Using a stand-level model to predict light absorption in stands with vertically and horizontally heterogeneous canopies

Abstract

Background: 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. Methods: 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.

Analysing the spatial and temporal dynamics of species interactions in mixed-species forests and the effects of stand density using the 3-PG model

“…However, even in those stands the extinction coefficient (k) can vary, for a given species, with age, season and site quality (Cannell et al, 1987;Binkley et al, 2013), so care must be taken using this approach because it can result in biased predictions of pa (see Fig. 5b in Forrester et al, 2014). Part of the bias due to horizontal canopy heterogeneity (e.g.…”

“…This variability in k can be accounted for to some extent by replacing k with an extinction coefficient for a homogeneous (not heterogeneous, see Eq. (A.19)) canopy, k H (Duursma and Mäkelä, 2007), and this was applied in the new light absorption sub-model of 3-PG mix (Forrester, 2014b;Forrester et al, 2014). This is described in detail in Appendix A.…”

“…Therefore, pa can also be predicted using a second approach (option L2 in Table 1) that can account for vertically and horizontally heterogeneous canopies e.g., thinned stands, mixed-species stands or uneven-aged stands where there is vertical overlap between the crowns of different cohorts or species, and/or horizontal variability in leaf area due to the mixing of cohorts or species, and gaps in the canopy resulting from disturbances (Forrester, 2014b;Forrester et al, 2014). For this approach pa for the ith species in the jth (vertical) canopy layer ( pa ij ) is predicted using Eq.…”

“…Eq. (A.20) has been tested for a wide range of crown architectures, species, species combinations, stand densities and vertical stand structures (Forrester, 2014b;Forrester et al, 2014).…”

“…It accounts for differences in crown architecture in terms of crown shapes, crown length and width, leaf area, the vertical distribution of leaf-area density, the leaf angle distribution and the optical properties of leaves. The parameterisation of Maestra for these plots is described in detail in Forrester et al (2014), which also calculated the extinction coefficient parameter (k H ) for these species and stands. These data were combined with data from four other experiments including mixtures and monocultures with a wide range of species, species compositions, species proportions and stand densities to develop the light sub-model used in 3-PG mix (see Forrester et al, 2014).…”