Sand-fixing and windbreak forests are widely used to protect or/and improve the ecological environments in arid and semi-arid regions. A full understanding of wind flow characteristics is essential to arranging the patterns of these protective forests for enhancing the effectiveness. In this study, the wind velocity over the underlying surface with sand-fixing forests and windbreak forests at the heights of 1-49 m was monitored from two 50-m high observation towers in an oasis of Minqin, Gansu Province of China. The wind velocities were simulated at different locations over these protective forests between those two towers by a two-dimensional Computational Fluid Dynamics (CFD) model. The results showed that at the heights of 1-49 m, the wind velocity profiles followed a classical logarithm law at the edge of the oasis and a multilayer structure inside the oasis. With increasing number of sand-fixing forest and windbreak forest arrays, the wind velocity at the heights of 1-49 m generally decreased along the downstream direction of the prevailing wind. Specifically, below the height of windbreak forests, the wind velocity decelerates as the airflow approaches to the windbreak forests and then accelerates as the airflow passes over the windbreak forests. In contrast, above the height of windbreak forests, the wind velocity accelerates as the airflow approaches to the windbreak forests and then generally decelerates as the airflow passes over the windbreak forests. Both the array number and array spacing of sand-fixing and windbreak forests could influence the wind velocity. The wind protection effects of sand-fixing forests were closely related to the array spacing of windbreak forests and increased with the addition of sand-fixing forests when the array of the forests was adequately spaced. However, if the array spacing of windbreak forests was smaller than seven times of the heights of windbreak forests, the effects were reduced or completely masked by the effects of windbreak forests. The results could offer theoretical guidelines on how to systematically arrange the patterns of sand-fixing and windbreak forests for preventing wind erosion in the most convenient and the cheapest ways.

As an important component of ecosystem carbon (C) budgets, soil carbon dioxide (CO₂) flux is determined by a combination of a series of biotic and abiotic processes. Although there is evidence showing that the abiotic component can be important in total soil CO₂ flux (R_total), its relative importance has never been systematically assessed. In this study, after comparative measurements of CO₂ fluxes on sterilized and natural soils, the R_total was partitioned into biotic flux (R_biotic) and abiotic flux (R_abiotic) across a broad range of land-cover types (including eight sampling sites: cotton field, hops field, halophyte garden, alkaline land, reservoir edge, native saline desert, dune crest and interdune lowland) in Gurbantunggut Desert, Xinjiang, China. The relative contribution of R_abiotic to R_total, as well as the temperature dependency and predominant factors for R_total, R_biotic and R_abiotic, were analyzed. Results showed that R_abiotic always contributed to R_total for all of the eight sampling sites, but the degree or magnitude of contribution varied greatly. Specifically, the ratio of R_abiotic to R_total was very low in cotton field and hops field and very high in alkaline land and dune crest. Statistically, the ratio of R_abiotic to R_total logarithmically increased with decreasing R_biotic, suggesting that R_abiotic strongly affected R_total when R_biotic was low. This pattern confirms that soil CO₂ flux is predominated by biotic processes in most soils, but abiotic processes can also be dominant when biotic processes are weak. On a diurnal basis, R_abiotic cannot result in net gain or net loss of CO₂, but its effect on transient CO₂ flux was significant. Temperature dependency of R_total varied among the eight sampling sites and was determined by the predominant processes (abiotic or biotic) of CO₂ flux. Specifically, R_biotic was driven by soil temperature while R_abiotic was regulated by the change in soil temperature (ΔT). Namely, declining temperature (ΔT<0) resulted in negative R_abiotic (i.e., CO₂ went into soil) while rising temperature (ΔT>0) resulted in positive R_abiotic (i.e., CO₂ released from soil). Without recognition of R_abiotic, R_biotic would be overestimated for the daytime and underestimated for the nighttime. Although R_abiotic may not change the sum or the net value of daily soil CO₂ exchange and may not directly constitute a C sink, it can significantly alter the transient apparent soil CO₂ flux, either in magnitude or in temperature dependency. Thus, recognizing the fact that abiotic component in R_total exists widely in soils has widespread consequences for the understanding of C cycling.

A knowledge of soil permeability is essential to evaluate hydrologic characteristics of soil, such as water storage and water movement, and soil permeability coefficient is an important parameter that reflects soil permeability. In order to confirm the acceptability of the one-dimensional horizontal infiltration method (one-D method) for simultaneously determining both the saturated and unsaturated permeability coefficients of loamy sand, we first measured the cumulative infiltration and the wetting front distance under various infiltration heads through a series of one-dimensional horizontal infiltration experiments, and then analyzed the relationships of the cumulative horizontal infiltration with the wetting front distance and the square root of infiltration time. We finally compared the permeability results from Gardner model based on the one-D method with the results from other two commonly-used methods (i.e., constant head method and van Genuchten model) to evaluate the acceptability and applicability of the one-D method. The results showed that there was a robust linear relationship between the cumulative horizontal infiltration and the wetting front distance, suggesting that it is more appropriate to take the soil moisture content after infiltration in the entire wetted zone as the average soil moisture content than as the saturated soil moisture content. The results also showed that there was a robust linear relationship between the cumulative horizontal infiltration and the square root of infiltration time, suggesting that the Philip infiltration formula can better reflect the characteristics of cumulative horizontal infiltration under different infiltration heads. The following two facts indicate that it is feasible to use the one-D method for simultaneously determining the saturated and unsaturated permeability coefficients of loamy sand. First, the saturated permeability coefficient (prescribed in the Gardner model) of loamy sand obtained from the one-D method well agreed with the value obtained from the constant head method. Second, the relationship of unsaturated permeability coefficient with soil water suction for loamy sand calculated using Gardner model based on the one-D method was nearly identical with the same relationship calculated using van Genuchten model.

With increasingly intensifying degradation of natural grasslands and rapidly increasing demand of high quality forages, natural grasslands in China have been converted into planted grasslands at an unprecedented rate and the magnitude of the conversion in Inner Mongolia is among the national highest where the areal extent of planted grasslands ranks the second in China. Such land-use changes (i.e., converting natural grasslands into planted grasslands) can significantly affect carbon stocks and carbon emissions in grassland ecosystems. In this study, we analyzed the effects of converting natural grasslands into planted grasslands (including Medicago sativa, Elymus cylindricus, and M. sativa+E. cylindricus) on ecosystem respiration (F_eco) in Inner Mongolia of China. Diurnal F_eco and its components (i.e., total soil respiration (F_ts), soil heterotrophic respiration (F_sh) and vegetation autotrophic respiration (F_va)) were measured in 2012 (27 July to 5 August) and 2013 (18 July to 25 July) in the natural and planted grasslands. Meteorological data, aboveground vegetation data and soil data were simultaneously collected to analyze the relationships between respiration fluxes and environmental factors in those grasslands. In 2012, the daily mean F_eco in the M. sativa grassland was higher than that in the natural grassland, and the daily mean F_va was higher in all planted grasslands (i.e., M. sativa, E. cylindricus, and M. sativa+E. cylindricus) than in the natural grassland. In contrast, the daily mean F_ts and F_sh were lower in all planted grasslands than in the natural grassland. In 2013, the daily mean F_eco, F_ts and F_va in all planted grasslands were higher than those in the natural grassland, and the daily mean F_sh in the M. sativa+E. cylindricus grassland was higher than that in the natural grassland. The two-year experimental results suggested that the conversion of natural grasslands into planted grasslands can generally increase the F_eco and the increase in F_eco is more pronounced when the plantation becomes more mature. The results also indicated that F_sh contributed more to F_eco in the natural grassland whereas F_va contributed more to F_eco in the planted grasslands. The regression analyses show that climate factors (air temperature and relative humidity) and soil properties (soil organic matter, soil temperature, and soil moisture) strongly affected respiration fluxes in all grasslands. However, our observation period was admittedly too short. To fully understand the effects of such land-use changes (i.e., converting natural grasslands into planted grasslands) on respiration fluxes, longer-term observations are badly needed.

Studies of energy balance that rely on eddy covariance (EC) are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux (LE) and sensible heat flux (Hs). The Bowen ratio (BR) and energy balance residual (ER) approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches (i.e., BR and ER) were compared to correct the LE measured at six EC sites (i.e., three vegetated, one mixed and two non-vegetated sites) in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio ((LE+Hs)/(Rn-Gs); where Rn is the surface net radiation and Gs is the surface soil heat flux) was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas.

Meteorological drought has been an inevitable natural disaster throughout Mexican history and the northern and northwestern parts of Mexico (i.e., the studied area), where the mean annual precipitation (MAP) is less than 500 mm, have suffered even more from droughts in the past. The aim of this study was to conduct a meteorological drought analysis of the available MAP data (1950-2013) from 649 meteorological stations selected from the studied area and to predict the drought features under the different IPCC-prescribed climate change scenarios. To determine the long-term drought features, we collected 1×10⁴ synthetic samples using the periodic autoregressive moving average (PARMA) model for each rainfall series. The simulations first consider the present prevailing precipitation conditions (i.e., the average from 1950 to 2013) and then the precipitation anomalies under IPCC-prescribed RCP 4.5 scenario and RCP 8.5 scenario. The results indicated that the climate changes under the prescribed scenarios would significantly increase the duration and intensity of droughts. The most severe impacts may occur in the central plateau and in the Baja California Peninsula. Thus, it will be necessary to establish adequate protective measures for the sustainable management of water resources in these regions.

Land disturbance and land restoration are important factors influencing runoff production and sediment yield in the semi-arid loess regions of China.This study compared the runoff production and sediment yield during the early stage after land disturbance (ESLD) with those during restoring stage after land disturbance (RSLD). Grey relational analysis was used to analyse the importance of each one of the influencing factors (vegetation, rainfall, soil and topography) in affecting the runoff production and sediment yield. Our results showed that during ESLD, topography was the most critical factor controlling the runoff production, while soil was the most important factor controllingthe sediment yield. DuringRSLD, vegetation was more important in affecting runoff production, while rainfall was more important in affecting sediment yield. In additional, this study demonstrated that both the runoff production and the sediment yield can be effectively reduced by restoring vegetation on severely-disturbed lands, thus providing an important theoretical basis for better implementations of the Grain for Green Program.Our results revealed that the vegetation types of Hippophaerhamnoides+Pinustabulaeformis and H.rhamnoides are better plant selections for land restoration in this area, especially for relatively gentle slopes (i.e., less than 20 degrees).

Monitoring glacier mass balance is crucial to managing water resources and also to understanding climate change for the arid and semi-arid regions of Central Asia. This study extracted the inter-annual oscillations of glacier mass over Central Asia from the first ten principal components (S-PCs) of filtered variability via multichannel singular spectral analysis (MSSA), based on gridded data of glacier mass inferred from Gravity Recovery and Climate Experiment (GRACE) data obtained from July 2002 to March 2015. Two significant cycles of glacier mass balance oscillations were identified. The first cycle with a period of 6.1-year accounted for 54.5% of the total variance and the second with a period of 2.3-year accounted for 4.3%. The 6.1-year oscillation exhibited a stronger variability compared with the 2.3-year oscillation. For the 6.1-year oscillation, the results from lagged cross-correlation function suggested that there were significant correlations between glacier mass balances and precipitation variations with the precipitation variations leading the response of glacier mass balances by 9-16 months.

Groundwater is a significant component of the hydrological cycle in arid and semi-arid areas. Its evapotranspiration is an important part of the water budget because many plants are groundwater-dependent. To restore the degraded ecosystems, the need is pressing to further our understanding of the groundwater evapotranspiration (ET_g) in arid and semi-arid areas. This study employed the White method to estimate ET_gat four sites in the Mu Us Sandy Land in northern China, and the four sites are covered by Salix psammophila(SP site), Artemisia ordosica(AO site), Poplar alba (PA site), and Carexenervis(CE site), respectively. The depth of groundwater table and the duration of drainage were taken into account in calculating the specific yield (S_y) to improve the accuracy of the ET_gestimats. Our results showed that from late May to early November 2013 the ET_g were 361.87 (SP site), 372.53 (AO site), 597.86 (PA site) and 700.76 mm (CE site), respectively. The estimated ET_g rate was also species-dependent and the descending order of the ET_g rate for the four vegetation was: C. enervis, P. alba, A. ordosica, and S. psammophila. In addition, the depth of groundwater table has an obvious effect on the ET_g rate and the effect varied with the vegetation types. Furthermore, the evapotranspiration for the vegetation solely relying on the water supply from unsaturated layers above the groundwater table was much less than that for the vegetation heavily relying on the water supply from shallow aquifers.

Shada Mountains in Saudi Arabia, separated by Maleel Wadi into twin mountains (i.e., Shada Alalah and Shada Asfal), are rich in biodiversity. We investigated the diversity of endemics and endangered species of the mountains based on the data collected from 38 stands falling in 8 elevational zones ranging from 500 to 2215 m a.s.l. Results indicated that 495 plant species falling in 314 genera and 76 families occurred in the Shada Mountains, including 19 endemic species and 43 endangered species, and accounting for 22% of the total flora in Saudi Arabia. Canonical correlation analysis indicated that physiographic features, particularly altitude play an important role in the frequency and abundance of species. Endemics were not evenly distributed in the Shada Mountains and mostly restricted in the elevation zone of 1000-1500 or >1500 m a.s.l. When the altitude reached up to 2000 m a.s.l., the endemics decreased substantially due to the significantly low temperature. The endangered species existed three different distribution patterns: (i) most endangered species distributed in the altitudes of 1000-1800 m a.s.l.; (ii) endangered trees, lianas, and shrubs occupied the altitudes 1000-1300 m a.s.l.; and (iii) endangered subshrubs and herbs inhabited the altitudes 1500-2100 m a.s.l. The results demonstrated that the biodiversity of endangered species in the Shada Mountains is high and undisturbed by invasive species, and protective measures should be taken against human disturbances to the small-scale hotspot.

Reforestation or natural forest regeneration is an alternative measure for controlling soil erosion in degraded land on the Chinese Loess Plateau (CLP). However, our understanding of the temporal dynamics and the spatial patterns of forest regeneration remains inadequate. Two oak forests at different development stages were investigated to determine the spatial patterns of competitions (intraspecies and interspecies) during different successional stages. The intraspecies and interspecies spatial relationships among different tree diameters at breast height were analyzed at multiple scales by Kriging interpolation method and univariate and bivariate O-ring statistics. Our analytical results indicated that self-correlation and competition intensity were relatively high between oak and pine trees in the early development stage of oak forests due to their clumped distributions of heavy seeds. Birch trees had a lower competition in comparison to oak trees although birch was the dominant species. Therefore, asymmetric competition of oak trees was most likely to have led to their edge dispersal and their success in replacing the pioneer species. Asymmetric competition means that larger individuals obtained a disproportionately large share of the resources and suppressed the growth of smaller individuals. Kriging interpolation analysis showed a tendency towards homogenization caused by interspecies competition during the succession of oak forests. Our results demonstrated that the competition was the driving factor in the spatial distribution of oak forests on the CLP.

This study investigated the seasonal variations of the normalized difference vegetation index (NDVI) and its relationships with climatic variables and topography in a small-scale (20 km×20 km) area (i.e., Tsogt-Ovoo village) within the desert steppe zone of Mongolia using in-situ observed climate data and satellite remote sensing data. We found that the topography is very important for vegetation growth in the desert steppe although the summer precipitation is the constraining factor. The unexpectedly high NDVI (upto 0.56), as well as the high aboveground biomass, in the valley bottom was primarily resulted from the topography-modulated redistribution of overland flow after relatively heavy precipitation events during the growing season. This makes the valley bottoms in desert steppes not only reliable feeding resources for livestock but also heavens for wild lives. But, the detected large standard deviation of annual maximum NDVI (NDVI_max) from 2000 to 2013 in the valley bottom in response to rather variable precipitation implies that the valley bottoms under desert steppe climates are more vulnerable to climatic change.

Chenopodiaceae is one of the most important families in arid and saline environments. Several studies have observed the mycorrhizal structure in Chenopodiaceae plants (i.e., chenopods), but the mycorrhizal colonization status of chenopods in saline habitats and the influencing factors are still not well understood. The mycorrhizal colonization of twenty chenopod species in three different saline habitats (a saline alkaline meadow in the Songnen Plain of northeastern China, a saline desert in the Junggar Basin of northwestern China, and a saline alpine meadow in the Tibetan Plateau of western China) and the chenopod-associated environmental factors (including soil moisture, soil available phosphorous (P) concentration, pH, and salt content) were analyzed. Our results showed that approximately 60% of the studied chenopods were colonized by arbuscular mycorrhizal (AM) fungi with a colonization percentage ranging from 5% to 33%. Structural analysis of mycorrhizal association indicated that vesicles were quite common, while arbuscules and hyphal coils were relatively rare. In addition, a positive correlation between mycorrhizal colonization rate and soil electrical conductivity (r=0.920, P<0.01) and two negative correlations of mycorrhizal colonization rates with soil moisture (r= -0.818, P<0.01) and the soil available P concentration (r= -0.876, P<0.01) confirmed that mycorrhizal colonization rate in the roots of chenopods was environment-dependent.

This study aimed to reveal the microbial diversity in the fecal samples of bactrian camels using the 16S rRNA sequencing analysis on the Illumina MiSeq platform. Three fecal samples were collected from two geographical regions in China. Operational taxonomic unit (OTU) clustering was performed by identifying an OTU at 97% sequence identity. The alpha and beta diversities were applied to estimate the differences in microbial diversity among the three fecal samples. Totally, 4409, 3151 and 4075 OTUs in the fecal samples were identified in the Lop Nor wild camel (Camelus ferus), the domestic camel (C. bactrianus) and Dunhuang wild camel (C. ferus), respectively. The majority of bactreria were affiliated with phylum Firmicutes and Bacteroidetes in the three samples. The wild camels had higher gastrointestinal tract microbial diversity than the domestic one, while the microbial composition of the Lop Nor wild camel shared higher similarity with domestic camel at the genus and family levels than that of the Dunhuang wild camel did. Our results may provide a theoretical basis for assessing their health conditions and may thus be useful for protecting the critically endangered species of C. ferus.