Identification And Assessment Of Forest Disturbance In Southwestern China Based On Multisource Remote Scensing Products

Excessive disturbances can lead the reversion of forest succession or tree mortality,trigger a series of ecological and environmental problems such as ecological imbalance and soil erosion,and further weaken terrestrial forest ecosystem function services.Southwest China is the second largest forest region in China with the frequent forest disturbances in the context of global changes which have exacerbated the vulnerability of regional ecosystems and further affected the Chinese forest ecological function services.Therefore,forest disturbance identification and assessments are very meaningful for developing forest recovery measures and preventing eco-environment deteriorations in southwest China.Some reported forest disturbance identification methods were mostly developed by high spatial resolution remote sensing products,and they were often applied to forest disturbance identification at small spatial scales.However,these methods perform a lower accuracy of forest disturbance identification with larger spatial scales.Meanwhile,large-scale and long-term forest disturbance identification by higher spatial resolution products has shortages of huge amounts of data requirement,time-consuming processing,and lower identification efficiency.Moreover,current characteristics after forest disturbances such as burned area,cutting area and number of trees cut were wildly used as classification criterion of forest disturbance levels,which failed to describe the dynamic changes of cumulative forest disturbance and forest ecosystems service functions recovery process.To address the issues above,we creatively developed the Paired Point Approach,a quick and accurate method based on multi-resource remote sensing products(e.g.Landsat and MODIS images)to identify the forest disturbance lands(e.g.forest wildfire and forest logging)at larger spatial scales.We also creatively proposed the Cumulative Forest Disturbance Index(CFDI)which can describe the forest cover and forest ecosystem service functions dynamics after forest disturbances to quantify the cumulative forest disturbance levels.This study took southwest China as an example,which mapped the spatial distributions of forest burned lands and forest logging lands in southwest China from 2002 to 2017 based on Paired Point Approach.Then we evaluated the cumulative forest disturbance levels in southwest China and the spatial-temporal characteristics of forest burns and harvested sites with different topography,climate and forest cover conditions and combining the GIS spatial analysis,Mann-Kendall trend analysis,Spearman test and other statistic methods.The main conclusions are as follows:The general accuracy in 2010 was 92.27%and 88.05%for identified fire and logging disturbances by the Paired Point Approach in southwest China,respectively,and the kappa coefficients were 0.85 and 0.75,respectively.The general accuracy of the Paired Point Approach was up to about 16%higher than that of the traditional methods,which indicated that the Paired Point Approach did better in forest disturbances identification at larger scales.We derived some sampling points from spatial distribution maps of forest burned and logging lands in 2017 to validate in the field,and the results showed that there was an accuracy of more than 80%.The total forest burned land area was 37.69×10~4hm~2 in southwest China from 2002to 2017,and they mostly distributed in the western Sichuan province,northwest and central Yunnan province,and central and southeast Guizhou province.In the 16-year period,the forest burned land area in the southwest regions was Guizhou,Sichuan,Yunnan and Chongqing in descending order.The spatial distribution of forest burned lands varies with topographical,climate,and forest cover conditions.The forest burned lands were mainly distributed in areas with an altitude of less than 4000m,a slope of less than 25 degrees,annual precipitation of 800～1000mm,and average temperature of10～20℃.In the southwest China,the largest area of the burned land was mostly covered by shrubs and least by coniferous forests.The average annual logged forest land was 4.28×10~4hm~2 in southwest China from2002 to 2017,which mostly logged from 2010 to 2013 with an area of 24.08×10~4hm~2.In the overall study period,the forest logging land area in the southwest regions was Yunnan,Sichuan,Guizhou and Chongqing in descending order.The logged forest land is mostly distributed in western and southeastern Sichuan province,central,northwestern,western and southern Yunnan province,and central,northern and eastern Guizhou province.In terms of the distribution of logged forest land in different topographical and climatic conditions,logged forest land in southwest China is mostly located in areas with an altitude of less than 2500m,a slope of less than 35°,an average annual temperature of10～20°C and a greater annual precipitation.In the southwest China,the largest area of the logged land was mostly covered by mixed forests and least by coniferous forests.The F-CFDIs,L-CFDIs and CFDIs showed significant decline trends(p<0.05)in southwest China from 2002 to 2017,and F-CFDIs decreased more quickly than L-CFDIs.It indicated that the cumulative forest disturbance of southwest China weakened from2002 to 2017,and vegetation on burned land recovered more quickly than that on logged land.In terms of different forest types,disturbed shrub lands and evergreen broadleaf forests recovered more quickly than other forest types.The forest land with low intensity of disturbances mainly located in the western Sichuan province,central and southern Yunnan province,and Guizhou province,while forest disturbances land with moderate to high intensity mainly distributed in west Sichuan province,and central and northwest Yunnan province.Areas where cumulative forest disturbance is significantly reduced are mostly found in watershed areas dominated by the Yunnan-Guizhou Plateau,such as the Jinsha River Basin and the Yuanjiang-Hongjiang River Basin,where mostly covered by shrubs and evergreen broadleaf forests and less human activities.The Paired Point Approach proposed in this study improves the accuracy of identifying different types of forest disturbances on a large spatial scale,providing a scientific method for forest disturbance identification in other regions.Meanwhile,the evaluation of forest disturbances in southwest China can provide a scientific support for forest resource managers to design forest ecological restoration measures.