Structural complexity is an important forest characteristic for habitat assessment, forest management
and planning. However, monitoring how forest structural complexity evolves over
time in various forest types has not been widely explored. In this study we investigate the
feasibility of bi-temporal low-density airborne laser scanning (ALS) for the assessment of
changes in light availability conditions within forest canopy, considered to imply changes in
forest structural complexity. We used ALS data acquired in 2012 and 2019 to generate canopy
vertical profiles by slicing the point clouds into 4 × 4 ×1 m voxels which were then rasterized
and reclassified into four light availability categories. To understand structural development
over time in different forest types we used field-measured tree heights and tree species
information to stratify sample plots in different stand complexity categories. Stands with higher
structural complexity represented increased proportions of space occupied by vegetation as
well as decreased proportions of empty space below the canopy. The experiments showed the
ability of low-density ALS to characterize the dynamics in canopy layering structure, implying
changes in forest structural complexity. The presented methodology could potentially be
upscaled and applied in the landscape-level monitoring of the development of boreal forest
structural characteristics.