Abstract The three-dimensional (3D) distribution of light within forest ecosystems is a major driver for species competition, coexistence, forest ecosystem functioning, productivity, and diversity. However, accurate knowledge about the 3D distribution of light within the canopy is difficult to obtain. Recent advances in 3D forest reconstruction as well as the use of radiative transfer modelling provide new insights into spatio-temporal variations of light distribution within a forest canopy. We used high resolution laser scanning data coupled with in-situ leaf optical properties (LOP) measurements to parameterize the DART radiative transfer model for a temperate deciduous forest on the Laegern mountain, Switzerland, and for a tropical rain forest located in the Lambir Hills national park, Borneo, Malaysia. Combining terrestrial and unmanned aerial vehicle (UAV) laser scanning acquisitions allowed a high detailed, 3D reconstruction of forest canopies. We analyse the impact of the two contrasting forest canopies, both in terms of structure as well as optical properties, on the 3D extinction of photosynthetic active radiation (PAR, 400 nm - 700 nm) for a whole diurnal cycle. We show that PAR extinction is mainly driven by the canopy structure, resulting in an exponential light extinction profile for the temperate and a more linear extinction profile in the tropical site. The larger 3D heterogeneity in canopy structure for the tropical site also resulted in larger variability in light extinction throughout the whole canopy. We found that contrasting LOPs between the two forests had a minor influence on light extinction. However, approximating light extinction profiles with layered Beer-Lambert or Big-Leaf models only poorly represented the 3D heterogeneity of light extinction within the canopy, illustrating the need for more detailed 3D modelling of light distribution within forest ecosystems. This can give us important insights into light-related mechanisms driving species coexistence, competition and diversity in complex forest ecosystems.

中文翻译：

---

两个对比鲜明的森林中的三维昼夜消光建模

摘要 森林生态系统内光的三维 (3D) 分布是物种竞争、共存、森林生态系统功能、生产力和多样性的主要驱动力。然而，很难获得关于树冠内光的 3D 分布的准确知识。3D 森林重建的最新进展以及辐射传输模型的使用为森林冠层内光分布的时空变化提供了新的见解。我们使用高分辨率激光扫描数据结合原位叶片光学特性 (LOP) 测量来参数化 DART 辐射传输模型，用于瑞士莱根山温带落叶林和位于兰比尔山国家森林的热带雨林。公园，婆罗洲，马来西亚。结合地面和无人机 (UAV) 激光扫描采集，可以对森林冠层进行高度详细的 3D 重建。我们分析了两个对比鲜明的森林冠层在结构和光学特性方面对整个昼夜循环中光合有效辐射（PAR，400 nm - 700 nm）的 3D 消光的影响。我们表明 PAR 消光主要是由冠层结构驱动的，导致温带呈指数光消光剖面，热带地区呈线性消光剖面。热带地区冠层结构的较大 3D 异质性也导致整个冠层的光消光变化较大。我们发现两片森林之间对比的 LOP 对光消光的影响很小。然而，使用分层 Beer-Lambert 或 Big-Leaf 模型近似光消光剖面仅能很好地代表树冠内光消光的 3D 异质性，说明需要对森林生态系统内的光分布进行更详细的 3D 建模。这可以让我们深入了解在复杂的森林生态系统中驱动物种共存、竞争和多样性的光相关机制。