| Desert steppe is the most arid type in grassland compared with other grassland types.In this region,steppe degradation and land desertification are common phenomena due to the limited precipitation and excessive use of grassland.Since the beginning of the new century,the government has vigorously promoted the ecological restoration in the desert steppe area through various measures.As a simple and easy recovery measure,fence enclosure has been implemented for many years to promote the continuous optimization of the ecological environment in this region.The clear ownership of the fence provides support for the mobilization of farmers' enthusiasm.The vegetation in the same natural village has a similar recovery rate,however,the fence divides the original continuous natural environment into discrete patches.At the same time,the multi-year accumulation of the intensity difference between the fences causes great differences in recovery rates between the different fences.Therefore the difference in vegetation coverage between the fences is increasing and presents a landscape distribution pattern that is bordered by fences eventually.In this paper,we select a typical desert steppe area in Yanjigou Village,Yanchi County,Ningxia.we use high,medium and low resolution remote sensing data to analyze the temporal and spatial changes landscape types and NDVI.Afterwards,we explore the influencing factors of the spatial distribution of surface vegetation coverage in the study area by using the questionnaires of the households and the analysis of microscopic soil samples.The final result is as follows:(1)The natural grassland is the most important landscape type in the study area.The proportion of natural grassland and shrub lands has gradually increased while the proportion of sandy land has been declining from 2000 to 2015.The spatial location of the sand land has changed from a wide distribution around the South Wangquan Village and Yangzhaizi Village to a small area in the southern part of the study area.The shrub lands gradually formes the northeast-southwest,northwest-southeast continuous shrubbery areas where located in the boundaries of natural villages.Fences promote the increasingly uniform distribution of natural grassland within the study area and the rapid increase of the area coincides with the reduction of sand and saline areas.(2)After the fences,the patches of sandy land are increasingly fragmented,while the patches of natural grassland and shrub lands become agglomeration.From 2000 to 2015,the LPI index of the sand land showed a declining trend,while natural grassland and shrub lands showed an increase trend.In the average patch area,shrub lands increased similarly during the two periods while the sandy lands and natural grasslands showed a sharp transformation from 2010 to 2015.(3)The total number of patch in the study area decreased first and then increased,meanwhile the overall shape index of the area gradually decreased.The results of landscape diversity represent by SHDI,SHEI,resolution and spread index showed that the overall landscape diversity in the study area decreased while the evenness of different landscape types increased.(4)The multi-year average of pre-fence NDVI was 0.172 while the multi-year average of NDVI after the fence was 0.199.Fence promotes overall vegetation in the study area improved at the rate of 0.059/10a.From 2000 to 2015,the main vegetation coverage in the study area changed from 0.1-0.3 to 0.3-0.5.At the same time,the moderately improved area accounts for 59%of the total area.Fences promote the overall improvement of vegetation in the study area.On this basis,the difference in vegetation coverage between fences gradually increases.Finally,there is a radial pattern of landscape reflected by the spatial differences in vegetation coverage.(5)Vegetation coverage within the enclosure is most closely related to the fractal dimension of 0-10cm soil,total soil nitrogen of 10-20cm,soil organic carbon of 0-10cm and fence grazing.The change of landscape pattern was caused by the different grazing intensity between the fences and the interaction with other factors.The above results are obtained by analyzing the relationship between grazing intensity and the above influencing factors. |
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