Methods to assess tropical rain forest canopy structure: an overview

Forest canopy structure (sensu latu) is the combination of forest texture ( the qualitative and quantitative composition of the vegetation as to differ ent morphological elements), and forest structure (sensu strictu, the spati al arrangement of these elements). Scale is an aspect of major importance. At a regional scale forest types can be distinguished, like broadleaf or co niferous forest. At local scale, distribution and size and shape of tree cr owns, and the spatial distribution of leaves and branches within tree crown s determine to a large extent the canopy structure. Which components and su b-components are used, and also the scale at which their spatial arrangemen ts are studied, is of great importance for the possible outcome of the anal ysis of canopy structure. This is specially the case when canopy structure is needed as a correlate to ecological questions, e.g., on habitat specific ity of animals, or epiphytes. Methods available for describing and analysin g canopy structure are discussed. At large scale levels remote sensing data are used to describe differences in structure. High-resolution radar image s are used to describe canopy structure in detail and over large areas. Rep eated measurements over time can be used for monitoring purposes. Ways to m easure the three dimensional structure of (components within) individual tr ees in detail are being developed, and are coupled to physiological models. Currently, use of such methods is only feasible for small plants. Forest t omography (where the vegetation occupation and empty spaces are determined in horizontal and vertical slices of the forest) is proposed as a way to de scribe vertical and horizontal structure. Vegetation cover and occupation i s analysed above grid points in a forest. As an example the vertical struct ure of a Cameroonian forest is described at several levels of detail. The r esearch question asked should govern completely the choice of the parameter s and the methods used for the description of forest canopy structure.