Modelling the radiative transfer in discontinuous canopies of asymmetric crowns. I. Model structure and algorithms

Abstract

A versatile model for mechanistic simulation of the radiative transfer in discontinuous canopies is presented. Canopy structure is described with arrays of asymmetric crown envelopes, adaptable to various tree geometries and based on the following parameters: total tree height, height at crown insertion and at the greatest width of the crown, crown radii in four orthogonal directions, and shape coefficients of vertical crown profiles. Unlike previous models, this canopy model can simulate the high level of asymmetry in crown shape and displacement typical of natural and semi-natural forests. Within an individual crown, the vertical distribution of leaf area density (LAD) is modelled using the Beta or Weibull equation. The effect of the spatial pattern of leaf area is taken into account by simulating random, regular or clumped distributions. Parameters related to the canopy architecture (vertical and spatial distribution of the leaf area, angular distribution of the leaf normal) are treated as species-specific, so that mixed forests can be represented. Light penetration is modelled by the ‘turbid medium hypothesis’; that is, by computing the beam path length and the LAD within single crowns with a high angular resolution (0.05–5 deg). Diffuse fluxes generated by reflection and transmission of intercepted radiation are simulated by the Adding method, on the basis of the leaf scattering coefficients of each plant species. Using this model, the distribution of radiation intensity and spectra beneath heterogeneous canopies may be analysed in time and space at the required resolution, and could complement research activities involving remote sensing, plant physiology and ecology. The software has been written in C + + with an object oriented approach, and it does not impose any limit to the number of trees or species in an experimental plot.