| Vegetation has important ecological and economic value.However,due to climate change and human destruction,the vegetation has been seriously degraded in the past century.Since 1999,the Chinese government has been piloting a series of ecological construction projects,including returning farmland to forest/grassland(RFTFG),and has continuously strengthened vegetation construction and carried out ecological restoration.It is of great significance to carry out the evaluation of vegetation restoration effect and clarify the implementation effect and driving mechanism of previous ecological policies.However,the existing remote sensing data cannot take into account the temporal and spatial resolution,and the accuracy is low,which is insufficient to support the research on the fine scale.At the same time,conventional vegetation indicators have great limitations in evaluating the effect of ecological policies.In addition,there is also controversy in the academic circles about the implementation effect of RFTFG.In view of the above shortcomings,this study took Yan’an City,a key area for the implementation of the RFTFG policy,as an example to carry out the following work.Firstly,high-quality vegetation remote sensing data with a resolution of 30 m in 16 days was obtained by using spatio-temporal fusion technology.In this study,the ESTARFM model was selected for fusion,and the spatial distribution trend of the low-value and high-value areas of the fusion image and the original image,as well as the boundary line of the object contour basically the same.After multi-angle accuracy verification,the correlation coefficient between the fusion image and the original image is high,showing that the fusion result is reliable.Secondly,the vegetation restoration potential achievement degree(VRPAD)index and Hurst index were introduced to comprehensively evaluate the vegetation restoration effect in the study area from 2000 to 2020 from the perspectives of potential improvement and sustainability.At the same time,taking into account the problem of spatial heterogeneity,this study mainly used geographically weighted regression(GWR)model to explore the impact of ecological policies on vegetation restoration from the above two perspectives,and provide suggestions for better implementation of ecological policies.The main conclusions are as follows:(1)Since the implementation of the ecological project of returning farmland to forests,the vegetation coverage in Yan’an City has shown an overall upward trend.From 2000 to2020,the vegetation coverage of 13 counties in Yan’an City increased.Among them,Zichang,Yanchang and Yanchuan counties in the north of Yan’an had the largest vegetation growth slope,and Huangling and Huanglong counties in the south had the smallest vegetation growth slope,but their vegetation coverage The degree has been higher than other counties.In terms of stages,the growth rate of vegetation coverage in Yan’an City in the four stages of 2000-2005,2005-2010,2010-2015,and 2015-2020 showed an alternating phenomenon of "fast-slow-fast-slow";the four stages of Yan’an The areas where the vegetation coverage of the city showed an increasing trend accounted for 99.5%,86%,96.7% and 84.9% respectively.(2)The center of gravity of vegetation in Yan’an City is in the south.From 2000 to2020,the center of gravity of vegetation continued to move northward,and the moving distance gradually shortened,and the effect of vegetation restoration in the north gradually weakened.The low-low concentration area and the high-high concentration area of vegetation cover are distributed contiguously,the low-low concentration area is located in the north,and the high-high accumulation area is located in the south.(3)The comparison results of the NDVI difference and the VRPAD difference in each county from 2000 to 2020 show that the VRPAD is more sensitive in highlighting the effect of policies on vegetation restoration.For example,Wuqi county is the county with the most intensive implementation of the RFTFG policy in Yanan city,whose difference ranking of VRPAD from 2000 to 2020 was significantly higher than that of NDVI difference ranking.The Hurst index of Yan’an City is between 0.4346-0.8202,with an average value of 0.57.14.3% of Yan’an’s regional Hurst index is less than or equal to 0.5,which is unsustainable.At the same time,there are obvious differences in the sustainability of vegetation restoration in Yan’an City,and the sustainability of vegetation restoration in the south is significantly lower than that in the north.(4)The results of the GWR model showed that the impact of each explanatory variable on the improvement degree of VRPAD had obvious spatial heterogeneity.The effect of ecological policy on the improvement of VRPAD in northern Yan’an is obviously stronger than that in the south;precipitation plays a role in promoting the improvement of vegetation restoration potential in most areas;population density has a promoting effect on the improvement of vegetation restoration potential in the key counties of northern counties where farmland is returned to forest,but it is inhibited in other areas;in the central and southern regions of Yan’an,the better the traffic conditions,the higher the degree of improvement of vegetation restoration potential,and the opposite is true in northern Ansai and other regions.(5)The implementation of ecological policies in the central,eastern,and northeastern parts of Yan’an is conducive to improving the sustainability of vegetation restoration,but in Wuqi and Zhidan counties in the northwest,the implementation of ecological policies has reached "oversaturation",and continued implementation is not conducive to sustainable vegetation restoration.Precipitation can promote the sustainability of vegetation restoration in central and eastern Yan’an,but has a negative impact on the sustainability of vegetation restoration in the peripheral areas of Yan’an.In the south of Yan’an,the better the socio-economic conditions,the stronger the sustainability of vegetation restoration,while in the north of Yan’an,the stronger the socio-economic activities,the more unfavorable to the sustainable restoration of vegetation.According to the conclusions of the study,the following suggestions are put forward.First,the implementation of the RFTFG policy should be tailored to local conditions and implemented precisely.Secondly,the implementation of the ecological projects like RFTFG might have side effects on the sustainability of vegetation restoration,which should be alerted,and the protection of existing forest and grass vegetation should be strengthened intensity;Finally,the government must pay attention to the coordinated development of ecology and economy.This study is expected to provide reference for research related to vegetation restoration and ecological policy evaluation,and provide decision-making reference for policy optimization. |
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