No abstract

Understanding the water use characteristics of vegetation is crucial for guiding the rational allocation of water resources and the restoration of sustainable vegetation in natural oases in arid desert areas. To analyze the water use characteristics of Populus euphratica Oliv. and Tamarix chinensis Lour. at different stages of growth in the Daliyabuyi natural pristine oasis in northwestern China, the δ18O values of plant xylem of 15 sample trees with different sizes per species, potential water sources (i.e., river water, soil water, and groundwater), and the δ13C values of plant leaves of sample trees were measured in August 2019 and June 2020. The results show that the δ18O values of the xylem water of P. euphratica at different growth stages were similar, but water uptake proportions from deep soil water changed in P. euphratica trees at different growth stages between years with (2019) and without (2020) river water. In contrast, the main water source of young T. chinensis shrubs was deep soil water, and those of mature and overmature shrubs were deep soil water and groundwater in 2020. However, the plant leaf water use efficiency (WUE) of the P. euphratica and T. chinensis were higher without river water. Overall, the water uptake proportion from groundwater and the WUE for T. chinensis were higher than those for P. euphratica, and thus, T. chinensis was more dependent on groundwater. This means that vegetation types and growth stages are the essential factors to be considered in ecological restoration management, which can enhance the effectiveness of vegetation restoration strategies.

Populus euphratica is an important tree species in desert ecosystems. The protection and restoration of natural Populus euphratica forests requires accurate positioning information. The use of Sentinel-2 images to map the Populus euphratica distribution at a large scale faces challenges associated with discriminating between Populus euphratica and Tamarix chinensis. To address this problem, this study selected the Daliyabuyi Oasis in the hinterland of the Taklimakan Desert as the study site and sought to distinguish Populus euphratica from Tamarix chinensis. First, we determined the peak spectral difference period (optimal time window) between Populus euphratica and Tamarix chinensis within monthly Sentinel-2 time-series images. Then, an appropriate vegetation index was selected to represent the spectral difference between Populus euphratica and Tamarix chinensis within the key phenological stage. Finally, the maximum entropy method was used to automatically determine the threshold to map the Populus euphratica distribution. The results indicated that the period from 22 April to 1 May was the optimal time window for mapping the Populus euphratica distribution in the Daliyabuyi Oasis. The combination of the inverted red-edge chlorophyll index (IRECI) and the maximum entropy method can effectively distinguish Populus euphratica from Tamarix chinensis. The user’s accuracy of the Populus euphratica distribution extraction from single-data Sentinel-2 images acquired within the optimal time window was 0.83, the producer’s accuracy was 0.72, and the F1-score was 0.77. This study verified the feasibility of mapping Populus euphratica distribution based on Sentinel-2 images, and analyzed the validity of exploiting spectral differences within the key phenological stage from a single-data image to distinguish between the two species. The results can be used to extract the distribution of Populus euphratica and serve as an auxiliary variable for other plant classification methods, providing a reference for the extraction and classification of desert plants.

Soil is the largest carbon reservoir in terrestrial ecosystems, and thus minor changes in it can dramatically affect atmospheric CO2 concentrations. In the northwestern arid zone of China, the prediction of soil organic carbon (SOC) changes is often limited by the scarcity of soil samples and the scale and depth of research, which limit the understanding of carbon cycling processes in arid zone terrestrial ecosystems. Therefore, this study produced digital soil maps of SOC stocks (SOCS) for two periods (1980‐1990 and 2010‐2020) at a 90‐m resolution based on historical soil profile data and a random forest model. The results showed that the prediction accuracy for SOCS in the topsoil (0–30 cm) was superior to that of the subsoil (30–100 cm). Among them, the mean annual evapotranspiration, normalized difference vegetation index during the growing season, multi‐year mean temperature, and clay content were the main environmental factors affecting the spatial distribution of SOCS. In the past 30 years, the SOCS of the northwestern arid zone has decreased by 585.50 Tg, with a mean decline of 19.52 Tg C yr−1. The changes in SOCS caused by land‐use conversion and reductions in SOCS were further shown to be attributable to grassland desertification and agricultural reclamation. These findings are valuable for exploring the carbon cycle in terrestrial ecosystems in the context of global climate change and for achieving China's goal of carbon neutrality.

An oasis is a unique natural landscape in arid and semi-arid areas, significant for regulating regional microclimates and hydrological processes in deserts. However, little is known regarding the response of natural oasis plants communities to various environmental factors. Nineteen sample plots (50 m × 50 m) were selected in the Daliyabuyi Oasis in the Taklimakan Desert hinterland based on the location of groundwater monitoring wells and 76 vegetation quadrats (25 m × 25 m) were established. A two-way indicator species analysis, Mantel test, detrended correspondence analysis, canonical correspondence analysis (CCA), and hierarchical partitioning were used to provide an in-depth analysis of community classification, species composition, and environmental interpretation of the oasis. A generalized linear model was used to verify the results which showed that the current oasis community could be divided into four types according to the dominant species, which is controlled by soil moisture. Measurement of species composition and distribution of communities showed significant differences between species diversity of individual community types. Variations in groundwater depth affects patterns of species diversity which is sensitive to richness, while the degree of surface water disturbance affects the pattern of species evenness. Moreover, the CCA ordination map showed that community distribution and diversity characteristics have their own preferences in habitat gradients. The study concluded that the species dominance of the community and the composition and distribution are not dominated by a single factor. There are differences in the scale and effect of different water resource types in maintaining community characteristics.