The effects of hierarchical levels of needle clumping on the canopy transmittance of a conifer stand are examined using a 3D radiative transfer model. Canopy architecture in an experimental plot is described by the tree spatial distribution, crown shape, shoot geometry and needle morphology. Various assumptions about canopy structure (homogeneous or discontinuous; measured or random tree distribution) and basic foliage elements (needles or shoots) are tested. The vertical profiles of unintercepted direct and diffuse radiation, and the spatial variability of the fluxes within and between tree crowns are examined. In the case of a homogeneous canopy, most of the incoming radiation would appear to be absorbed when leaf area index (LAI) reaches a value of 5, while leaf clumping in crowns increases the average canopy transmittance at the base of the canopy (LAI 7.84) up to 4.9 % for direct and up to 10.9 % for diffuse radiation. The effect of needle clumping in shoots on light penetration rapidly decreases if needle clumping in crowns is also assumed. The impact of needle clumping on the indirect LAI estimates obtained by a LI-COR LAI 2000 plant canopy analyser is quantified by simulating the device within the modelled tree canopies. Needle clumping in crowns induces an LAI underestimation of 54 % if the observed tree distribution is assumed, and this increases to 61 % in the case of a random distribution. In a homogeneous canopy, needle clumping in shoots induces an LAI underestimation of 36 %, while in discontinuous canopies the negative bias is only 4 %. ((C) Inra/Elsevier, Paris.).
