Modeling light availability and growth of Norway spruce trees (Picea abies, L. Karst) in uneven-aged forests of the French Northern Alps

This paper presents a tree distant dependent growth model for uneven-aged Norway spruce stands in the northern French Alps. The competition between the trees is modeled calculating the available light at 90% and 60% of the height of the trees. For each subject tree, the incident radiation is first spatially divided in elementary beams. The interception of each beam is calculated using a geometrical description of the crown of the surrounding trees and the available (non-intercepted) light is summed up over all beams. This light submodel is validated by comparing predicted light with observations made on hemispherical photographs. Height and diameter increments are predicted from tree age and light availability by empirical functions calibrated on two experimental plots: root mean squared error of individual increments is about 50% of their initial standard deviation. The growth model allows simulation of the development of elementary plots and elaboration of a local silviculture strategy. These dynamics can be synthesized by simple links between representative stand structures. This could be a way to simulate the evolution of a forest analyzed as a mosaic of elementary plots. / Cet article présente la réalisation d'un modèle de croissance pour l'épicéa en peuplement irrégulier. A partir de la carte d'une placette représentative d'un élément de peuplement, le modèle permet de calculer la disponibilité en lumière pour chaque arbre et de prévoir son accroissement annuel. Par itération, il permet de simuler la dynamique de la placette à moyen terme. Le rayonnement lumineux incident est discrétisé en une centaine de rayons élémentaires. La représentation géométrique des houppiers des arbres permet d'évaluer le nombre d'interceptions subit par chaque rayon au cours de sa traversée du peuplement. On peut alors calculer l'intensité résiduelle du rayonnement pour chaque point de calcul