Quantitative assessment of the impact of Lymantria dispar larvae on forest-floor light environments: distribution patterns on host tree species and UAV‑LiDAR analysis

Folivorous insect disturbances can substantially alter forest structure, yet their effects on forest-floor light environments remain poorly quantified. We investigated how the distribution patterns of spongy moth (Lymantria dispar) larvae among host tree species and the associated cumulative feeding impact (CFI) influence canopy structure and understory light conditions in a larch plantation containing Larix kaempferi, L. gmelinii, and hybrid larch (L. gmelinii × L. kaempferi). During the 2025 outbreak, larval abundance was surveyed from May to July, and UAV-LiDAR point-cloud data were collected from April to November. Canopy openness was quantified using the laser penetration index (LPI), and a state-space model was used to estimate the effects of CFI on LPI and predict forest-floor relative light intensity. At the study site, larval density differed among tree species, with L. kaempferi supporting higher densities than the other two species. Increased CFI significantly increased LPI in L. kaempferi during August–September and in L. gmelinii during August–November. Correspondingly, forest-floor relative light intensity was estimated to increase by 1.23–1.24-fold in L. kaempferi and by 1.19–1.46-fold in L. gmelinii. This increase in canopy openness increased the proportion of observations with photosynthetic photon flux density exceeding 150 μmol m−2 s−1 from 0.23–7.35% to 7.67–12.96%.