Impacts of canopy structure on the sub-canopy solar radiation under a deciduous forest based on fisheye photographs

Forest canopy in a deciduous forest has significant sheltering effects on the sub-canopy solar radiation, significantly influencing the energy balance of snow and permafrost beneath the forest and their spatial distribution. This study employs a digital camera mounted with a fisheye lens to acquire photographs at various times in a growth cycle of the forest canopy at three selected sites in a deciduous forest near the Greater Khingan Mountains Forest Ecological Station, Northeast China. The vegetation types and conditions at the selected sites include P1 in Ledum-Claopodium-L. dahurica, P2 in Carex tato-L. dahurica, and P3 in Betula fruticosa-L. dahurica. After necessary image processing, these photographs were used to identify the canopy structure and its impacts on the sub-canopy solar radiation. Results show that fisheye photographs can successfully capture the forest canopy structure and are useful in estimating the sub-canopy solar radiation. The order of sheltering effects from the largest to the smallest on sub-canopy solar radiation at three selected sites is P3, P1, and P2, highly depending on the canopy density. Then sub-canopy solar radiation was calculated using fisheye photographs and an algorithm validated by in-situ observed solar radiation beneath the canopy at P1 and P3. The results are reasonable, although the accuracy seems compromised due to the mismatch of conditions for calculation and observation. Results also show that the mean annual solar radiation above the canopy was about 148.3 W/m² in 2018, and the mean annual solar radiation values beneath the canopy were about 90.0, 123.8, and 61.0 W/m² at P1, P2, and P3, with only 60%, 84%, and 42% of the total solar radiation penetrating through the canopy, respectively. Even in winter, when the trees are leafless, the canopy sheltering effects cannot be ignored in dense forests. Despite the limitations, fisheye photographs and related algorithms are useful in investigating the forest canopy structure and its impacts on sub-canopy solar radiation.