Canada-wide foliage clumping index mapping from multiangular POLDER measurements

In this paper, vegetation canopy structural information is retrieved over Canada from multiangular Advanced Earth Observing Satellite (ADEOS-1) polarization and directionality of the earth's reflectance (POLDER) data based on canopy radiative transfer simulations using the Five-Scale model. The retrieval methodology makes use of the angular signature of the reflectance at the hot spot, where the sun and view angles coincide, and at the dark spot, where the reflectance is at its minimum. The POLDER data show that the normalized difference hot spot dark spot (NDHD) constructed from the hot spot and dark spot reflectances has no correlation with the nadir-normalized normalized difference vegetation index (NDVI), from which vegetation properties are often inferred, indicating that this angular index has additional information. Five-Scale simulations are used to assess the effects of foliage distribution on this angular index for different crown sizes, spatial distribution of crowns and foliage inside crowns, and foliage density variations. The simulations show that the NDHD is related to canopy structure quantified using a clumping index. This latter relationship is further exploited to derive a Canada-wide clumping index map at 7 km by 7 km resolution using spaceborne POLDER data. This map provides a critical new source of information for advanced modelling of radiation interaction with vegetation and energy and mass (water and carbon) exchanges between the surface and the atmosphere.