Remote sensing resistance and resilience estimation of a Japanese cedar plantation vs. native broad-leaved trees, in Shuang-lian-Pi forest, after 2015 typhoons.

Typhoons cross Taiwan almost every year. This study quantifies the impact of two typhoons that passed through a forest located in northeastern Taiwan, near Shuang-lian-pi Lake, in August and September 2015.The assessment was conducted using remote sensing techniques, specifically the Normalized Difference Vegetation Index (NDVI) and the NDVI ratio calculated with the formula ((NDVIafter_event - NDVIbefore_event) / NDVIafter_event) * 100, in order to evaluate changes in photosynthetic activity before and after the typhoons. SPOT 6, SPOT 7 and Sentinel-2 satellite images, aerial photographs and field validation were used to carry out the analyses. The NDVI index showed a decrease of 11% following typhoons Soudelor (08/07/2015) and Dujuan (09/28/2015). According to the NDVI index ratio, the forest recovered from these extreme weather events within 20 months. In addition, a comparison between the photosynthetic responses of conifers (Cryptomeria japonica) which were planted over 100 years ago, and those of native broad-leaves trees showed that Japanese cedars experienced less damage and greater recovery than broadleaved trees in response to the two typhoons. Future studies using radar images such as those taken by the sentinel 1 satellite can overcome the difficulties of acquiring cloud-free images before and after typhoons.