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 different morphological elements), and forest structure (sensu strictu, the spatial arrangement of these elements). Scale is an aspect of major importance. At a regional scale forest types can be distinguished, like broadleaf or coniferous forest. At local scale, distribution and size and shape of tree crowns, and the spatial distribution of leaves and branches within tree crowns determine to a large extent the canopy structure. Which components and sub-components are used, and also the scale at which their spatial arrangements are studied, is of great importance for the possible outcome of the analysis of canopy structure. This is specially the case when canopy structure is needed as a correlate to ecological questions, e.g., on habitat specificity of animals, or epiphytes. Methods available for describing and analysing canopy structure are discussed. At large scale levels remote sensing data are used to describe differences in structure. High-resolution radar images are used to describe canopy structure in detail and over large areas. Repeated measurements over time can be used for monitoring purposes. Ways to measure the three dimensional structure of (components within) individual trees in detail are being developed, and are coupled to physiological models. Currently, use of such methods is only feasible for small plants. Forest tomography (where the vegetation occupation and empty spaces are determined in horizontal and vertical slices of the forest) is proposed as a way to describe vertical and horizontal structure. Vegetation cover and occupation is analysed above grid points in a forest. As an example the vertical structure of a Cameroonian forest is described at several levels of detail. The research question asked should govern completely the choice of the parameters and the methods used for the description of forest canopy structure