vegetation dominated by Tamarix chinensis, Karelinia caspia, Alhagi camelorum, and Apocynum venetum.

The results of the various growth indexes of vegetation in different sections of the green corridor are shown in Figure 4. In the river flat, the water condition is good, and annual herbs become the main dominant species. The grassland under the forest shows a relatively complete three-layer structure of tree, crown, and grass community. With the increase in the distance from the river channel, perennial herbs and shrub species increase in the shrub grassland, and annual herbs gradually disappear. In the desert forest land, drought-tolerant and salt-tolerant shrubs are dominant, and species richness is also reduced.

It can be seen from Figure 4 that there are obvious differences between the vegetation richness and various indicators of the sample plots at different distances perpendicular to the river. The water and soil conditions of understory shrub grassland are relatively suitable, so the vegetation richness index and dominance index show higher value than those of other sample plots (Figure 4b).

3.3. Non-Zonal Distribution Characteristics of Vegetation

As far as the overall trend in vegetation distribution is concerned, vegetation grows better closer to the river channel and has higher growth indicators. However, during the field observation, we found that there are non-zonal distribution patterns of vegetation in the study area. ① Desert grassland is widely distributed in the peripheral area of riparian forest, and the main plant species include Kareliniacaspia, Apocynum venetum, Nitraria sibirica Pall., Tamarix hispida, Tamarix taklamakanensis, Calligonum roborowskii Losinsk, Cistanche tubulosa, and Cynanchum kaschgaricum, as well as a small amount of coarse old grey poplar and Populus euphratica. A small number of shrubs and grasslands are distributed in the areas with shallow groundwater depth on both sides of the river. ② The constructive species mainly include Alhagi sparsifolia Shap, Karelinia caspia, and T. ramosissima, accompanied by Halostachys caspica, Salicornia europaea, Suaeda glauca, Poacynum hendersonii, Glycyrrhiza inflata Batalin, and Aeluropus pungens. ③ The lower reaches of the Hotan River are close to the Tarim River, and the constructive vegetation is the Halocnemum strobilaceum, accompanied by Salicornia europaea, Aeluropus pungens, Suaeda glauca, Phragmites australis, and Tamarix chinensis. In addition to the green corridor, the lower reaches of the Hotan River are mostly grasslands with low levels of vegetation. Areas near the river have high vegetation coverage, while areas away from the river have low vegetation coverage. Further research can be conducted on the association between vegetation types and soil conditions in this non-zonal distribution pattern.

3.4. Physiological Indexes of Populus Euphratica in Different Growth Conditions

In this study, various physiological indexes of P. euphratica under different growth conditions were analyzed, and the results are presented in Figure 5.

It can be seen from Figure 5 that under the condition of “medium” (VS3) growth, various physiological indexes of P. euphratica reached the highest value. The RWC of leaves was 79.5% in the growth grade of VS1; this gradually increased with the increase in growth grade and reached a higher value in the VS3, with a change range of 1.8% (p > 0.05), and then gradually decreased. Compared with VS3, the RWC of leaves in the growth grade of VS5 decreased by 13.9% (p < 0.05) (Figure 4a). The LSD test results showed that the RWC of leaves of VS1, VS2, and VS3 grades was significantly different from that of VS5 (p < 0.05), while the RWC of leaves of the other two growth grade was not significantly different (p > 0.05).

The SS content of the leaf increased with the growth grade (from VS1 to VS3), reached the highest value of 0.22 in the VS3, and then decreased gradually. Compared with VS3, the SS content of leaves of dying P. euphratica (VS5) decreased by 48.8% (p < 0.05). There was no significant difference in the SS content of P.euphratica leaves between VS2 and VS4 (p > 0.05), but there was a significant difference in the SS content of other growth grades (p < 0.05). Pearson correlation analysis showed a negative correlation between SS content and growth grade (p < 0.05).

With the gradual decline of the growth conditions (from VS1 to VS5) of P. euphratica, the Pro content of its leaves increased first and then decreased (Figure 5c). The Pro content in VS3 is 25.757 μg·g⁻¹, compared to the VS1, increased by 37.5% (p < 0.05), and the Pro content in VS4 decreased by 30.1% compared with VS3.

Figure 5d shows the POD activity of leaves of P. euphratica of different growth grades (VS1–VS5). The POD activity increased sharply at VS3 and then decreased; the change range of rapid increase and decrease was 183.9% and 80.4%, respectively.

Further, the ANOVA is used to determine the variation in the physiological indexes of P. euphratica in different growth conditions. The ANOVA for means of the physiological indexes was calculated. The determination coefficient (R-Sq.) is equal to 98.5%, which implies that all of the indexes were significant parameters at the 95% confidence level. Comparing the variation in the leaf RWC at different growth grades, the VS3 is the most significant, with a 76.35% the contribution, followed by the VS2, with a 13.15% contribution. Using the same method, the variations in SS content, Pro content, and POD activity of P. euphratica at different growth conditions were also analyzed. For the SS content, the VS3 is the most significant, with a 79.45% contribution, followed by the VS1, with a 19.25% contribution; for the Pro content, the VS3 is the most significant, with a 66.35% contribution, followed by the VS5, with 22.55% contribution; for the POD activity, the VS3 is the most significant, with an 87.85% contribution, followed by the VS2, with a 9.15% contribution. The results of the ANOVA further confirm that the various physiological indexes of P. euphratica reach the highest value when the growth grade is “medium” (VS3). The chlorophyll content in plant leaves, which is a highly sensitive proxy for external environmental stress, is also one of the main indicators of vegetation growth and adaptability. In this paper, the chlorophyll of P.s euphratica under different growth conditions was also compared and analyzed. The results are shown in Figure 6.

According to Figure 6, the chlorophyll a (Chla), chlorophyll b (Chlb), total chlorophyll content (Chlt), and the ratio of chlorophyll a and b (Chla/b) differ according to the growth grade of P. euphratica. It was found that the contents of Chla, Chlb, and Chlt of P. euphratica increased first and then decreased from exuberant (VS1) to death (VS5) conditions. In comparison with VS1, each of these contents has increased by 0.30 mg/g, 0.15 mg/g, and 0.45 mg/g with the growth condition of V3, respectively. It can also be seen from the value of Chla/b that, from VS1 to VS3, it decreased by 2.0% (p > 0.05), and the increase in Chla content was less than that in Chlb, indicating that the effect of stress on Chla was less than that on Chlb. At VS5, it reached the highest point and increased by 4.7% compared with VS3 (p > 0.05), and the decrease in Chla was smaller than that in Chlb, which also showed that the effect of stress on Chla was smaller than that on Chlb. The LSD results indicated that the Chla, Chlb, and Chlt of VS3 were significantly different from the corresponding indicators for VS1, VS2, VS4, and VS5 (p < 0.05), but there was no significant difference between the last four growth grades (p > 0.05), and the Chla/b of the five growth grades was not significantly different (p > 0.05). The Pearson correlation between Chlt, Chla, Chlb, and Chla/b, and growth grade did not reach a significant level (p > 0.05). The chlorophyll index gave information about the greenness of the leaf, enabling a comparison of the physiological status of plants and an evaluation of whether grass covering caused stress. A relatively low value of the Chla was found when the growth condition was medium (VS3), whereas other indicators showed a higher value in this growth grade. To alleviate drought stress, Populus euphratica has strong adaptive and defensive abilities, including physiological adjustments and changes in biomass allocation as root growth increases. The mutual transformation of plant chlorophyll is disturbed by different environmental factors. Chlorophyll a is converted into chlorophyll ester under the action of chlorophyll synthase. Under the catalysis of chlorophyll a oxygenize, chlorophyll ester a undergoes a two-step reaction to form chlorophyll ester b, which is formed under the action of chlorophyll synthase. During the process of converting chlorophyll a into chlorophyll b, the utilization of soil by plants decreases because plants have already begun to utilize chlorophyll a for photosynthesis to produce organic nutrients, meet their own needs, and reduce their dependence on soil materials. In the growth conditions of VS3, both the soil and water conditions are more suitable for the acceleration of the conversion between Chla and Chlb, resulting in a reduction in the Chla content in the leaf. So, the opposite responses to environmental conditions were detected simultaneously.

4. Discussion

Considering the arid climate conditions of the study area, groundwater is the main determinant of vegetation growth, including P. euphratica [39,40,41,42]. As a result of the water diversion in the upstream irrigation area, at present, water from the source stream flows downstream only during flood seasons, and the river channels remain dry most of the year. Consequently, the groundwater is largely responsible for the ecological water consumption of the riverbank forest and grass vegetation in the green corridor zone of the Hotan River’s lower reaches. Several studies have shown that desert vegetation relies heavily on groundwater for survival. Groundwater is the most important source of water for desert vegetation. It further underscores the impact of water diversion in the upstream irrigation area, resulting in downstream flow only during flood seasons, leaving river channels dry for most of the year. The dependence of the riverbank forest and grass vegetation in the green corridor zone on groundwater for ecological water consumption is consistent with the existing research, reinforcing the significance of groundwater as the predominant water source for desert vegetation survival [43,44,45].

Therefore, the groundwater depth in the desert riparian forests directly affects the soil moisture and nutrient dynamics closely related to the vegetation growth, and is the leading factor determining the distribution, growth, population succession, and survival of the oasis in the desert area. Previous studies have analyzed the changes in vegetation coverage, density, and biomass caused by the rise in the groundwater level, or described the response of plant water potential or physiological indicators to the rise or fall of the groundwater level [46,47,48,49]. Ling et al. studied vegetation growth in the arid areas of China and proposed that the vegetation coverage showed a decreasing trend with the increase in the distance from the river bank [50].

The results are consistent with the distribution of vegetation along the lower reaches of the Hotan River that is proposed in this paper. The groundwater level decreases gradually, and vegetation diversity diminishes with increasing distance from the riverbank. P. euphratica is a typical mesophytic plant in desert areas and has a strong adaptability to the extremely arid continental climate including greater temperature differences, less soil moisture, atmospheric drought, high temperatures, and cold. It can tolerate the extreme maximum temperature of 45 °C and the extreme minimum temperature of −40 °C. Zhou Junli et al. studied the growth characteristics and variation trends of desert riparian forest in the Hotan River basin and found that P. euphratica, with its developed root system, has a strong adaptability to cold, heat, saline-alkali stress, atmospheric drought, and strong wind [51]. Shi Junhui et al. studied the effects of different salt conditions on the physiological and biochemical indexes of P. euphratica, and proposed that the activity of P.s euphratica leaves increased under high salt concentration stress, which enhanced the ability of Populus euphratica seedlings to resist salt stress; under low salt stress, P. euphratica seedlings still maintain a high level of salt absorption and have a certain ability to resist salt stress [52]. In addition to phreatic water, its roots can extend into shallow water layers to obtain the water they need for growth. The plant’s roots have strong pressure, and its leaves contain sodium bicarbonate, allowing it to survive in drought conditions and salty soil [53]. Lu et al. examined the hydraulic constraints on leafing intensity in a desert riparian species in the lower reaches of the Tarim River and found that P. euphratica grows well on the sandy soils of seasonal floodplains and also survives in dry uplands. Its roots can reach depths of at least 10 m into groundwater, and it reaches a maximum height of 15 m above ground [54]. The findings contribute valuable information for informed decision making in the preservation and restoration of desert riparian ecosystems. Overall, this research offers a scientific foundation and technical guidance that are essential for effective conservation strategies in the face of ongoing environmental challenges.

It was found that P. euphratica exhibits saline-alkali tolerance characteristics, as the sample plots with the “medium” growth condition (V3) had a higher salt content than those with the “exuberant” growth condition (V1), and the various physiological indexes also showed the highest value with the medium growth condition (V3). The physiological indexes of P. euphratica reach their maximum value at VS3 (medium growth). It was found that the species richness of river flats and understory shrub grassland was higher and that it was lower in shrub grassland and desert forest, indicating that the closer the green corridor was to the river, the more varied it was. The variation trends of physiological indexes of P. euphratica indicated that the decrease in leaf water content and the increase in proline accumulation are due to a physiological response of P. euphratica to water deficit. Through proline accumulation in vivo, P. euphratica can improve its osmotic adjustment capacity and maintain an osmotic balance between the protoplasm and the environment when the groundwater level decreases, suggesting that P. euphratica has adapted to environmental changes through physiological changes. Our result agreed with those of Yu et al. (2012), who reported that the physiological indexes of P. euphratica leaves also underwent many changes to improve the viability under extreme drought stress [55]. As a result of analyzing five physiological indices, Sun et al. (2022) examined how different vegetation restoration methods affected the cold resistance of three restoration plants planted in the alpine mining areas of QTP in different habitats. A higher level of soluble sugars (SSs) was found in Elymus nutans in the sunny slope habitat after CS treatment [56]. Using appropriate indices and appropriate experimental systems, Negrao et al. suggested some guidelines for selecting appropriate experimental systems, imposing salinity stress, and obtaining and analyzing relevant physiological data [57].

As a result of P. euphratica’s physiological index response to water conditions, it can be concluded that water conditions are determining factors for vegetation growth. We present the distribution of vegetation in the lower reaches of the Hotan River and the response of physiological indexes of P. euphratica to growth conditions through field investigation and the determination of various physiological indexes. Additionally, P. euphratica’s heat and cold resistance characteristics are further confirmed. Based on the results of this study, an oasis protection forest system could be established in areas of salinized land where freshwater resources are increasingly scarce as a result of desertification. Additionally, furthering the understanding of salt tolerance mechanisms in plants and providing a theoretical basis for the rapid restoration of arid vegetation and the development of ecological industries would be helpful.

5. Conclusions

(1) The lower reaches of the Hotan River have desert riparian forests that provide ecological functions for biodiversity maintenance and environmental purification. As a result of human disturbances in the basin, the desert landscape has been seriously fragmented, and the environmental heterogeneity has been reduced, resulting in the decline of the P. euphratica population.

(2) The dominant plant in the study area was P. euphratica, which was distinguished by its heat and cold tolerance. Generally, phreatic water or overflowing river water is necessary for its growth, and its roots can extend to shallow water layers. It can survive drought and salty soil conditions for a longer period due to its strong root pressure. There were 45 species of shrubs and herbs belonging to 17 families in the understory vegetation.

(3) From the river channel to the desert, the water availability of different sections changed as follows: river flat > understory shrub grassland > shrub grassland > desert forestland. The closer to the river channel, the better the vegetation growth, and the higher the growth indicators of the vegetation. However, the existing non-zonal distribution pattern of vegetation needs to be further analyzed based on the relationships between the vegetation types and soil conditions.

(4) Physiological indices of vegetation vary significantly among the four selected sections. Among the various sections, the understory shrub-grassland dominance indices and richness indices are higher than those of the river flats, while the diversity indices and evenness indices of the river flats are the highest. As a result of the scarce water supply, all of the desert forestland index values are low.

(5) The physiological indices of P. euphratica reached the highest value when the growth grade was “medium” (VS3), which indicates that Populus euphratica can physiologically adapt to the changes in the environment. Further research can be conducted on the basis of anatomical structure, water and nitrogen use, oxidative stress and antioxidants under drought, salinity, and the combined stress of the Populus euphratica.