Résumé
Tree height and vegetation height distribution are structural characteristics of major importance when attempting to understand and monitor biological and ecological processes at tree, stand, and landscape scales. Tree height plays a central role in forest inventories, and height growth is a main driver for growth and yield models. Forest structure features, and in particular gaps, also play a major role in forest ecology because of their influence on microclimate and habitat quality, and thus on biodiversity potential.Accurate measurement of tree heights and careful monitoring of the dynamics of both canopy height and gaps at several scales in space and time is known to be crucial to improve both forest ecosystem monitoring and modeling and thus contribute to their sustainable management. But, as field evaluations of tree height and forest structure are labor-intensive and costly, they cannot fully meet data requirements. In this chapter we focus on two remote sensing approaches: ALS and digital photogrammetry, which are deemed to be mature enough to provide practical and accurate measurements of 3D forest structure.