Using Airborne Laser Scanning and Sentinel‐2 to Understand Subcanopy Light Regimes and Understory Diversity of Vascular Plants in Temperate Mountain Forests

Canopy cover modulates the subcanopy light regime in forests—variations in canopy cover alter both, the amount and spatial distribution of light, representing the source of energy for plants. Because plant species differ regarding their adaptations to light, canopy‐driven light regimes play a key role in structuring understory composition and diversity. Here, we evaluated if Airborne Laser Scanning (ALS) and Sentinel‐2 can map subcanopy light regimes across three study sites ( n = 244 plot locations) along the German Alps. We then examined how understory vascular plant species richness varies with subcanopy light regimes. We found light availability increases linearly (Adj. R 2 0.84/RMSE 0.09) with ALS‐derived gap proportion (i.e., inverse of canopy cover), whereas light variability (Adj. R 2 0.46/RMSE 0.05) follows a hump‐shaped pattern. The relationship between the subcanopy light regime and vascular plant species richness in the understory (Adj. R 2 0.39/RMSE 12.77), was equally well described by ALS‐derived gap proportion (Adj. R 2 0.40/RMSE 12.70). Canopy cover described by NDWI from Sentinel‐2 showed similar relationships and could be used to characterize light availability (Adj. R 2 0.43/RMSE 0.18), light variability (Adj. R 2 0.21/RMSE 0.06), and understory vascular plant species richness (Adj. R 2 0.31/RMSE 13.70), albeit with lower variance explained. Understory vascular plant species richness increased with higher light availability and variability, indicating that canopy cover heterogeneity is an important driver of biodiversity next to overall canopy cover. Using spaceborne sensor systems might enable repeatable, large‐scale mapping of canopy cover and thus also understory diversity, as our results suggest such sensors provide similar insights as gold‐standard ALS data, which remain scarce. Understanding the links between canopy cover, the subcanopy light regime, and understory diversity is therefore essential for predicting vegetation dynamics and supporting biodiversity‐oriented forest management.