关键词: 太阳短波辐射, 激光点云, 计算机模拟模型, 树木三维重建, 林木辐射传输

Abstract:

Objective: Tree crown architecture and the leaf optical properties and spatial distribution in the canopy are closely related to the absorption and interception of solar shortwave radiation. It is important to quantitatively simulate and characterize the solar radiation distribution and interception in the forest canopy, and invert the radiation flux changes of solar shortwave radiation in different forest canopy under spatiotemporal transformation, which would provide a theoretical foundation for forest cultivation practices and forest quality enhancement. Method: Firstly, the public dataset and the terrestrial laser scanned points of trees on campus were used, combined with the designed machine vision algorithm, to separate branches and leaves, and perform individual leaf segmentation from the scanned points. Meanwhile, the elliptical and cylindrical geometric primitives with proper size were employed to approximate the leaves and branches, respectively, and the real model of forest trees was reconstructed. Secondly, the computer graphics methods, combined with the longitude and latitude of the study area and the time of day, were used to simulate the incident solar beams with corresponding solar altitude and azimuth angles. Meanwhile, the bi-directional reflectance and transmittance distribution functions of physics coupled with Monte Carlo ray tracing method were employed to simulate the events of reflected and transmitted solar beams collided with leaf surfaces in the canopy. Finally, based on the average roughness and refractive index of the leaves of different tree species measured by the instrument, combined with ray tracing algorithm, the calculation of the distribution of shortwave radiation within the canopy and the assessment of the canopy light interception efficiency were conducted. Result: The direct, reflected and transmitted solar radiation fluxes for the four tree crowns of different tree species (mango, rubber, crape myrtle, and cherry) and a camphor forest in both spatial and temporal dimensions were calculated. The results showed that the direct radiation flux accounted for roughly 86%, the reflected radiation flux accounted for about 5%, and the transmitted radiation flux accounted for roughly 9%. The crown with higher leaf area index (LAI) intercepted more direct and transmitted light. When the solar altitude angle was small (in the morning or afternoon), the oblique solar beams after being reflected always had a high occurrence probability of collision with other leaves in the tree crown, resulting in an increment in the reflected radiation flux. Meanwhile, due to the phototaxis of forest trees, the interception rate of light by the crown at noon was approximately 10% higher than that in the morning and afternoon. The absolute error between the interception ratio of tree crown radiation flux calculated using this research method and the measured results of handheld photoelectric instruments in the sample site was less than 6%. Conclusion: By using the multidisciplinary perspectives incorporating surveying science, computer graphics and forestry, this study reconstructs the real morphological structure of the target trees from the scanned points. The physical models (the bi-directional reflectance and transmittance distribution functions) and Monte Carlo ray-tracing method are used to simulate the propagation process of solar beams in the tree crown. The field measurements verify the accuracy of the quantitative results regarding the distribution of solar shortwave radiation in the forest canopy and the intercepted canopy radiation flux by our method. The work has great significance for studying solar radiation absorption, investigating the coupling relationship between radiation regime and canopy morphological structure, and evaluating the radiation transfer model within tree canopies with varying phenotypic traits.

Key words: shortwave solar radiation, laser scanned points, computer simulation model, 3D reconstruction of trees, radiation transfer inside a forest canopy