Modelling the radiative transfer in discontinuous canopies of asymmetric crowns. II. Model testing and application in a Norway spruce stand

A spatially explicit 3-D model of crown geometry is combined with a radiative transfer model for heterogeneous media to predict the transmittance of a discontinuous Norway spruce (Picea abies Karst.) canopy under different architectural assumptions. The canopy gap fraction was sampled with a Plant Canopy Analyser (PCA; LAI 2000, LI-COR, Lincoln, NE) at 200 regularly spaced points in the understorey. A comparison of predicted and observed values of diffuse non-interceptance (DIFN) shows that the above model can predict most of the spatial variability of the diffuse fluxes (R2 = 0.97). In order to evaluate the variation in light penetration due to needle clumping in shoots, two architectural hypotheses are tested: random distribution of needles (scenario 1) and of shoots (scenario 2) within crowns. Results indicate that the variability in the radiative fluxes due to shoot architecture is rather limited in comparison with the variability generated by canopy discontinuities. However, assuming shoots as the basic foliage element slightly improves the model performance. The predictive accuracy for the two architectural hypotheses is similar, with a mean absolute percent error of 12.2 and 12.0% for scenarios 1 and 2, respectively. The predictive bias is almost equivalent in magnitude but opposite in sign (mean percent error of −4.82 and 3.55% for scenarios 1 and 2, respectively). The underestimation of the DIFN in the first scenario suggests that, when needles are considered as basic foliage elements, the spatial distribution is not random, but clumped. On the contrary, the overestimation of DIFN in scenario 2 indicates that the spatial distribution of shoots is more regular than random. The angular distribution of the predictive error in the bands of the PCA suggests that the angular distribution of the shoot normal was more ellipsoidal than the spherical one adopted in the simulatio, while leaf clumping in whorls appears to be the source of the high gap fraction at larger zenith angles.