Under-canopy UAV Solutions for Forest Inventory – Challenges and Opportunities

Abstract. Forest inventory underpins every facet of ecosystem management and monitoring by providing accurate, spatially explicit data on stand structure, species composition, and site conditions. Yet traditional inventories are frequently constrained by logistical challenges, financial limitations, methodological inconsistencies, and institutional hurdles that undermined the accuracy, completeness, and timeliness of these essential datasets. Over the past two decades, close-range sensing technologies have markedly reduced manual field effort while enhancing the digitization and automation of plot-level measurements. However, these systems remain reliant on human operators for deployment, limiting their ability to fully overcome logistical and technical constraints. Recent advances in under-canopy unmanned aerial vehicles (UAVs) have begun to address these limitations by integrating lightweight, UAV-borne LiDAR and photogrammetric sensors capable of semi-autonomous or autonomous flights beneath dense canopy cover. Such platforms extend the reach of close-range sensing into previously inaccessible forest interiors, enabling rapid, repeatable acquisition of tree- and stand-level metrics without the need for extensive ground crews. In this review, we dissect the technical architectures, sensor configurations, and performance metrics of emerging under-canopy UAV systems for forest inventory. We further identify the principal engineering and operational challenges to guide future research directions and accelerate the adoption of UAV-based forest monitoring solutions.