Scaling PAR absorption from the leaf to landscape level in spatially heterogeneous ecosystems

Realistic and efficient photon transport modeling at the landscape level is vital to understanding material and energy exchange processes that occur between the biosphere and atmosphere. Our knowledge of the magnitude and dynamics of chemical fluxes in spatially heterogeneous areas is constrained by an insufficient understanding of photon transport at scales larger than a single homogeneous canopy. To address this problem, we examine factors controlling the absorption of photosynthetically active radiation (PAR) at the leaf, canopy, and landscape levels. A description of a landscape PAR simulation model based on canopy radiative transfer theory and geometric–optical modeling is provided from a biophysical scaling perspective. Using this model, some of the scaled factors controlling fractional PAR absorption by plants and plant assemblages are discussed. Landscape characteristics influence PAR absorption beyond that which is exerted at the leaf level, but are not divorced from leaf and canopy characteristics. For instance, the spatial distribution of canopies can have a greater effect on PAR absorption than the leaf area index of those canopies, but at the same time, this sensitivity is integrally dependent on canopy leaf area index. Leaf area index has a greater effect on canopy-level PAR absorption than either leaf orientation or leaf optical properties. While some of these relationships can be elucidated through field studies, a comprehensive analysis requires a model that accurately accounts for each interaction based on physical scattering principles.