Vegetation Structure Retrieval in Beech and Spruce Forests Using Spectrodirectional Satellite Data

The structure of vegetation canopies largely controls the functioning of ecosystems. There is a substantial demand for spatial information on canopy structure. This paper examines the retrieval of an important forest structure property, leaf area index (LAI) from spectro-directional satellite observations (PROBA/CHRIS) using a forest reflectance model and a look-up table approach. Retrieved parameter estimates are compared to forest structure measured in 15 spruce stands (Picea abies L. Karst.) and 13 beech stands (Fagus sylvatica). For both species, off-nadir looking significantly reduced the normalized error (NRMSE) of forest LAI (spruce: NRMSE = 18.4%; beech: NRMSE = 26.1%) compared to near-nadir data (spruce: NRMSE = 32.6%; beech: NRMSE = 58.8%). At the same time acceptable R2-values were obtained. The best view angle for beech lies in forward direction due to foliar self shading in the canopy. With spruce, the forward direction is less favorable probably due to the very dark spruce leaves and dark shadows present in the canopy; instead the backward direction is more favorable as the canopy is brightly illuminated and shadows are minimal.