基于GEE和LandTrendr的宁夏“三山”森林干扰监测

Monitoring of forest disturbance in“Three Mountains”of Ningxia based on GEE and LandTrendr

宁夏六盘山、罗山和贺兰山自然保护区(简称“三山”),是我国西北重要的生态安全屏障。研究宁夏“三山”的森林干扰程度和趋势是否存在明显差异,以及造成“三山”森林干扰的主要原因,对于宁夏守好“三山”自然保护区,维护西北乃至黄河流域生态安全具有重要的参考意义。基于GEE(Google Earth Engine)遥感云平台中的Landsat 5—8时间序列堆栈(1990—2020年),通过LandTrendr时间序列分割算法监测近30 a来宁夏“三山”的森林干扰,借助Google影像对监测结果进行精度评价,统计分析森林干扰的时空分布特征。结果表明:1)LandTrendr算法的用户精度、生产者精度、总体精度均>90%,Kappa系数达到0.87,LandTrendr算法与人工目视解译结果无显著差异(McNemar s test>0.05);2)1991—2020年“三山”森林干扰总面积比:六盘山(1.77%)>罗山(0.07%)>贺兰山(0.05%);“三山”的年干扰面积比都有减少的趋势,且六盘山(52.60×10^(-4)%)>贺兰山(1.58×10^(-4)%)>罗山(0.40×10^(-4)%);3)“三山”的森林干扰主要发生在六盘山的北部及东侧2块“飞地”、罗山北部的“大罗山”和贺兰山的中部;“三山”的森林干扰持续时间都在1~3 a内,且主要集中在1 a。宁夏“三山”中,六盘山的干扰程度要远高于罗山和贺兰山,但“三山”的森林干扰面积比每年都有减少的趋势,六盘山减少最为明显,其次为贺兰山和罗山。六盘山、罗山森林干扰主要是人为活动造成,而贺兰山是人为活动和自然条件共同作用的结果。

[Background]The natural reserve of Liupan Mountain,Luo Mountain,and Helan Mountain in Ningxia(Abbreviated as“Three Mountains”)is an important ecological security barrier in Northwest China.Studying on whether there are obvious differences between the degree and tendency of forest disturbance in“Three Mountains”or not,and the main reasons causing forest disturbance has great reference value in protecting the natural reserve of“Three Mountains”,the Northwest region,even the ecological safety in The Yellow River basin.[Methods]Based on the Landsat 5-8 time series stack(1990-2020)in the GEE(Google Earth Engine)remote sensing cloud platform,the forest disturbance in the“Three Mountains”of Ningxia over the past 30 years was monitored by the LandTrendr time series segmentation algorithm,and the accuracy of the monitoring results was evaluated with the help of Google images,and the spatial and temporal distribution characteristics of the forest disturbance were statistically analyzed.[Results]1)The user accuracy,producer accuracy and overall accuracy of LandTrendr algorithm were all>90%,Kappa coefficient reached 0.87,and there was no significant difference between the result of LandTrendr algorithm and manual visual inspection(McNemar s test>0.05).2)The overall forest disturbance area ratio of“Three Mountains”in 1991-2020:Liupan(1.77%)>Luoshan(0.07%)>Helan(0.05%).The annual disturbance area ratio of the“Three Mountains”tended to decrease,and Liupan(52.60×10^(-4)%/a)>Helan(1.58×10^(-4)%/a)>Luoshan(0.40×10^(-4)%/a).3)The forest disturbance of“Three Mountains”mainly occurred in the northern part of Liupan Mountain and 2 exclaves to the east,“Daluo Mountain”in the northern part of Luo Mountain and the center of Helan Mountain.The duration of forest disturbance was 1-3 years,and mostly within a year.[Conclusions]Among the“Three Mountains”,the disturbance level of Liupan Mountain is much higher than that of Luo and Helan Mountain,but the disturbance area of the“Three Mountains”is decreasing year by year,with the most obvious decrease in Liupan Mountain followed by Helan Mountain and Luo Mountain.The disturbance in Liupan and Luo Mountain is mainly caused by human activities,but in Helan Mountain,it is caused by both human activities and natural conditions.