Forest canopies are characterized by the composition, organization, and location of aboveground biomass and of open spaces among these components, thus providing a wide range of habitats for other organisms. Canopies are also the interface between biomass and atmosphere, and thus affect the distribution of light, atmospheric turbulence, temperature, and humidity. We used a small footprint scanner with high pulse density for the assessment of the three-dimensional structure of forest canopies and propose a set of simple and interpretable parameters to quantify outer and inner canopy characteristics. This study is based on 1 ha sample plots in Fagus sylvatica L. and Picea abies (L.) Karst. age class and selectively cut forest and on un-managed old growth stands. The spatial resolution of the canopy height model has a large impact on the stability of the parameters. It should be less than 1 m. Laser pulse densities have less impact on parameter stabilities, but do affect the quantification of local conditions when they are less than 5 m-2. The proposed parameters for quantifying canopy structure are (max, min): surface area index (1.2 to 4.1 m2 m-2), canopy surface height (average 4.6 to 25.5 m; maximum 16.7 to 35.8 m), area fraction of north and west facing slopes (north: 0.8 to 20.4%, west: 2.8 to 22.3%), canopy gaps as fraction of surface area (7.5 to 16.1%), crown islands as fraction of surface area (42.9 to 95.9% of surface area), trunk-space height (2.9 to 19.5 m), euphotic zone depth (1.7 to 8.8 m), canopy entropy (2.1 to 3.4 dimensionless) and contrast (3.7 to 225 m2). Only some of these parameters are correlated but the degree of correlation depends on the conditions. The parameters have ecological significance in quantifying the topography of the canopy. There was no consistent pattern of canopy structural parameters related to forest management
