| The Liupan,Luo and Helan Mountain Nature Reserves in Ningxia are important ecological security barriers in northwest my country.How to timely and accurately obtain the forest changes and their temporal and spatial characteristics of the reserve has important theoretical and practical significance for Ningxia to keep the "Three Mountains" nature reserve and maintain the ecological security of the northwest and even the Yellow River Basin.In this paper,through the Google Earth Engine(GEE)platform,the imaging time is selected as the target year summer(June to September)Landsat remote sensing images,and the water body,snow,cloud and cloud shadow,and non-forest in the time series stack are masked.The composite image of minimum cloud cover in summer of the target year was reconstructed by the median method.Three algorithms,LandTrendr,Bfast and VCT,were used to monitor the forest disturbance recovery in Luoshan in the past 30 years.Combined with existing images,historical data,field survey records,and the visual interpretation,the results of the three monitoring algorithms for forest disturbance were verified.Accuracy,choose the optimal time series trajectory algorithm.Monitor the disturbance and restoration of the "Three Mountains" forest,including:disturbance and restoration year,forest disturbance and restoration amount,forest disturbance and restoration duration,etc make forest disturbance and restoration maps,and analyze the temporal and spatial distribution characteristics of monitoring results;explore possible This paper analyzes the driving mechanism of forest disturbance and restoration in the "Three Mountains" of Ningxia,and then proposes an effective data basis for forest management in the "Three Mountains" area of Ningxia.The result shows:(1)The overall accuracy of LandTrendr algorithm is 93%,and the Kappa coefficient is 0.87;the overall accuracy of VCT algorithm is 93%,and the Kappa coefficient is 0.87;the overall accuracy of Bfast algorithm is 80%,and the Kappa coefficient is 0.6.(2)The ratio of the total area disturbed by forest disturbance in The Three Mountains from 1991 to 2020 was:Liupan Mountain(1.77%)>Luo Mountain(0.07%)>Helan Mountain(0.05%).The annual disturbed area ratio of liupan Mountain 52.60×10-4%(3.65 hm2)>Helan Mountain 1.58×10-4%(0.30 hm2)>Luo Mountain 0.40×10-4%(0.014 hm2).(3)The ratio of total area of forest restoration in Liupan Mountain(1.64%)>Luo Mountain(0.20%)>Helan Mountain(0.06%)from 1991 to 2020.The annual restoration area ratio of Liupan Mountain and Luo Mountain showed a decreasing trend,with Liupan Mountain 59.60 ×10-4%(4.14 hm2)>Luo Mountain 7.78 ×10-5%(0.027 hm2).The yearly restoration area of Helan Mountain showed an increasing trend,with an average annual increase of 67.69 ×10-6%(0.13 hm2).(4)The forest disturbance of "Three Mountains" mainly occurred in two "enclaves" in the north and east of Liupan Mountain,"Daluo Mountain" in the north of Luo Nature Reserve and the middle of Helan Mountain.The duration of forest disturbance in the "Three Mountains" is located between 1 and 3 years.(5)The forest restoration of "Three Mountains" mainly occurred in the north and east of Liupan Mountain,the "Daluo Mountain" zone in the north of Luo Nature Reserve and the central and southern part of Helan Mountain.The duration of forest restoration in the "Three Mountains" is mainly three to nine years.(6)The order of explanation for the influence of natural factors on disturbance area is wind speed(7.7%)>air temperature(1.4%)>soil moisture(1.2%).Among them,wind speed index has significant influence;Soil moisture(10.0%)>wind speed(2.0%)>ground temperature(0.1%).Among them,soil moisture has a suggestive effect.In summary,based on the VCT algorithm and the Bfast algorithm,the LandTrendr algorithm is optimized to achieve timely and accurate information such as forest disturbance year,area,intensity,and disturbance duration.Over the past 30 years,the total disturbed area of the "Three Mountains" in Ningxia is less than the restoration area,the total forest area has increased,and the forest protection has achieved remarkable results.By improving the soil moisture content and reducing the wind speed,the restoration degree of the "Three Mountains" forest can be improved,thereby achieving the overall improvement of the "Three Mountains" ecological environment. |
