Evaluation of Fir Forest Die-Back and Regeneration After a Severe Bark Beetle Disturbance Using UAV-Based Remote Sensing

Understanding how forests recover after severe disturbances is essential for developing effective management strategies that promote stable forest regeneration. Disturbances are particularly significant in transition zones such as treelines, which are highly sensitive to climate change. In the subalpine treeline ecotone of Zao Mountains (northeastern Japan), a severe double-pest infestation devastated the Abies mariesii forest, triggering a treeline retreat of nearly 400 m. Prior to the infestation, the stand density was estimated at 3135 (based on the sum of standing living and dead standing trees and fallen trees detected in the orthomosaics generated for the year 2019). Of these, 3023 were standing trees (of which 2787 were dead). By 2025, the number of standing trees had declined to 2472 (18.2% reduction), a significant development for Abies seedlings, which appeared to establish in decaying fallen logs. In order to evaluate whether this disturbance has permanent or temporary effects, high-resolution unmanned aerial vehicle (UAV) imagery was collected annually over the study area, resulting in six orthomosaics from 2019 to 2025 (2020 data unavailable) for continuous and precise forest monitoring. Analysis of the monitored area revealed that in the 6.9 ha study site, entirely covered by 1.2–1.3 m tall sasa vegetation (Sasa kurilensis), the number of new young trees increased from 60 (2019) to 119 in 2025. These younger trees were mainly located near surviving mature trees. Sasa vegetation did not exert a negative effect on regeneration but instead appeared to function as a strong wind protection, facilitating Abies seedling growth. In conclusion, fallen logs and sasa vegetation appeared to have a positive effect on fir regeneration as suggested by the increasing number of young trees observed over time. The bark beetle outbreak functioned as a forest stand-replacing disturbance, where the subalpine fir forest at the treeline is expected to regenerate naturally within the coming decades.