THE CAUSES AND DEVELOPMENTAL TREND OF DESERTIFICATION IN THE MIDDLE REACHES OF THE YARLUNG ZANGBO RIVER AND ITS TWO TRIBUTARIES IN XIZANG

The middle reaches of the Yarlung Zangbo River and its two tributaries are an important development and construction region in Xizang(Tibet) in recent years, but the fast development of desertification has caused immense damages to its social and economic development, so the study on desertification in this region is significant. The paper studies the causes and development trend of desertification in this region through analysis of natural and human factors as well as their relationship. It can be concluded that desertification in the study area is one process which is accelerated and intensified by human activities on the basis of the slow natural desertification process. It is resulted from the dislocation of irrational and intensive human activities, fragile eec-environment and meagre natural resources, and tends to become more wide and severe.

PRESENT STATUS AND CAUSE OF LAND DESERTIFICATION IN THE YARLUNG ZANGBO RIVER BASIN

The Yarlung Zangbo River basin is a spacial zone in the south of Xizang, the types, distribution and causes of desertified lands have special features. The type, area, distribution and damage of desertification land in the Yarlung Zangbo River basin are firstly analysed in detail by using latest investigation information. According to the classification criteria of desertified land types and grades, the desertified land in the Yarlung Zangbo River basin can be divided into three grades and five types: fixed sand (dune) land, semi bare sand and gravel land, semi fixed sand (dune) land, base sand and gravel land, and shifting sand (dune) land. The desertified lands in the basin are mainly distributed in the wide valley floor and tributary junction area. The main factors affecting desertification are fragile eco environment, climate warming and drying and over exploitation of land resources. The man made factors leading to desertification in the Yarlung Zangbo River basin are over cutting and over grazing, dominated by over grazing.

A long-term record(1995-2019)of the dynamics of land desertification in the middle reaches of Yarlung Zangbo River basin derived from Landsat data

Widespread desertification in the middle part of the Yarlung Zangbo River(YZR)basin is threatening the sustain-able development of this region.To capture this process,a method was proposed for large-scale desertification monitoring by using Landsat images from 1995 to 2019.The method used an integrated classification method combined with a hierarchical decision tree and nearest neighbor classifiers.The spatio-temporal dynamics of the desertification pattern were analyzed to assist in the detection of possible driving forces.Using validation samples collected from Google Earth high-resolution images and field investigations,the overall accuracy of the classification in 2019 was 92.3%with a Kappa coefficient of 0.84.The major results were:(1)total sandy land area in 2019 was 734.1 km^(2),which accounted for 3.7%of the study area,prominently distributed along the wide river valleys and inlets of tributaries with a strip and discontinuous pattern.Sandy land tends to be distributed in the southern aspect regions with lower elevations and that are closer to rivers;(2)sandy land areas showed two temporal stages:a gradual increase of 102.4 km^(2)from 1995 to 2015 and a large decrease of 106.8 km^(2)from 2015 to 2019;(3)newly increased sandy land was distributed in the YZR Valley,while the revegetation on sandy land occurred mainly in the Lhasa River basin and some regions in the YZR Valley;and(4)increased sandy land area of 142.1 km^(2)was mainly distributed in the southern band of the two rivers.Correspondingly,revegetation on sandy land was more effective on the northern banks of the river valleys.These findings provide guidance for implementing vegetation recovery on sandy lands and provide important insights for maintaining sustainable development.

Diverse Climatic and Anthropogenic Impacts on Desertification in the Middle Reaches of Yarlung Zangbo River Catchment on the Tibetan Plateau

The unique desertification processes occurring under the Alpine climate and ecosystem on the Tibetan Plateau could provide critical clues to the natural and anthropogenic impacts on desertification.This study used the Landsat data to investigate the spatial and temporal distribution of desertification from 1990 to 2020 in two areas(Shannan and Mainling),within the Yarlung Zangbo River Basin.The results show not only distinct spatial patterns but also various temporal changes of desertification.In Shannan,aeolian sand was distributed over wide areas from valley floor to mountain slope,while in Mainling,it is distributed sporadically at the footslope.The aeolian sandy land initially expanded before undergoing long-term shrinkage in Shannan.While in Mainling,it steadily expanded followed by a rapid decrease.These changes are attributed to both climate change and human activites.The increase in temperature causes desertification expansion in Shannan,while favorable climate conditions coupled with decreasing human activity promoted desertification reversal.However,both the expansion and shrinkage of desertification were sensitive to human activity in Mainling.This highlights the diverse responses of desertification to natural and anthropogenic impacts on different backgrounds of climatic and vegetation coverage.A threshold of climatic conditions may control the dominance factors in desertification,shifting from natural to anthropogenic elements.

STUDY ON THE MOISTURE PASSAGE ON THE LOWER REACHES OF THE YARLUNG ZANGBO RIVER

A series of gorges on the lower reaches of the Yarlung Zangbo River is like an enormous crevice splitting the southeastern Qinghai-Xizang (Tibet)Plateau to create a passage through which warm, wet air flow from the Indian Ocean reaches the inner part of the plateau. This moisture passage was mainly strengthened after the Pleistocene epoch in the Quaternary period and has played a major role in affecting the beginning date of the rainy season and the amount of the plateau’s rainfall. It has also served as a corridor for the living things on the southern and the northern sides of the Himalayas to mingle with each other. The region along this passage is abundant in evidences of species of differentiation, variation and biological division. Also the passage has provided a favorable environment for human beings and affected natural phenomena there.

Land surface roughness impacted by typical vegetation restoration projects on aeolian sandy lands in the Yarlung Zangbo River valley,southern Tibetan plateau

Aeolian sandy lands are widespread and desertification is recognized as one of the main environmental issues in the Yarlung Zangbo River valley,southern Tibetan plateau.The surface microtopographic var-iations induced by the near soil surface characteristics of plant communities are important compositions of land surface roughness,which likely influence wind erosion.This study was conducted to quantify the effects of typical vegetation restoration on land surface roughness on the aeolian sandy lands,and to identify the main influencing factors of land surface roughness in the Yarlung Zangbo River valley.Two bare sandy lands(as controls)and eight vegetated sandy lands with different restoration communities and ages were selected,and land surface roughness(LSR),as represented by surface microtopographic variations in this study,was measured by photogrammetric surveys.The results showed that LSR significantly increased by 7.9-16.8 times after vegetation restoration on the aeolian sandy lands,and varied among different restoration communities and ages.The mostly restored communities of Sophora moorcroftiana and Populus L.had greater LSR as compared to Artemisia wellbyi and Hedysarum scoparium.With succession from 6 to over 30 years,LSR gradually increased in the Sophora moorcroftiana restored sandy lands,but decreased in the Populus L restored sandy lands.The variations of LSR were mainly attributed to the differences in near soil surface characteristics of vegetation(plant stem diameter and coverage,and plant residue density)and biological soil crusts(coverage and thickness).Mixed plantation of Populus L and Sophora moorcroftiana was considered as the best restoration communities because of their effectiveness in increasing land surface roughness on the aeolian sandy lands in the Yarlung Zangbo River valley.The results would facilitate the understanding of the benefits of vegetation restoration in controlling wind erosion on the aeolian sandy lands.

雅鲁藏布大峡谷植被垂直分布格局研究--以多雄河及邻近区域为例

通过对多雄河及邻近区域的现场调查,结合基于3S技术的遥感解译技术,以多雄河及邻近区域为例,对雅鲁藏布大峡谷植被垂直分布格局进行了初步研究,并对白马西路河3条支流(多雄河、比西日河以及丹戈着河)的植被类型相似性进行了分析。结果表明:多雄河及邻近区域的植被从江面往上依次为热带季雨林带,亚高山常绿、半常绿阔叶林带,亚高山针叶林带,高山灌草丛带,流石滩植被带以及永久冰雪带共6个垂直分布带;比西日河与多雄河的植被类型相似性比丹戈着河与多雄河的植被类型相似性更高。

雅鲁藏布江流域土地沙漠化现状与成因初步研究——兼论人为因素在沙漠化中的作用

利用最新的普查数据，首次详尽分析了该区土地沙漠化的类型、面积、分布及危害，通过对其沙漠化影响因素及其相互关系的分析，辅以定量方法系统阐述了该区土地沙漠化的成因，并探讨了藏南河谷农牧区人为因素在沙漠化过程中的作用及其机理。

雅鲁藏布江江当宽谷地区沙源物质与现代沙漠化过程

对雅鲁藏布江江当宽谷附近地表堆积物的粒度和矿物分析结果表明，沙丘沙的平均粒径、分选性、主粒组构成以及矿物成分与附近沙源物质具有一致性和继承性。河流冲积物是最主要的沙源，其次是洪积物和基岩风化物。河谷沙漠化从根本上就是沙源物质“就地起沙”导致的土地退化过程。同时。

西藏"一江两河"中部流域地区土地沙漠化变化的驱动因素分析

土地利用/覆盖变化及其驱动力是当前研究的热点问题,现代土地沙漠化变化的机制及其驱动因素的研究更倍受关注。选择西藏"一江两河"中部流域地区为研究区域,依据该区1991年与1997年的两期土地沙漠化类型与面积数据,利用定量方法分析了同期土地沙漠化的发展变化与区内各种自然与人文因子间的数量关系,以定量辩识其变化的驱动因素。分析结果表明,该区近期土地沙漠化的变化是在多种气候因子、社会因子与经济因子共同驱动下自然变化过程与人为变化过程相互影响的结果,其中,自然驱动因素主要是风力与干旱程度,人为驱动因素主要是农作物播种面积与牲畜(超载)数量,但比较之下自然驱动因素的作用相对稍重,具有相对主导性。

西藏雅鲁藏布江下游黄斑褶[鱼兆]的个体繁殖力研究

2018年3月和7月,在西藏地区雅鲁藏布江下游墨脱段(N 29°18′,E 95°16′,海拔680 m)沿岸石堆后的回水区或岩石之间的静水区,用定置刺网(网目大小1~3 cm)采捕黄斑褶[鱼兆]78尾,采用个体生物学测定方法,研究产卵前成熟雌鱼的个体繁殖力。研究结果表明,黄斑褶[鱼兆]繁殖群体由4~9龄共6个年龄组组成,个体绝对繁殖力为346.00~799.00粒,平均为(498.18±115.88)粒;体长相对繁殖力为3.36~5.53粒/mm,平均为(4.45±0.47)粒/mm;体质量相对繁殖力为20.69~92.80粒/g,平均为(41.33±16.18)粒/g。体长(L)与体质量(m)的关系式为:m=0.6632L-54.924(r^2=0.9215)。个体绝对繁殖力(F)与体长的相关方程式为:F=7.229lnL-302.01(r^2=0.913),个体绝对繁殖力与体质量的相关方程式为:F=10.343m+307.16(r^2=0.889)。个体绝对繁殖力与体长、性腺成熟系数(GSI)、肥满度(K)的多元线性标准化回归分析方程为:F=-446.369+6.567L+480.757GSI+10815864.49K(r^2=0.930),体质量相对繁殖力(Fm)与体长、体质量、净体质量(mn)、卵巢质量(mo)、性腺成熟系数、肥满度的多元线性标准化回归分析方程为:Fm=133.990-0.899L+2.819m-15.044mo+231.368GSI-3962785.189K(r^2=0.938)。根据性腺发育及卵径组成等分析,判断黄斑褶[鱼兆]为一次性产卵鱼类。

雅江河谷防沙治沙工程近地表特性——林下植被特性、生物结皮及土壤养分变化特征

采用对比分析和时空互代法,从植被配置模式(杨树+砂生槐、砂生槐、花棒、藏沙蒿)、工程年限(6,10,30a)2个方面,研究了雅鲁藏布江(简称雅江)河谷防沙治沙生态工程实施过程中林下植被特性、地表生物结皮及土壤养分等变化特征.结果表明,工程年限为10a的乔灌(杨树+砂生槐)、灌木(砂生槐、花棒)型样地较草本(藏沙蒿)更有利于提高林下植物多样性,且杨树+砂生槐样地的林下植被地上生物量、枯落物量、地下生物量、土壤有机质、全氮与全磷含量均显著高于其他3种样地.藏沙蒿样地的生物结皮最为发育,花棒样地次之,杨树+砂生槐样地无结皮.随年限延长,杨树+砂生槐样地林下植物多样性、枯落物量、地下生物量、土壤有机质、全氮及全磷含量呈增加趋势,生物结皮仅在6a样地中有发育;砂生槐样地地上生物量、地下生物量、生物结皮逐渐增加,但植物多样性、全氮含量呈下降趋势,枯落物量、有机碳及全氮含量呈先增加后减小规律.不同配置模式中,杨树+砂生槐乔灌型样地各项指标综合表现最佳,并随工程年限延长,对促进河谷沙地植被演替、提高植被生产力、提升沙地肥力等作用愈强.建议该区防沙治沙工程中应推广杨树+砂生槐乔灌型模式为主.

雅江河谷中段典型防沙治沙生态工程对沙地持水性能的改良效应

雅鲁藏布江河谷中段是西藏自治区社会经济文化政治的中心区域,严重风沙灾害对雅江中段区域经济发展构成了直接威胁。雅江河谷实施的一系列防沙治沙生态工程对减轻河谷地区的风沙灾害起到了显著作用。然而,对高寒地区生态工程实施引起的沙地土壤持水性能改良效应仍然缺乏定量认知。本研究选择雅江河谷4种典型植被类型(藏沙蒿、花棒、砂生槐、杨树+砂生槐)生态工程样地及1个活跃沙丘对照样地,开展原位观测与试验研究,测定了0~30 cm深度土壤基本理化性质和持水性能,分析了土壤持水性能与理化性质的相关性。研究结果表明,4种生态工程对沙地土壤理化性质及持水性能均有一定改良作用:(1)与活跃沙丘相比,生态工程实施驱动表层0~10 cm土壤颗粒明显细化,粉粒、极细砂含量提高25.75~54.61倍和2.31~5.56倍,中砂、粗砂含量降低55.47%~91.57%和39.73%~90.23%;容重显著降低8.67%~34.00%,总孔隙度、毛管孔隙度提高16.06%~45.46%和8.74%~23.39%;有机质、全氮含量增加17.77~72倍和9.50~32倍;而10~30 cm土层的上述理化性质仅在杨树+砂生槐样地有显著变化。(2)生态工程实施驱动沙地表层0~10 cm持水性能显著提升,与活跃沙丘相比,表层土壤饱和持水量、毛管持水量和田间持水量分别提高了28.04%~124.45%、20.09%~88.75%和25.56%~104.80%;而10~30 cm土层持水能力仅杨树+砂生槐样地有显著提升。(3)相关分析表明,持水性能指标与容重、中砂粒含量极显著负相关,与总孔隙度、毛管孔隙度、粉粒、极细砂、有机质、全氮含量呈极显著正相关(P<0.01)。本研究结果可为雅江河谷防沙治沙生态安全屏障建设成效评估提供基础数据支撑,同时为区域沙地治理工程类型选择提供依据和参考。

西藏“一江两河”流域土地沙化空间分布研究

采用TM影像对西藏"一江两河"流域1989、2002和2009年沙化土地空间分布、沙化土地类型分布格局和沙化程度分布格局进行分析。结果表明,流域现有沙化土地面积占总土地面积的6.44%,其中沙砾质类占2/3。但从区域来看,东部(46.64%)、中部(54.94%)明显高于西部(18.15%),说明"一江两河"流域沙化治理工作集中在东部、中部开展。

西藏“一江两河”地区生态环境地质问题与防治对策

从生态环境地质的角度出发 ,指出西藏“一江两河”地区由于生态环境条件脆弱、生态系统的不稳定 ,在缺乏生态屏障保护下 ,对资源的大量开发利用、天然植被的大量掠取 ,诱发和加重了一系列重大生态环境地质问题 ;有针对性地提出了生态地质环境保护措施和防治对策 ,并提供了典型实例和经验。

雅鲁藏布江中部流域沙化地牧草引种适应性试验

为利用草本植物在雅江中部流域沙化地进行植被恢复,引进19个草本植物品种在雅江中部流域沙化地进行生态适应性种植试验。通过对引进植物的出苗、越冬以及产草量进行测定,筛选出了适宜在雅江中部流域沙化地种植且生物量较高的牧草品种,即菊苣、WL168HQ、高冰草和多年生黑麦草,其二茬鲜草产量分别为27110.00、24046.00、5960.00、5823.30 kg/hm^(2)。

基于碎屑锆石特征的雅鲁藏布江山南宽谷风成沙丘物源示踪

雅鲁藏布江河谷(尤其是“一江两河”区域)风沙化较为严重,制约了当地社会经济的可持续发展,追踪河谷风沙来源对当地风沙治理意义重大。本研究聚焦于拉萨贡嘎机场附近的山南宽谷风沙地貌,采集2个河沙、 3个风成沙丘和1个坡积物样品,以及引用前人研究的2个河沙样品。基于碎屑锆石U-Pb年龄及微量元素地球化学特征,进行沙源示踪研究,旨在为当地风沙治理提供科学依据。对360颗锆石进行LA-ICP-MS U-Pb定年,获得322个谐和年龄。分析结果表明,河沙、风成沙丘及坡积物样品碎屑锆石U-Pb年龄分布特征高度相似,主要峰值与冈底斯岩浆弧新生代的两期岩浆活动一一对应;非矩阵多维标度统计技术(MDS)分析也证实了样品与冈底斯岩浆弧有良好的亲缘性;地球化学判别图解显示锆石具有亏损轻稀土元素、富集重稀土元素、明显正Ce异常和明显负Eu异常的特征,亦指示其岩浆成因,表明冈底斯岩浆弧为沙丘、河沙与坡积物中碎屑锆石的主要来源。依据不同类型样品碎屑锆石年龄对比以及锆石年龄相似程度对比,结果发现:1)雅鲁藏布江地区风沙地貌主要是由于枯水季河床水位下降,导致河岸和河漫滩出露地表,就地起沙吹扬到山坡上所形成;2)研究区河沙并非主要从上游搬运而来,更多是拉萨河搬运汇入的泥沙。依据沙源分析结果,欲解决河谷风沙化问题,首要应在河岸和河漫滩种植适宜当地气候的耐旱植物固定河床上的泥沙,这是最直接有效的手段。

雅鲁藏布江中部流域土地沙化遥感识别

雅鲁藏布江中部流域长期遭受土地沙化侵蚀,采取有效手段进行沙化土地信息快速识别,跟踪土地沙化现状和动态发展,是土地沙化防治的基本前提。遥感数据因其快速、大范围、高精度监测等特点已被广泛应用于土地沙化监测。为降低该区域沙化土地破碎化分布特征以及广泛分布的稀疏植被地表对沙化土地遥感识别带来的不确定性,本文利用Google Earth Engine平台获取2019年秋季雅鲁藏布江中部流域Landsat无云遥感影像,基于面向对象的分类思想,充分提取沙化土地的光谱、几何和地形特征,根据不同的分类器构建4种分类方案,包括单一分类器(支持向量机、决策树、最近邻)分类以及组合分类法分类,提取雅江中游河谷地区沙化土地信息并验证不同方案的提取精度。结果表明:①利用面向对象组合分类模型提取的沙化土地信息效果最佳,总体精度高达91.38%,Kappa系数为0.82;②相较于采用单一分类器(支持向量机、最近邻和决策树分类)的面向对象分类方法,组合分类模型能更有效地识别破碎化的小面积沙化土地,降低沙化土地与稀疏植被地表的混淆情况,提高分类可靠性;③基于面向对象组合分类模型反演得到雅鲁藏布江中部流域2019年沙化土地分布信息,土地沙化面积达299.61 km^(2),总体上呈现沿河谷的带状不连续分布,且集中分布于河流北岸以及靠近河道的阳坡、低海拔地区。本研究可为土地沙化遥感监测提供新思路,其应用可服务于雅鲁藏布江中部流域土地沙化预防和治理工作。