Mapping of Forest Understory Using Multi-angular Misr Data for Improvement of Global Leaf Area Index Products

Studies of the bidirectional behavior of forest canopy have shown that the total reflectance of a forest canopy is the combination of illuminated and shaded components of the tree crown as well as the background. The vegetation background (soil/moss/grass/shrub in forests) is a recognized problem that limits the accuracy of satellite-estimated forest LAI. Previous LAI validation efforts have resulted in the recognition that understory cannot be neglected in reflectance modelling [1], especially in the case of low to intermediate canopy covers. Based on the field-tested methodology with airborne data, we estimate the background portion from the bidirectional reflection observed by Multi-angle Imaging SpectroRadiometer (MISR) instrument, which scans the earth in nine different view angles in an oblique plane relative to the sun. For the first time, monthly maps of the reflectivity of the forest background were created and used as an input into the global LAI algorithms [2]. We document the non-constant reflectivity of the forest background with changing canopy LAI, biome type, and the present spatial and temporal patterns. The new LAI maps are compared with other global LAI products. The inclusion of the derived background reflectivity maps into LAI algorithtms led to an improved quality of the LAI product and its agreement with field measurements of LAI and high resolution LAI maps obtained via the ORNL DAAC Mecury system.