Windfalls in the Forests of the Southwestern Sakhalin under the Influence of Cyclones

Tropical cyclones operating in the western Pacific Ocean in the summer and autumn periodically go far north, transforming into extratropical ones. In 2014–2015, significant areas of forests were disrupted during the passage of cyclones in the southwestern part of Sakhalin. The aim of the research has been to investigate the scale and nature of damage in forest stands subjected to catastrophic windfalls during the passage of extratropical cyclones over the southwestern regions of Sakhalin Island. Forest damage assessment has been carried out using both ground-based and remote sensing methods. In the sample plots established in various types of forest, the trees have been tallied, the necessary forest inventory characteristics have been established, and then the stocks of the remaining stand and its dead part have been determined. Forest stands have been divided into 3 categories according to the degree of damage caused by windfalls: complete loss of a stand or with a large (above 60 %), average (30–60 %) or insignificant (less than 30 %) proportion of dead trees from their original volumes. The available images from the Sentinel-2 spacecraft, obtained from 2016 to 2017, have been used for the spatial analysis of windfall glades. Satellite images of the study area in a combination of channels 11-8-4 have been classified “with training”. The training sample has included a set of pixels representing the image of reference sites where field geobotanical and forest inventory studies have been performed. As a result of image interpretation, it has been revealed that the total area of windfalls in the Kholmsky and Nevelsky Districts of the southwestern part of Sakhalin Island has amounted to 22,735 ha. For the study area, the normalized difference vegetation index (NDVI) has been calculated and its values have been analyzed to isolate areas with varying degrees of damage. At the same time, it has been established that in the southwestern part of Sakhalin, completely or largely dead forest stands occupy 19.85 % of the area, moderately damaged – 41.86 %, and slightly damaged stands account for 38.29 %.