Characteristics of Wind Erosion and Deposition in Oasis-desert Ecotone in Southern Margin of Tarim Basin,China

The oasis-desert ecotone is a fragile ecological zone that is affected both by oasis and desert conditions. To understand the impact of the differences in wind power, and the influence of wind erosion and deposition on the ecotone, meteorological data and con- temporaneous wind erosion and deposition data were collected on the southern margin of Tarim Basin with serious sand-blown hazards. The wind velocity, average wind velocity, sand drift potential （DP）, resultant sand drift potential （RDP）, and sand transportation rate decrease significantly and successively across four landscape types with increasing vegetation coverage （VC）. Flat surfaces and areas of shifting sandy ground experience intense wind erosion with fast movement of mobile sand dunes; semi-fixed sand areas experience ex- tensive wind deposition but only slight wind erosion; and fixed sand areas experience only slight wind erosion and deposition. Volume of wind erosion on bare newly reclaimed farmland is up to 6.96 times that of bare shifting sandy ground. Wind erosion volume per unit area and VC follow an exponential function relationship in natural conditions, while wind deposition volume per unit area does not conform to any functions which has close relationship with vary topography and arrangement patterns of vegetation besides for VC. The results indicate that the volume of wind erosion has a close correlation with VC, and different types and distribution patterns of topog- raphy and vegetation also profoundly influence the wind deposition volume in the field, and underground water tables in different land- scape types control the plant community distribution. Keywords： wind erosion; wind deposition; oasis-desert ecotone; vegetation coverage （VC）; topography; Cele County

Quantification of driving factors on NDVI in oasis-desert ecotone using geographical detector method

Within oasis-desert ecotone regions,the normalized difference vegetation index(NDVI)is an important parameter for evaluating the growth of vegetation.An accurate quantitative study between NDVI and environmental and anthropogenic factors is critical for understand the driving factors of vegetation growth in oasis-desert ecotone.In 2016,four periods Landsat 8 OLI_TIRS images,relevant climatological parameters data(air temperature,air relative humidity,wind velocity and accumulated temperature),land cover type data and soil data were selected as proxies.In order to quantify the explanatory power for NDVI spatial and temporal distribution in the southern edge of Dunhuang City and northern side of the Mingsha Mountain,the geographical detector model was used to explain the potential influences of factors versus the spatial distribution of NDVI,and each explanatory variable's relative importance can be calculated.The factor detector results disclose that the spatial distribution of NDVI is primarily dominated by land cover type.The risk detector results show that,high NDVI region is located within woodland.The mean value of NDVI displays an increase and then decrease trend with air temperature increase.With the increase of wind velocity and decrease of air relative humidity,the NDVI value shows a decrease trend.The interactive q values between the two factors are higher than any q value of separated factors.Results also indicate that the strongest interactive effects of NDVI are different in distinct seasons.Consequently,anthropogenic activity is more important than environmental factors on NDVI in oasis-desert ecotone.We also demonstrate that air relative humidity rather than air temperature have played a greater role in NDVI spatial distribution.

Soil properties and herbaceous characteristics in an age sequence of Haloxylon ammodendron plantations in an oasis-desert ecotone of northwestern China

Haloxylon ammodendron, a typical desert shrub with C4 pathway of photosynthesis, possessing a strong ability to adapt to an extreme drought environment, has a rapid growth rate in sandy lands and is widely used in sand-fixing shelter-forest systems in oasis-desert ecotones. To assess the effects of H. ammodendron plantation on the soil, we measured soil properties and herbaceous characteristics along a nearly 40-year chronosequence after H. ammodendron was planted in shifting sand dunes in an oasis-desert ecotone. Results showed that silt and clay fractions increased significantly in the topsoil. The accumulation rates of soil organic carbon （SOC）, total nitrogen （TN） and total phosphorus （TP） were faster in the early stages （0-9 years） and slower in the late stages （9-39 years）. The soil pH and electrical conductivity （EC） were higher than those in the non-vegetation dunes. Moreover, the soil properties in the topsoil （0-5 cm） showed larger variation scope than those in the deeper soil layers （5-20 cm）. The significant relationships of the soil silt＋clay content with the chemical properties mainly appeared in the topsoil. The wind erosion susceptibility of the soil, evaluated by erodible fraction （EF）, decreased significantly with increasing H. ammodendron plantation age. Additionally, the annual pioneer herb, Agriophyllum squarrosum, was gradually substituted by the annual salt-tolerant herb, Bassia dasyphylla, with increasing plantation age. These results showed beneficial effects of H. ammodendron plantation on improving soil conditions. However, the dynamics of the herbaceous species also reminded us that the long- term effects of H. ammodendron plantation, especially on changes in vegetation composition, still need further evaluation.

Desert vegetation distribution and species-environment relationships in an oasis-desert ecotone of northwestern China

Environmental heterogeneity significantly affects the structure of ecological communities.Exploring vegetation distribution and its relationship with environmental factors is essential to understanding the abiotic mechanism(s)driving vegetation succession,especially in the ecologically fragile areas.In this study,based on the quantitative analysis of plant community and environmental factors in 68 plots at 10 different transects in the Minqin oasis-desert ecotone(ODE)of northwestern China,we investigated desert vegetation distribution and species-environment relationships using multivariate analysis.Two-way indicator species analysis(TWINSPAN),detrended correspondence analysis(DCA),and canonical correspondence analysis(CCA)methods were used.A total of 28 species,belonging to 27 genera in 8 families,were identified.Chenopodiaceae,Zygophyllaceae,Gramineae,and Leguminosae were the largest families.Annual and perennial herbs accounted for 28.60%of the total number of plants,while shrubs(42.90%)were the most dominant.Nitraria tangutorum was the constructive species of the desert plant community.We divided the 68 plots surveyed in this study into 7 community types,according to the results of TWINSPAN.The distribution of these 7 communities in the DCA ordination graph showed that species with a similar ecotype were clustered together.Results of CCA indicated that groundwater was the dominant factor influencing vegetation distribution,while distance between plot and oasis(Dis)and soil electrical conductivity(EC)were the local second-order factors.Our study suggests that optimizing the utilization of groundwater in oases is key to controlling the degradation of desert vegetation.The favorable topographic conditions of sand dunes should be fully utilized for vegetal dune stabilization,and the influence of soil salinity on the selection of afforestation tree species should be considered.

Carbon assimilation characteristics of plants in oasis-desert ecotone and their response to CO_2 enrichment

Six species of more than 20-year-old desert woody plants in the oasis-desert ecotone were selected for study. The results showed that: (1) in different growing seasons δ13C values of assimilating organ varied between -14‰ and -16‰ for Haloxylon ammodendron (HA),-14‰-15‰ for Calligonum mongolicum (CM) and-25‰- -28‰ for Caragana korshinskii(CK), Nitraria sphaerocarpa (NS) and Hedysarum scoparium (HS). (2) The net photosynthetic rate (Pn) of HA and CM was significantly higher than those of the other species. With the decrease in Pn for the six species, their intercellular CO2 concentration increased, but stomatal limitation value decreased under the intensive light. At the same time, the photochemical efficiency of PS II dropped to different degrees. (3) The CO2 enrichment experiment demonstrated that, Pn of HA and CM increased to different extent under 450 umol/mol, but their Pn reduced or approximated to the current condition under 650 umol/mol. Under 450 umol/mol the efficiency for solar energy utilization of CK and HS significantly reduced and under 650 umol/mol their respiration rate exceeded photosynthesis rate. It can be concluded that HA and CM have some function of pathway for C4, but the other three species have the function for C3. The decline in their Pn is mainly caused by non-stomatal factors. HA, CM, CKand HS exhibited photoinhibition, which disappeared in a short time. This is a kind of positive readjustment to adapting to the desert environment. HA and CM can adapt to the high CO2 environment, but CK and HS cannot. With the rise in atmospheric CO2 concentration and climate warming, the latter two species in the oasis-desert ecotone may be gradually degraded or even disappear.

1973—2010年塔里木河中游土地利用变化特征分析——以肖塘绿洲-荒漠交错带为例

在对塔里木河中游肖塘绿洲-荒漠交错带1973年、1990年、2000年和2010年的4期Landsat卫星遥感影像进行解译的基础上,利用GIS技术和RS技术,建立该地区土地利用空间数据库,定量分析了研究区37年来土地利用变化特征,并从自然因素和人类活动2个方面对交错带土地利用变化的驱动力进行了较为深入的探讨。结果表明:肖塘交错带土地利用类型以未利用地和荒漠林地为主。与1973年相比,2010年荒漠林地面积大幅下降,占研究区的5.06%,而农田则急剧增加,占研究区的4.67%,未利用地有所增加,占0.63%,草地有所减少,占0.21%。研究区荒漠林地主要转化为农田,草地转化为农田和荒漠林地,农田面积的增长主要来源于荒漠林地。交错带内生态退化加剧,生态破坏严重。人类的生产生活方式对研究区的土地利用方式有着深刻的影响。

Comprehensive evaluation on the ecological function of groundwater in the Shiyang River watershed

With an arid climate and shortage of water resources,the groundwater dependent ecosystems in the oasis-desert ecotone of the Shiyang River Watershed has been extremely damaged,and the water crisis in the oasis has become a major concern in the social and the scientific community.In this study,the degene-ration characteristics of the groundwater ecological function was identified and comprehensive evaluated,based on groundwater depth data,vegetation quadrat and normalized difference vegetation index(NDVI)from Landsat program.The results showed that(1)the suitable groundwater depth for sustainable ecology in the Shiyang River Watershed is about 2-4 m;(2)the terms of degenerative,qualitative and disastrous stages of the groundwater ecological function are defined with the groundwater depths of about 5 m,7 m and 10 m;(3)generally,the groundwater ecological function in the oasis-desert ecotone of the lower reaches of Shiyang River Watershed is weak with an area of 1397.9 km2 identified as the severe deterioration region,which accounted 74.7%of the total area.In the meantime,the percentages of the good,mild and moderate deterioration areas of groundwater ecological function are 3.5%,5.5%and 16.3%,respectively,which were mainly distributed in the Qingtu lake area and the southeastern area of the Shoucheng town;(4)the degradation and shrinkage of natural oasis could be attributed to the dramatic groundwater decline,which is generally caused by irrational use of water and soil resources.This study could provide theoretical basis and scientific support for the decision-making in environmental management and ecological restoration of the Shiyang River Watershed.