| The cold and dry region represented by Eurasian grassland is very important to the sustainable development of the world or China.In the past few decades,due to the impact of human activities and climate change,the region has faced the risk of desertification caused by ecological degradation.In this paper,Horqin sandy land is selected for this study,and the dune-meadow area in the southeast edge of Horqin sandy land is taken as the research area.Based on a large number of vegetation and habitat factor data collected,structural equation model(SEM)was used to simulate the dynamic change process of vegetation community.According to the results of community dynamic analysis,a land cover classification system based on community scale suitable for desertification ecosystem health analysis was developed.Combined with vegetation phenology,habitat information and multi temporal remote sensing data,a decision tree suitable for long-time series land cover recognition in dune-meadow area was established.Based on the evaluation framework of vigor-organization-resilience-services(VORS),the ecosystem health temporal and spatial variation characteristics in the process of periodic changes of hydrological conditions were analyzed.Finally,the effects of natural factors and human activities on ecosystem health were quantified by constructing SEM.The main research results are as follows:1.Based on SEM,the influence of habitat conditions on the vegetation community composition and structure was analyzed.In the the process of model construction,the community was distinguished by the difference of important values of dominant plant species,and the community composition was reflected by richness and diversity.It is an innovation in the research of vegetation community composition and structure.The results showed that soil moisture and texture had the most significant effect on community composition in sand dune habitat.Under the condition of meadow habitat,the most significant effect on community composition is the topographic characteristics.In addition,the dynamic changes of vegetation community composition were simulated based on SEM statistical results.The results showed that although Artemisia frigida +Agropyron cristatum community and Caragana microphylla community with high community diversity and richness can play a role in fixing sand dunes,Artemisia frigida+ Agropyron cristatum community has a better effect on the restoration of sand dune soil,which is more conducive to the healthy and sustainable development of the ecosystem.Therefore,it is necessary to develop a land cover classification system based on community scale for the analysis of ecosystem health changes in desert areas.2.Based on Sentinel-2 A/B and Landsat-5/7/8 multi temporal data,long time series land cover in the study area was accurately identified at the community scale.The results show that,for the complex underlying surface of dune-meadow area,referring to the growth environment of vegetation,the mixed distribution between sand vegetation and meadow vegetation can be reduced.Referring to the phenological information of vegetation,the mixed division among arbor forest,shrub community and semi shrub community can be reduced.In addition,according to the succession process of vegetation community,the diversity can be identified,and then the semi shrub community can be distinguished.For the lack of high-precision spatial resolution historical image data,the best segmentation results based on high spatial and spectral resolution Sentinel-2 data can improve the interpretation accuracy when interpreting the historical cover type of the study area based on low spatial resolution Landsat series data.3.The transformation law and spatial-temporal variation characteristics of each cover type in the study area were analyzed.The results showed that from period I(1984~1988)to period II(1999~2004),land desertification intensified and landscape fragmentation and heterogeneity decreased.The change of land cover has a certain rule.The mobile sandy land is mainly transformed into the semi shrub community with low diversity,while the semi shrub community with high diversity is transformed into the semi shrub community with low diversity or directly degenerated into the mobile sandy land due to man-made reclamation and abandonment.From period II(1999~2004)to period III(2018),the trend of land desertification in the study area has been effectively curbed,and the degree of landscape fragmentation and heterogeneity are increasing.However,affected by man-made control activities,the change intensity of each cover type to mobile sand and low diversity semi shrub community was significant.4.Based on the interpretation results of three periods,the temporal and spatial variation characteristics of ecosystem health were evaluated using VORS framework.From period I to period II,the vitality of ecosystem increased,the organizational capacity and restoration capacity of ecosystem decreased,and the service capacity in northwest region gradually decreased.From period II to period III,ecosystem vitality,organization and service capacity increased,while ecosystem resilience decreased slightly in the southeast region and increased slightly in the northwest region.When ecosystem health deteriorates and recovers,the change trend of comprehensive ecosystem health reflecting service function is greater,which indicates that ecosystem change is more sensitive to human impact.5.The effects of natural and human restorations on ecosystem health were analyzed quantitatively.From period I to II and period II to III,the changes of ecosystem health in the study area are mainly caused by the changes of natural factors.For ecological health,natural factors are mainly reflected in the terrain conditions and the land area of n atural restoration.For integrated ecosystem health,before period II,the natural factors are mainly reflected in the rainfall conditions and the land area of natural restoration.After period II,the effect of artificial restoration increased significantly,while that of natural restoration decreased.To sum up,it is suggested that in the process of desertification ecosystem health restoration,we should continue to promote the enclosure measures and strengthen the supervision of grazing prohibition and rotational grazing.Maintain the current situation of land cover and manage the arbor and shrub communities reasonably.It further shows that scientific and systematic analysis of land cover evolution process and ecosystem health changes can lay a theoretical information foundation for promoting ecological hydrology and protecting and utilizing fragile grassland resources in cold and arid areas. |
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