Abstract
Forest ecosystem functioning is strongly influenced by the absorption of photosynthetically active radiation (APAR), and therefore, accurate predictions of APAR are critical for many process-based forest growth models. The Lambert-Beer law can be applied to estimate APAR for simple homogeneous canopies composed of one layer, one species, and no canopy gaps. However, the vertical and horizontal structure of forest canopies is rarely homogeneous. Detailed tree-level models can account for this heterogeneity but these often have high input and computational demands and work on finer temporal and spatial resolutions than required by stand-level growth models. The aim of this study was to test a stand-level light absorption model that can estimate APAR by individual species in mixed-species and multi-layered stands with any degree of canopy openness including open-grown trees to closed canopies. The stand-level model was compared with a detailed tree-level model that has already been tested in mixed-species stands using empirical data. Both models were parameterised for five different forests, including a wide range of species compositions, species proportions, stand densities, crown architectures and canopy structures. The stand-level model performed well in all stands except in the stand where extinction coefficients were unusually variable and it appears unlikely that APAR could be predicted in such stands using (tree- or stand-level) models that do not allow individuals of a given species to have different extinction coefficients, leaf-area density or analogous parameters. This model is parameterised with species-specific information about extinction coefficients and mean crown length, diameter, height and leaf area. It could be used to examine light dynamics in complex canopies and in stand-level growth models.
Highlights
Forest ecosystem functioning is strongly influenced by the absorption of photosynthetically active radiation (APAR), and accurate predictions of APAR are critical for many process-based forest growth models
The absorption of photosynthetically active radiation (APAR) by trees is an important determinant of their growth and accurate estimates of APAR are often critical for process-based growth models
F or APAR was not predicted well in Stand 2 where the crown architectures were more variable and it appears unlikely that fi could be predicted in such stands using tree- or stand-level models that do not allow different trees of the same species to have different kH or other architectural variables such as LAD
Summary
Forest ecosystem functioning is strongly influenced by the absorption of photosynthetically active radiation (APAR), and accurate predictions of APAR are critical for many process-based forest growth models. Tree-level light absorption models have been developed to deal with this canopy heterogeneity and some have been shown to give comparable predictions to field measurements of APAR (Norman and Welles 1983; Oker-Blom et al 1989; Wang and Jarvis 1990; Bartelink 1998; Brunner 1998; Canham et al 1999; Bartelink 1998; Courbaud et al 2003; Gersonde et al 2004; Abraha and Savage 2010; Ligot et al 2014b) Inputs for these models may be the leaf area of each tree, vertical and horizontal leaf area distributions, leaf angle distribution, leaf and soil optical properties, and x and y coordinates to indicate the tree positions. The accuracy of these tree-level models increases with the level of detail used to describe the tree crowns and canopy structure (Brunner 1998; Sinoquet et al 2000; Parveaud et al 2008)
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