Modeling the view-angle dependence of the gap fraction in subtropical forests by using terrestrial laser scanning

Determining how the view angle relates to the directional gap fraction (DGF) of natural forests with varying canopy structure and terrain is beneficial for assessing the radiation transfer and understory growth. However, it remains challenging to determine how DGF depends on the view angle due to the limitation to obtain the DGF at arbitrary view angles. This study verifies a point number-based method based on terrestrial laser scanning (TLS) data to estimate the DGF at arbitrary view angles for subtropical forests on either sloped or flat terrain. We then explore how the vertical DGF in the non-nadir direction relates to that in the nadir direction, and analyze the effects of forest canopy structure and terrain on the relationship. Finally, we quantify the dependence of DGF on both view zenith angle (VZA) and view azimuthal angle (VAA). The results show that the TLS-based method captures over 67% of the variations (root mean square error & LE;0.12) of the optically measured DGF. The vertical DGF in the non-nadir direction is related by a power function model to that in the nadir direction. Sloped terrain weakens the dependence of DGF on the VZA especially when VZA is large than 60 in this study, and the low forest density, low average tree height, and small crown diameter strengthen the dependence of DGF on the VZA. The DGF depends on the VZA more than the VAA for forests on flat terrain. However, the VAA also plays a key role in DGF estimation for forests on sloped terrain. This study proves that TLS is a useful tool to acquire reference data of DGF which can help to understand aerial and satellite estimation analyses in subtropical forested areas.