1 Introduction
Desertification has attracted much international attention as one of the most serious land degradation problems, and establishing suitable land rehabilitation projects in sensitive areas would be the best strategy for slowing desertification worldwide (D’Odorico et al., 2013; Huang et al., 2020). Vegetation restoration and reconstruction have become the most important and effective measures and methods for mitigating desertification in drylands (Li et al., 2014; Qi et al., 2018). Shrubs are often used as priority individuals for planting in desertification restoration projects, as they can generally maintain a high coverage rate and strong protective performance under the most severe wind erosion conditions in spring and winter. Once pioneer plant species have been established on mobile dunes, aeolian sediments tend to be fine-grained and biological crusts begin to appear under the canopy (Zuo et al., 2008; Li et al., 2013). Yet, vegetation coverage undergoes a self-thinning process that declines during the succession of vegetation from artificial ecosystems to natural ecosystems (Wang et al., 2005). A previous study showed that vegetation coverage is close to being maximally coordinated with local soil moisture carrying capacity, rendering soil moisture a key limiting factor leading to shrubs’ mortality and leaving herbs to gradually become the dominant species in a semi-arid desert in China (Li et al., 2014). Nevertheless, that would be a long-term succession process, one that is still poorly understood in alpine deserts. There is no doubt that vegetation–soil dynamics are the principal focus of study for understanding the stability of a desert ecosystem, where characteristics of vegetation and soil properties strongly interact with each other (Maxwell et al., 2018). On one hand, artificial measures can weaken aeolian activities and promote organic matter’s increase on the surface due to the interception of dust; on the other hand, soil structure can influence the rainfall infiltration rate and pattern of plants’ water and nutrient absorption by roots (Noy-Meir, 1979; Silva and Lambers, 2018). Therefore, understanding the dynamics of both the plant community and soil at the initial restoration stage is useful to gain insight into the evolution of alpine desert ecosystems.
Qinghai Lake is the largest inland saltwater lake in China. It is not only the convergence zone of China’s three major climate types but also a buffer zone between the Qaidam Basin and Huangshui Valley in the Qinghai–Tibet Plateau (Li et al., 2018). Providing ecological security for the northeastern Qinghai–Tibet Plateau, and a natural barrier blocking off desertification that is spreading eastward, Qinghai Lake plays a paramount ecological role in the stable development of the Qinghai–Tibet Plateau. Currently distributed around Qinghai Lake is about 134 295 ha of desert, which has expanded by 2.2% annually since the 1950s (Wu et al., 2019a). Desertification of the Qinghai Lake watershed provides a hotspot for research investigating the influence of climate change on the Tibetan Plateau (Li et al., 2018). The local Qinghai Province government has encouraged afforestation and conducted several sand-fixation projects to impede the spread of mobile dunes along the nearby Qinghai–Tibet Railway and Eastern Surrounding Road (Tian et al., 2019). Desertification has been effectively curbed by a relatively stable pattern of “fixed lowlands,” “semi-fixed dunes,” and “mobile dunes” from the lakeshore to the foothills of Riyue Mountain, the eastern boundary of Qinghai Lake.
Recent studies in this area have mainly focused on desert formation, dynamic changes of desertification, desertification control measures, soil particle composition, soil moisture dynamics, effective control and evaluation of straw checkerboards, and individuals’ effects on reducing wind-driven sand erosion (Tian et al., 2015; Tian et al., 2019; Tian et al., 2020), in addition to water use sources of afforestation species (Wu et al., 2016; Wang et al., 2022). Work by Wu H et al. (2019) has analyzed species characteristics (richness and traits) and other afforestation community features over several years. Earlier, Lu et al. (2013) focused on the effect of rainfall on replenishing soil moisture in newly afforested areas. Yet, little attention has been paid to the interaction between community features and soil properties in the alpine desert ecosystem of the Qinghai–Tibet Plateau, especially in the Qinghai Lake watershed.
To fill these knowledge gaps, here we illustrate community characteristics and soil physicochemical properties during growing seasons in an alpine semi-arid desert in the northeastern region of the Tibetan Plateau. The tested hypotheses were that 1) plant community diversity and coverage and soil physicochemical properties showed differences in different sand-binding communities; 2) soil moisture content was higher in natural dunes than in afforested dunes. This study’s findings will facilitate a mechanistic understanding of plant–soil–water relationships in alpine desert ecosystems, while also providing information for screening introduced species for sand-fixation effectiveness according to local soil conditions and soil water content. This could have far-reaching significance for the ecological engineering of desertification control in the Qinghai–Tibet Plateau.
4 Discussion
In drylands, abiotic processes such as wind- and water-driven erosion significantly influence surface processes (Turnbull et al., 2012). Before artificial measures were taken, sand movements were extremely strong on mobile dunes because of forceful winds in winter and spring. Once plantations were established on dunes, wind-related hazards decreased considerably (Tian et al., 2020).
4.1 How does vegetation respond to the local soil regime in the alpine desert?
Soil texture is a key factor in the coupled relationships between climate, soil, and vegetation, which affect soil erosion, soil accumulation, and soil physicochemical processes (Fernandez-Illescas et al., 2001; Renne et al., 2019). In other words, soil particle composition is one of the most important soil physical properties because it directly affects soil moisture transport and vegetation distribution patterns (Pan and Wang, 2009; Bayat et al., 2015). Medium and fine sands were the dominant particles in the study area (Table 6), whose content was more than 72.36% at some dune sites. Also, sand has a stronger infiltration capacity for precipitation than either silt or clay, thus creating an “inverse texture effect” in arid and semi-arid ecosystems (Noy-Meir, 1973; Porporato et al., 2002). Overall, the coarser the sand, the greater the water infiltration capacity that is exploitable by plant individuals in a water-limited environment (Pan et al., 2015). In our study area, the particles of natural fixed dunes were significantly finer than those of mobile or semi-fixed dunes, both artificial and natural, which ultimately resulted in less infiltration into deeper soils, allowing direct uptake by vegetation for survival (Wang et al., 2022).
Soil is the most basic and important part of an ecosystem, and fertile soil is an important guarantee for the healthy and sustainable development of plant communities (Noy-Meir, 1973). Our analysis of the physical and chemical characteristics of soil in the alpine desert uncovered a low content of organic matter and nutrients, particularly phosphorus, and the deposits mainly consisted of fine sand or medium sand with little clay present (Table 6; Figure 2). However, in this study, plantations on the artificial semi-fixed dunes were in an early stage of succession, and their deposits were still coarse in soil texture. With more years of vegetation restoration, the physical and chemical properties of soil become more robust indicators of soil development (Yu and Jia, 2014); good examples of this include soil texture refinement, soil organic matter, and nutrient content increases, soil bulk density decreases, and biological crust development.
The groundwater level at a dune’s topmost position is above 10 m in depth in our study area (Wang et al., 2022). Furthermore, there were few rainfall events from January to April, so precipitation had little effect on soil water content. However, the amount of soil water replenishment via soil melting increases significantly in May. Moreover, Qinghai Lake is located in the distribution area of seasonally frozen soil. The high-water zone formed by gaseous water condensation in this seasonally frozen soil, namely, a cold-generated wet sand layer, is beneficial for the growth of desert plants (Wan et al., 2005). However, in this area, the depth of the groundwater in the sand dunes is too significant for gaseous water transport to be utilized by artificial sand fixation vegetation.
Desertification control measures consist mainly of biological measures, or these are combined with mechanical measures, and soil moisture is one of the main limiting factors for vegetation restoration in the study area (Lu et al., 2013). Precipitation quantity and seasonality interact with soils to determine the spatiotemporal patterning of plant-available water in the soil, thereby influencing the differential success of plant growth forms in water-limited systems (Renne et al., 2019). Soil moisture seasonal dynamics in our study area could be divided into three periods. The first is a stage where stagnant water is frozen from December to the following February when the soil moisture content is low and relatively stable. The second stage lasts from March to June when losses in soil moisture are compensated by increases in soil moisture. The third and final stage occurs during a period of dehydration from July to November when high soil moisture levels gradually decrease. Overall, the highest soil moisture occurs in summer and autumn, when plants need water the most to maintain their growth, as proven by Rodriguez-Iturbe et al. (1999).
4.2 How do plantations affect soil properties after afforestation in the alpine desert?
Community features and soil properties are mutually reinforcing (Maxwell et al., 2018). On one side, vegetation cover reduces radiation and wind speed at the soil surface, thus reducing evaporation (Porporato et al., 2002). On the other side, once the vegetation has established on a mobile dune, leafy plants begin to control, to some extent, the loss of their own resources and create a favorable microenvironment for other plants and animals (Noy-Meir, 1973). Furthermore, vegetation also affects soil particle size composition, mainly in two ways. First, soil layers under the influence of different vegetation will have different root systems and their associated rhizosphere secretions, which in turn change the physicochemical properties of soil and its particle composition (Zhu et al., 2014). Second, the capacity of vegetation to intercept rainfall and lessen surface soil erosion depends on the canopy density and coverage of plants, leading to different retention capacities of surface soil particles (Gu et al., 2018). By comparing the particle size characteristics of the five dune types (Table 6), we conclude that artificial measures can significantly impact the soil surface by promoting the deposition of fine particles and by effectively reducing surface wind erosion.
In terms of the vegetation’s vertical structure, its above-ground components, such as plant type, canopy structure, surface cover, and litter, can reduce water erosion by intercepting rainfall and attenuating the energy of raindrops. Furthermore, root systems can slow surface runoff and weaken soil erodibility by improving soil structure and its biophysical properties (Xu et al., 2006). A recent study shows that precipitation in the studied region can strongly influence the moisture content at the surface (0–20 cm), while deeper soil moisture is less replenished (Wang et al., 2022), as was the case in our study.
In addition, vegetation restoration affects soil properties and hydrological processes in drylands by increasing soil organic matter content (Wu et al., 2023). In arid and semi-arid areas, individual plants’ canopies and collective vegetation cover are the main factors affecting soil water content, which is negatively correlated with soil water content in the community (Pan and Wang, 2009). The greater the width of a plant’s canopy, the more water it requires and the greater its water consumption (Wang et al., 2011). Yet, the soil water content of natural fixed dunes in the study area exceeded that of mobile dunes (Figure 4), due to the fast precipitation infiltration rate of mobile dunes and the lack of effective interception of precipitation by ground vegetation and shielding of surface evaporation by the canopy. Seasonal variation in soil moisture was similar among the dune types, but differences in water consumption among individual plants can also lead to changes in soil water content (Tian et al., 2021).
The shade and higher surface soil moisture under a shrub canopy enhance biocrust formation, which can significantly bolster the resistance of the soil surface to wind erosion by increasing the wind friction velocity threshold of the soil (Li et al., 2021). Once the straw checkerboard barrier is established, the prerequisite microenvironment for physical crust formation has been created. When H. rhamnoides is planted in the checkerboard’s corners, rainfall is intercepted by straw, and its mulched leaves form hard black physical crusts. We find that crust formation is strongly related to the kind of vegetation growing locally (Table 7). Furthermore, the length of formed crusts on dunes was generally longer in the north–south direction than the east–west direction, likely due to the prevailing westerly and northwesterly winds that disrupt the presence of crusts (Jiang et al., 2018). Previous research has shown that the amount of biocrust cover has a negative effect on rainfall infiltration, leading to the death of mature shrubs in Chinese temperate deserts (Li et al., 2013). The hydrological roles of biocrusts in the alpine desert warrant further investigation.
4.3 Implications for the ecological management of the alpine desert
Given the short recovery times of three-five years for SJD and WLD, fewer herbs will have settled on these dunes in the WPSER. In winter and spring, the contribution of herbaceous cover to the plant community is almost negligible, unlike that from shrubs and subshrubs, chiefly due to strong winds and low temperatures that hasten graminoid mortality. For example, the above-ground segments of L. racemosus and C. tristachya dried out in October and were later blown off in winter; hence, both species had to produce new branches from their roots in May of the following year. As a result, the Poaceae family has a very low density and contributes little to the composition of the desert community.
With the successive establishment of plantations, higher surface cover from plants leads to excessive moisture and intense local competition among individuals, which eventually leads to poor shrub growth (Yu et al., 2016; Yu et al., 2020). Based on a sound understanding of the spatiotemporal distribution of soil moisture, it is necessary to select appropriate plant species and allocate their densities rationally to achieve robust long-term effects on blocking alpine desert winds and reducing sand erosion (Yu et al., 2017; Li and Sawada, 2022). The differences in plant morphology and community characteristics of dune plantations reflect the ecological adaptations of sand-fixing species in alpine deserts (Wu et al., 2019b). Under similar climatic and soil conditions, species diversity in communities is determined by physiological and innate ecological adaptation mechanisms. Nonetheless, afforestation practices and structural patterns deserve more attention in future desert research and management.
An ecosystem is a collection of organic and inorganic components that are open to being influenced by human disturbances (Hill, 1975). In this very complex dune ecosystem, there is a self-regulating, buffering, and repair mechanism operated by the external environment (Noy-Meir, 1979–80). If the pressure from the external environment reaches a tolerable threshold, the vegetation can restore itself to some stable state after normal environmental conditions recover. Once the threshold is exceeded, however, both soil properties and vegetation will likely undergo a series of changes (Zhou et al., 2020). During this process, the alterations to the physical and chemical properties of the soil are not as pronounced as those of the vegetation, but when they do occur, it is difficult to restore them to their original state (Dormaar and Willms, 1990). Soil properties and vegetation are closely related; for example, high levels of ecosystem biodiversity with a complex vegetation composition are resilient, that is, able to withstand external disturbances. By contrast, plantations that are monoculture systems are less resilient and thus easier to change because their resistance to disturbance is minimal. In the study area, the reduced vegetation cover in SD was mainly caused by human damage to the biological crusts and the expansion of bare areas detected by yearly field surveys in spring during the study period (Figure 1). Finally, strong wind erosion occurred in spring and winter, ultimately leading to reductions in vegetation cover. However, soil moisture infiltration may increase, and vegetation could likely recover in the coming years. Thus, plant community management should be carefully carried out in the future.
Regardless of the dominant woody plant species, planting increases soil water content. Previous studies have shown that plants have prominent regulating and storage functions and that the water regulation exercised by plants becomes more pronounced as restoration is prolonged (Li et al., 2014). Afforestation increases vegetation cover and suppresses surface evaporation, thereby augmenting soil water content, which ultimately improves sand-driven wind speed and effectively reduces the impact of wind erosion in alpine deserts. Hence, the reasonable planting of artificial vegetation plays a pivotal role in improving the ecological environment in terms of its soil water content balance during alpine desert restoration.