| As the "roof of the world",the Qinghai-Tibetan Plateau(QTP),in parallel with the south and north poles,is known as the "third pole of the world".Tibetan Plateau is the main part of the QTP,with an average altitude of more than 4000 m.The ecological environment is extremely fragile and sensitive,and vulnerable to human activities and climate warming.The middle reaches of the Yarlung Zangbo River Basin is the social and economic center of Tibet Autonomous Region.And the urban life and production of agricultural and animal husbandry are seriously affected by the seasonal sand disaster because of the abundant sand on the surface,the dry and cold climate,the sparse and low vegetation coverage,etc.Vegetation recovery is an important part of ecological restoration,which can improve the function of regional ecological protection and the ability of social and economic sustainable development.Among them,artificial restoration and natural restoration are two important approaches to the restoration and management of degraded ecosystems,and they are also effective measures to increase the value of assets of forest and grass resources.In this study,the vegetation on the valley sandy land in the Tibetan Plateau was taken as the research object.Through field experiments,the relationship between vegetation structure,species diversity and habitat factor changed under different topographies of sandy land,treatments of sand barrier and gradients of altitude was revealed from three scales of quadrat,transect and region under the pattern of artificial recovery.For the first time in the alpine sandy land,through the Terrestrial Laster Scanning(TLS),the community structure and spatial distribution changed with the topography / micro-topography under the pattern of natural recovery were revealed from the perspective of horizontal and vertical vegetation structure.The changes of plant growth and soil properties from the typical communities in sandy land under different patterns of vegetation recovery were discussed and the influence of patterns of vegetation recovery on changes of plant and soil was clarified.The study on vegetation recovery on valley sandy land in Tibetan Plateau was further improved,and the corresponding countermeasures and suggestions were put forward,in order to provide scientific basis for the research on vegetation restoration and reconstruction.The results showed that:(1)There were great differences for vegetation recovery characteristics among different types of sandy land(flood plain,barchan dune and valley-slope).A total of 14 species were tested in all these types and a significant difference was observed among the growths of species in each type.Specifically,summarizing the species composition and plant growth,the important value(IV)of Artemisia sphaerocephala was 25% and the growth was the best on the flood plain,Artemisia wellbyi and Artemisia younghusbandii,of which IV was 28% and 16% respectively,favor the barchan dune,and Hedysarum scoparium,A.wellbyi and Heteropappus gouldii grew the best on the valley-slope.These species could be considered as the suitable plants for the early stage of revegetation,and as the best choice for the process of revegetation of different types of sandy land.For the sandy land of valley-slope,there are obvious improvements in the plant growth and soil quality during the vegetation recovery from 2011 to 2017.With the significant increase of vegetation coverage and height,the improvement of soil properties is very obvious,and the soil particle size changed from predominantly medium-fine sand at the period of pioneer(2011)to very fine sand at the period of development(2017).Higher plants were distributed mainly in the midslope area at the period of development(2017).A significant positive correlation was observed between pH and plant height at the period of development(2017).The sand-protecting barriers play an important role in revegetation.The effectiveness of revegetation is different among different sand-protecting barriers of the moving sandy land on flood plain.The straw barrier is more beneficial to the growth of individual species,while the stone and plastic checkerboard barriers improve plant diversity,promote the stability of ecological ecosystem and enhance the potential of vegetation restoration for aeolian sandy land.(2)The effectiveness of natural restoration for S.moorcroftiana population was significant.In different topographies,the middle of alluvial fan has highest individual growth.The positive correlation between height and crown projection area(CPA)reached the 99% confidence level(p < 0.01).The plant height was significantly correlated with the elevation and gradient(r = 0.167 and 0.145,respectively;p < 0.01).While the distribution of S.moorcroftiana population decreased along increasing elevation,and the trend of distribution was decreasing firstly,increasing secondly and decreasing finally with increasing slope.The habitat conditions on the southwest slope of approximately 20°-25° with altitudes of 3593-3643 m most favor to the distribution and growth of S.moorcroftiana population.(3)The highest silt content,at the depth of 0–20 cm,was 50.77% and 62.16% in Sophora moorcroftiana shrub and Stipa bungeana Trin.grassland under the pattern of natural restoration,respectively.The highest silt content at 20-40 cm(44.28%),as well as the lowest soil bulk density(SBD)(1.55 g/cm3),was observed in S.bungeana Trin.grassland.There was no significant difference in the soil organic matter(SOM)of the four vegetation types at 0-20 cm;however,it was significantly different at 20-40 cm.The SOM content was highest in S.bungeana Trin.grassland(23.37 g/kg),followed by S.moorcroftiana shrub(17.42 g/kg),Artemisia wellbyi shrub(14.85 g/kg),and Salix L.(8.43 g/kg),in decreasing order.NH4-N content in A.wellbyi shrub was 18.78 mg/kg,which was higher than in other types(5.95–9.10 mg/kg).Therefore,natural restoration is essential to prevent land degradation.The natural restoration should be as the main measure in the process of ecological restoration and combined with artificial recovery.The SOM was significantly correlated with the total phosphorus(TP)and available phosphorus(AP)contents of the aeolian sandy land(P < 0.05).The leaf TP was positively correlated with SBD,AP,and available potassium(P < 0.05),and the leaf total nitrogen was positively correlated with AP(P < 0.05).(4)According to the change of vegetation characteristics of different types of sandy land in the alpine valley,the corresponding management countermeasures are taken.For the sandy land of flood plain,the good conditions of water and gas are conducive to the construction of windbreak and sand fixation system combining with tree,shrub and grass along the river,and at the same time,the implementation of ground measures such as sand barriers should be paid attention;for the sandy land of barchan dune,the policy of "natural restoration should be as the main measure in the process of ecological restoration and combined with artificial recovery." should be implemented as the main pattern of vegetation recovery,and natural restoration should be mainly fencing closure,supplemented by suitable measures of artificial recovery for shrub such as replanting,aerial seeding,grass fixation,etc.;for the sandy land of valley-slope,the new method of manually stepping on and back sowing along the contour line should be adopted to artificial recovery,and the influence of topography factors should be fully considered to build a complete restoration system.At the same time,the measures of fencing closure and prohibition for grazing should be strictly implemented.Moreover,the process of vegetation recovery in the alpine valley cannot be separated from the support of science and technology and the improvement of the security system. |
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