Seasonal variations in surface reflectance patterns in the boreal region of Alaska the interplay between canopy structure and topography and its implication for surface radiation budget

Forests are critical in regulating the worlds climate and maintaining the overall Earths energy balance. The variability in forest canopy structure, topography, and underneath vegetation background conditions create uncertainty in modeling solar radiation at the Earths surface, particularly for boreal regions in high latitude. We analyzed seasonal variation in visible, near-infrared, and shortwave infrared reflectance with respect to land cover classes, canopy structures, and topography in a boreal region of Alaska. We did a fusion of Landsat 8 image and LiDAR-derived canopy structural data and statistical analysis to accomplish the objective of this study. We found that canopy structure and topography interplay and influence surface reflectance spectra in a complex way, particularly during the snow season. Deciduous trees, also tall with greater rugosity, are dominant in the southern slope than in the northern slope. Taller trees are typically seen in higher elevations regardless of vegetation types. Surface reflectance in visible, near-infrared, and shortwave infrared wavelengths shows similar relationships with canopy cover, height, and rugosity, mainly due to the close connection between these parameters. Visible and near-infrared reflectance decreases with canopy cover, tree height, and rugosity, especially for the evergreen forest. Deciduous forest shows more considerable variability of surface reflectance in all studied wavelengths, particularly in March, mainly due to the mixing effect of snow and vegetation. Topography has significantly impacted the relationship between vegetation structure and surface reflectance in three studied wavelengths. The topographic shadow effect is stronger for deciduous forests than from tree shadowing, particularly for winter seasons. For evergreen forests, surface reflectance is primarily independent of aspects, suggesting that the shadow effects from tree crowns on surface reflectance are more significant, especially during March. The generalized additive models based on non-linear relationships between surface reflectance, canopy structures, and topography confirm such observations. These results provide a great insight into understanding the role of vegetation structure and topography in surface radiation budget.