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Desert ecosystems possess an astonishing biodiversity and are rich in endangered species. This study investigated characteristics of species diversity and soil environmental factors in three major deserts of China's Alxa Plateau. The Alxa Desert included 183 plant species belonging to 109 genera and 35 families. The highest numbers of plant species belonged to the Compositae, Gramineae, and Chenopodiaceae families. The research area belongs to the semi-shrub and small semi-shrub deserts in temperate deserts. Species diversity was low, with the Shannon–Wiener index (H′) of shrub-herb = shrub > herb > tree. The Pielou evenness index (E) of shrub herb vegetation was the lowest, indicating more enriched species and fewer sparse species in the community, and that these types of vegetation had the characteristics of rich and obviously dominant species. Redundancy analysis (RDA) and correlations between the comprehensive plant community biodiversity index and soil factors indicated that soil-available phosphorus (NP), organic matter (SOM), and electrical conductivity (EC) had significant impacts on community species diversity. The herbaceous shrub community exhibited the highest H′, Simpson index (D), species richness index (S), soil moisture (SW), and soil nutrients. Planting Calligonum mongolicum, Ephedra membranacea, Artemisia annua, and Phragmites australis to form a typical desert shrub community for community diversity protection is recommended to effectively protect and restore desert ecosystems. [ABSTRACT FROM AUTHOR]

It is of great significance to study the effects of precipitation events on carbon exchange in the ecosystem for an accurate understanding of the carbon cycle. However, the response of net ecosystem CO2 exchange (NEE) in the desert to precipitation events is elusive. In this study, the NEE in response to precipitation events of varying intensities in the Badain Jaran Desert (BJD) in China was continuously monitored using the eddy covariance (EC) technique. The following results were obtained: (1) The BJD ecosystem was a net CO2 sink throughout the study period, with NEE values of -113.4, -130.7, and -175.4 g C m-2a-1 in 2016, 2018, and 2019, respectively. The total precipitation yielded a higher carbon sequestration capacity in 2019 than in the other two years. In addition, the intensity, time, and frequency of precipitation had significant impacts on CO2; (2) the threshold value of the NEE response to precipitation was ~1.4 mm, indicating the extreme sensitivity of the BJD to precipitation events; (3) the variations in the NEE response to precipitation events conformed to a dual exponential model. The analytical results of the model indicate that precipitation intensity was positively correlated with the carbon sequestration capacity of the desert. The model revealed that the greater the precipitation intensity, the longer it takes the NEE to reach the maximum, and the lengthier the duration of the residual effects. With an increase in the total precipitation and frequency of extreme precipitation events under warm and humidification climates, the carbon sequestration capacity of the BJD will likely be enhanced. The results of this study are of great significance for revealing the carbon cycle mechanism of the desert ecosystem. [ABSTRACT FROM AUTHOR]

The distinct effects of the lakes on the maximum and minimum temperatures cause a large discrepancy in diurnal temperature differences between the dry basin and lake basins. Discussion The above findings demonstrate that daily average temperature and the monthly minimum temperature inside the lake basins are warmer than those in the dry basin, particularly during the cold season. This suggests that, once the lake area reaches 1.91km SP 2 sp , the temperature difference between the lake basin and the dry basin (warm island intensity) becomes significant, but only in winter. Figure 4(a) shows that, the daily maximum temperature is lower in the lake basins than in the dry basin but differences between lake basins and the dry basin are not significant. [Extracted from the article]

Ancient cities are excellent spatiotemporal indicators for the study of historical human activities and environmental change. The ancient city of Sanchahe is located at the southern margin of the Mu Us Desert, China. It is an ideal location for studying the complex relationships between historical desertification and human activities. Field observations of the ancient city walls, a well, and a spring, as well as 14C dating, grain size, spatial analysis of archaeological sites, and analyses of historical seismicity, indicated that neotectonics may have contributed to crustal uplift and accelerated fluvial incision of the Wuding River. This rapid incision caused a decline in groundwater levels, which is an important reason for the irreversible environmental degradation around Sanchahe city over the past 800 yr. This study provides new evidence for such environmental degradation and may contribute to a better understanding of historical desertification in the Mu Us Desert. [ABSTRACT FROM AUTHOR]

Accurate estimation of groundwater evapotranspiration (ETG) is the key for regional water budget balance and ecosystem restoration research in hyper-arid regions. Methods that use diurnal groundwater level (GWL) fluctuations have been applied to various ecosystems, especially in arid or semi-arid environments. In this study, groundwater monitoring devices were deployed in ten lake basins at the hinterland of the Badain Jaran Desert, and the White method was used to estimate the ETG of these sites under three main vegetation covers. The results showed that regular diurnal fluctuations in GWL occurred only at sites with vegetation coverage and that vegetation types and their growth status were the direct causes of this phenomenon. On a seasonal scale, the amplitudes of diurnal GWL fluctuations are related to vegetation phenology, and air temperature is an important factor controlling phenological amplitude differences. The estimation results using the White method revealed that the ETG rates varied among the observation sites with different vegetation types, and the months with the highest ETG rates were also different among the sites. Overall, ETG was 600∼900 mm at observation sites with Phragmites australis during a growing season (roughly early May to late October), 600∼650 mm in areas with Achnatherum splendens, and 500∼650 mm in areas with Nitraria tangutorum and Achnatherum splendens. Depth to water table and potential evapotranspiration jointly control the ETG rates, while the influence of these two factors varied, depending on the specific vegetation conditions of each site. This study elucidated the relationship between diurnal GWL fluctuations and vegetation in desert groundwater-recharged lake basins and expanded the application of the White method, providing a new basis for the calculation and simulation of regional water balance. [ABSTRACT FROM AUTHOR]

For lakes in desert hinterlands that are not recharged by river runoff, sediment is input solely by wind transport; however, the mechanisms that control the spatial variations in geochemical characteristics of sediments in such lakes remain unclear. Moreover, it remains uncertain whether some element ratios can be used as proxies for environmental characteristics. In this study, 100 surface sediment samples were collected from five lakes in the hinterland of the Badain Jaran Desert, NW China, and examined for their major, trace and rare earth element contents and mineral assemblages. Chemical composition and mineral components of the sediments differed significantly between lakes with different total dissolved solids (TDS). Geochemical characteristics of the lake sediments and their spatial variations are influenced by eolian sands, saline minerals, and sediment sorting. Eolian sands dominate the immobile and nonsoluble elements, whereas authigenic minerals are important in influencing variations in the mobile and soluble elements. Sediment sorting is reflected in the spatial variation of the sediment chemical composition. The positive relationship between the SiO2/Ti of sediment and lake area indicates that changes in the SiO2/Ti ratio can reveal approximate lake area. The Na2O/K2O ratio in sediments can reveal the TDS of salt lakes more effectively than can the Rb/Sr ratio. However, the key factors and mechanisms governing the chemical composition of sediments at different brine evolution stages vary among lakes with different TDS levels, which should be carefully considered when interpreting the environmental significance of the chemical composition as a proxy for paleoenviroment change. Key Points: The geochemical characteristics of lake sediments in deserts are influenced by eolian sands, saline minerals, and sedimentary sortingChanges in the SiO2/Ti ratio can reveal lake level fluctuations, and the Na2O/K2O ratio can reveal the TDS of salt lakesAt different stages of evolution in lakes with different TDS, the environmental significance of chemical composition should be considered [ABSTRACT FROM AUTHOR]

In the hinterland of the desert, valuable archives of paleoenvironmental evolution are scarce. Calcareous root tubes (CRTs) have a strong potential for reconstructing paleoenvironmental conditions. It is still unclear, however, whether chemical elements in the CRTs can provide insights into paleoenvironmental conditions. In this study, the major- and trace-element composition of 32 CRT samples from the Alashan Desert were analyzed by X-ray fluorescence spectrometry. Results showed that the elemental composition and content change were controlled by the parent material and climatic conditions at the time of CRT formation. Ca, Mg, and Sr were significantly affected by climate, whereas the enrichment of P is likely related to the growth of plants. Higher (lower) Mg/Ca and Sr/Ca ratios corresponded to higher (lower) effective moisture and a higher (lower) Mg/Sr ratio indicated a higher (lower) temperature during the middle Holocene (8–5 cal ka BP). The reconstruction results for effective moisture were consistent with those in the Asian monsoon margin of northwestern China, which were caused by higher monsoon precipitation and lower evaporation. Therefore, chemical elements in the CRTs can reflect changes in paleo-effective moisture and paleotemperature at a millennial resolution in this area. [ABSTRACT FROM AUTHOR]

Water resources play a vital role in ecosystem stability, human survival, and social development in drylands. Human activities, such as afforestation and irrigation, have had a large impact on the water cycle and vegetation in drylands over recent years. The Badain Jaran Desert (BJD) is one of the driest regions in China with increasing human activities, yet the connection between human management and the ecohydrology of this area remains largely unclear. In this study, we firstly investigated the ecohydrological dynamics and their relationship across different spatial scales over the BJD, using multi-source observational data from 2001 to 2014, including: total water storage anomaly (TWSA) from Gravity Recovery and Climate Experiment (GRACE), normalized difference vegetation index (NDVI) from Moderate Resolution Imaging Spectroradiometer (MODIS), lake extent from Landsat, and precipitation from in situ meteorological stations. We further studied the response of the local hydrological conditions to large scale vegetation and climatic dynamics, also conducting a change analysis of water levels over four selected lakes within the BJD region from 2011. To normalize the effect of inter-annual variations of precipitation on vegetation, we also employed a relationship between annual average NDVI and annual precipitation, or modified rain-use efficiency, termed the RUEmo. A focus of this study is to understand the impact of the increasing planted vegetation on local ecohydrological systems over the BJD region. Results showed that vegetation increases were largely found to be confined to the areas intensely influenced by human activities, such as croplands and urban areas. With precipitation patterns remaining stable during the study period, there was a significant increasing trend in vegetation greenness per unit of rainfall, or RUEmo over the BJD, while at the same time, total water storage as measured by satellites has been continually decreasing since 2003. This suggested that the increased trend in vegetation and apparent increase in RUEmo can be attributed to the extraction of ground water for human-planted irrigated vegetation. In the hinterland of the BJD, we identified human-planted vegetation around the lakes using MODIS observations and field investigations. Four lake basins were chosen to validate the relationship between lake levels and planted vegetation. Our results indicated that increasing human-planted vegetation significantly increased the water loss over the BJD region. This study highlights the value of combining observational data from space-borne sensors and ground instruments to monitor the ecohydrological dynamics and the impact of human activities on water resources and ecosystems over the drylands. [ABSTRACT FROM AUTHOR]

The mid-latitudes of East Asia are characterized by the interaction between the Asian summer monsoon and the westerly winds. Understanding long-term climate change in the marginal regions of the Asian monsoon is critical for understanding the millennial-scale interactions between the Asian monsoon and the westerly winds. Abrupt climate events are always associated with changes in large-scale circulation patterns; therefore, investigations into abrupt climate changes provide clues for responses of circulation patterns to extreme climate events. In this paper, we examined the time scale and mid-Holocene climatic background of an abrupt dry mid-Holocene event in the Shiyang River drainage basin in the northwest margin of the Asian monsoon. Mid-Holocene lacustrine records were collected from the middle reaches and the terminal lake of the basin. Using radiocarbon and OSL ages, a centennial-scale drought event, which is characterized by a sand layer in lacustrine sediments both from the middle and lower reaches of the basin, was absolutely dated between 8.0–7.0 cal kyr BP. Grain size data suggest an abrupt decline in lake level and a dry environment in the middle reaches of the basin during the dry interval. Previous studies have shown mid-Holocene drought events in other places of monsoon marginal zones; however, their chronologies are not strong enough to study the mechanism. According to the absolutely dated records, we proposed a new hypothesis that the mid-Holocene dry interval can be related to the weakening Asian summer monsoon and the relatively arid environment in arid Central Asia. Furthermore, abrupt dry climatic events are directly linked to the basin-wide effective moisture change in semi-arid and arid regions. Effective moisture is affected by basin-wide precipitation, evapotranspiration, lake surface evaporation and other geographical settings. As a result, the time scales of the dry interval could vary according to locations due to different geographical features. [ABSTRACT FROM AUTHOR]

Mega-dunes in the lake group area of the Badain Jaran Sand Sea, China, are generally taller than dunes in the non-lake group area. This spatial distribution of dune heights may provide a new perspective on the controversy regarding the dunes' formation mechanism. In this study, we calculated the relative heights and slopes of individual dunes based on a digital elevation model, and we confirmed the height distribution of abnormally tall dunes in the lake group area of the sand sea. It was also found that slopes of more than 10° in the lake group area are more common than those in the non-lake group area. Based on meteorological observations, coupled with the measurement of water content in the sand layers, we propose a conceptual model demonstrating that moisture exchange between the lakes and soil via non-rainfall water will humidify dune slopes and form a more favorable accumulation environment for aeolian sand, thus increasing dune heights. Although long-term observations are yet to be carried out, the present study can be used as evidence for understanding the basis of dune formation in the lake group area and assessing groundwater utilization in deserts. [ABSTRACT FROM AUTHOR]

The northeastern part of the Tengger Desert accommodates several lakes. The effect of these lakes on local temperatures is unclear. In this study, the effects of the lakes were investigated using land surface temperature (LST) from MODIS (Moderate Resolution Imaging Spectroradiometer) data from 2003 to 2018 and air temperatures from meteorological stations in 2017. LST and air temperatures are compared between the lake-group region and an area without lakes to the north using statistical methods. Our results show that the lake-group region is found to exhibit a warm island effect in winter on an annual scale and at night on a daily scale. The warm island effect is caused by the differing properties of the land and other surfaces. Groundwater may also be an important heat source. The results of this study will help in understanding the causative factors of warm island effects and other properties of lakes. [ABSTRACT FROM AUTHOR]

The relationship between the variation of CO 2 flux and the pH of inorganic factors. [Display omitted] • The difference of CO 2 absorption and release is obvious during the day and night. • Yindeer Lake was a net sink of CO 2 during the study period. • The lakes can absorb large amounts of CO 2 every year to slow down global warming. • Meteorological factors controlling CO 2 fluxes differed by time scale. • pH is the main influencing factor controlling CO 2 absorption and release. Carbon sequestration levels in lakes are often underestimated despite being vital for the comprehensive analysis of CO 2 budget and significantly influencing inland lakes in arid areas. In this study, the eddy covariance (EC) technique was used to measure CO 2 flux for six consecutive years during 2012–2017 in a small salt-lake (Yindeer Lake) replenished by groundwater in southeastern Badain Jaran Desert, China. The results indicate that: (1) Yindeer Lake was a net sink of CO 2 , which led to the estimate that the lakes in the Badain Jaran Desert can absorb approximately 1.91 × 103 t of CO 2 per year and help slow down the rate of global warming. (2) The effects of meteorological factors on the CO 2 flux of the lake vary with timescales. (3) Inorganic processes, in which pH is the main influence mechanism, control CO 2 absorption and releases in Yindeer Lake. Thus, the study results provide a new guide for solving the "missing sink" problem. [ABSTRACT FROM AUTHOR]

Plant phenology is a commonly used indicator representing the impacts on vegetation by the climate and other environmental factors. The use of repeated digital photography provides an opportunity to conduct long-term monitoring of plant phenology and to extract phenological transition dates. Here, we tracked the phenological changes in the flowering tree, Elaeagnus angustifolia L., in the Cherigele (CRGL) of the Badain Jaran Desert (BJD) and the nearby Lianggejing (LGJ); using one-year near-surface digital repeat photography and meteorological data, the phenological difference between the two sites was revealed. We found that the use of digital cameras allowed for the monitoring of plant phenology with high temporal and spatial accuracy in these dryland ecosystems. Furthermore, in the lake group region of BJD, the onset of greening occurred 23 days earlier, the onset of dormancy began 13 days later and the growing season was 36 days longer, compared to those of the surrounding area. This difference is partly related to the higher altitude in the LGJ; however, it is dominantly related to the warm island effect in the lake group region. This effect resulted in the mean annual temperature in the CRGL being ~1.6 °C higher than in the LGJ. • This study fills the gap in phenological research using digital cameras in desert ecosystems in China. • The onset of greening was earlier, that of dormancy was later in the lake group region of BJD than in the surrounding area. • Phenological disparity between the two sites was mainly caused by the warm island effect in the lake group region of the BJD. [ABSTRACT FROM AUTHOR]

The Badain Jaran Desert (BJD) is characterized by the coexistence of over 110 perennial lakes and thousands of megadunes in its southeast part. Unlike the cold island effect, we found a special phenomenon of the warm island effect in the lake group region of the BJD. However, the concept and formation mechanism remains unclear. In this study, based on observations of land surface processes in the area, we first used the daily mean temperature from 23 automated meteorological stations from 2010 to 2017 to calculate the mean daily temperature (T) ≥ 0 °C, T ≥ 10 °C accumulated temperature and negative accumulated temperature. Furthermore, using the net radiation from two eddy covariance measurement systems, characteristics of the net radiation between the lake and megadunes were analyzed. When comparing observed data in the lake group region to surrounding areas, accumulated temperature from all three meteorological stations in the lake group region were higher; the duration days of T ≥ 0 °C and T ≥ 10 °C were longer, whereas duration days of negative accumulated temperature were shorter. In addition, the initial dates for T ≥ 0 °C and T ≥ 10 °C accumulated temperature were earlier, whereas the end dates were delayed. Variations in heat were observed between stations in the lake group region that may be reflective of microclimate environments between lakes. The authors relate warm island formation in the BJD lake group region to (1) the heat carried by groundwater recharge to the desert lake groups has a great impact on the local temperature. (2) Net heat radiation to the atmosphere through sensible heat flux owing to sparse vegetation in the desert areas. Hence, heat resources are richer in the lake group region. This study aims to improve our understanding of the warm island effect from a comprehensive analysis of its intensity and distribution pattern around the lake group region as compared to its surroundings. In addition, the results from this study will provide a scientific basis for determining the source of lake water in the BJD. [ABSTRACT FROM AUTHOR]

The upper reaches of the Yellow River (URYR) comprise the main water-producing zone of the Yellow River Basin. Quantifying the contribution rates of climate change and human activities (CCHAs) to the changes in runoff in the URYR is critical for formulating rational water resource management strategies in the entire basin. The main objective of this study is to combine the advantages of flexible sample entropy and multiscale entropy to propose a new three-stage method (TSM) to quantify the contribution rates of CCHAs to changes in runoff in the URYR above the city of Lanzhou during 1956–2015. The results show that climate change is the dominant factor affecting the changes in runoff in the study area over the period of study. However, compared with the baseline period (BP; 1956–1986), the variation period (VP; 1987–2015) experienced notably different impacts of CCHAs on the runoff changes. Human activities constituted the dominant factor impacting the changes in runoff in the Huangshui River and the lower section of the URYR (average contribution rate of 70.9%). Meanwhile, climate change was the dominant factor affecting changes in runoff in other regions of the URYR (average contribution rate of 91.8%). Declining precipitation, increasing temperature and evaporation, changing watershed surfaces, the construction of reservoirs and water diversion projects, and increasing human water consumption have resulted in varying degrees of runoff complexity. The novelty and uniqueness of this study can be ascribed to the TSM that (1) considers the impacts of human activities on changes in runoff during the BP and combines the advantages of single time-scale and multiscale entropy for the first time; and (2) uses positive and negative contribution rates to represent the increase and decrease in runoff complexity, respectively. The method exhibits appreciable performance and applicability in the URYR and other watersheds to effectively calculate the contribution rates of the CCHAs to changes in runoff. Image 1 • A new complexity-based three-stage method (TSM) was proposed to quantify impacts of climate change and human activities (HAs) on runoff changes (RCs). • The TSM considers the impact of HAs on RCs during the baseline period (1956–1986). • The TSM uses positive/negative contribution rate (CR) to represent runoff complexity increase/decrease. • HAs are the dominant factor influencing RCs, with a 70.9% CR in the lower section of the upper Yellow River. • The TSM performs well and is applicable to this study and other areas. [ABSTRACT FROM AUTHOR]