Relationship of MISR RPV parameters and MODIS BRDF shape indicators to surface vegetation patterns in an Australian tropical savanna

The global coverage of bidirectional reflectance distribution function (BRDF) products from the Multi-angle Imaging Spectroradiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) has the potential to provide quantitative information on surface vegetation structure for input to process modelling and model-data assimilation schemes for regional and biome-scale assessment of carbon dynamics. The relationship of MISR Rahman-Pinty-Verstraete (RPV) model parameters, derived from inversion of MISR 275 m fine mode data, and BRDF shape indicators calculated from the latest MODIS 500 m MCD43 BRDF product to vegetation patterns in an Australian tropical savanna was examined for a time series covering the dry season period from April to October 2005. The bidirectional reflectance products were compared with geographical information system (GIS) data coverage combining floristic polygons with Landsat thematic mapper (TM) based estimates of canopy cover and height classes. The analysis showed that both the MISR RPV asymmetry parameter T and several MODIS BRDF shape indicators constructed using the red band were sensitive to local-scale anisotropic scattering and thus vegetation structure. The MISR RPV asymmetry parameter T showed consistent variation between grasslands, forest (closed canopies), and more open tree-grass mixtures over time. The MODIS indicators such as NDHD-R and ANIF-R produced distinctly different temporal profiles for major vegetation types such as rainforest, Melaleuca woodland, and Dichanthium grassland. These indices also showed evidence of consistent discrimination between eucalypt savanna types that varied in canopy cover and tree height. A clumping index calculated from NDHD-R for a single period (day 177 in a time series) showed good correspondence with savanna vegetation canopy properties but was insensitive to dense canopy rainforest vegetation. These results indicate there is potential for both MISR and MODIS BRDF products to provide a quantitative description of vegetation types in global tree-grass systems. However, there is a pressing need for further study to calibrate the responses with fine-scale structural data derived from both field measurement and light detection and ranging (lidar).