Forest disturbance monitoring in the core area of Changzhutan urban agglomeration based on Landsat time series data

In order to quickly and accurately monitor the forest disturbance in the core area of the urban agglomeration, this study uses Landsat time series data to monitor the forest disturbance in Google Earth Engine (GEE) using LandTrend algorithm.

Based on the Landsat time series data of the vegetation growth season from 2000 to 2020, under the threshold restrictions of NDVI, NBR, and TCA indices, the LandTrender algorithm was used to process the time series trajectory to obtain information on the occurrence area and scale of forest disturbance in the core area of Changzhutan urban agglomeration. Combining continuous forest resource inventory data and Google Earth historical images, the accuracy evaluation and validation of forest disturbance monitoring results based on different indices were conducted, and the disturbance information was analyzed using forest cover data from land use data in the study area.

From 2000 to 2020, the total disturbed area of the forest in the core area of Changzhutan urban agglomeration was 264.35 km², with an average annual disturbed area of 13.22 km². Among them, the least disturbed area was 1.91 km² in 2002, and the largest disturbed area was 25.52 km² in 2011. The accuracy of forest disturbance information results under the three index thresholds of NBR, NDVI, and TCA under continuous forest resource inventory data was 90.91%, 81.72%, and 65.08%, respectively; In Google Earth’s historical images, the accuracy of random point distribution was 86.00%, 77.88% and 78.36%, respectively. There were significant disturbances in the forest from 2009 to 2011 to 2013. After processing and analyzing land use data nationwide, it could be seen that the forest area gradually decreased over time over the past 20 years, with the percentage of the total area decreasing from 27.10% to around 22.00%.

NBR is the most suitable forest disturbance index for the core area of Changzhutan urban agglomeration. The results of forest disturbance detection are consistent with the results of field survey and visual interpretation of historical images of Google Earth. The distribution boundary of forest disturbance patches can be fully proposed, and the trend of disturbance area change is the same as that of forest area change in national regional land use data, mainly caused by urbanization process, forest fires, and artificial logging. In the past 20 years, the area of forest disturbance has shown a fluctuating trend, the disturbance area fluctuated greatly in 2001, 2011 and 2013.