Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2006
Publication Date: 9/9/2006
Citation: Chopping, M.J., Su, L., Laliberte, A.S., Rango, A., Peters, D.C., Martonchik, J.V. 2006. Mapping woody plant cover in desert grasslands using canopy reflectance modeling and MISR data. Geophysical Research Letters. 33:L17402, doi:10.129/2006GL027148. Interpretive Summary: Eighty-four percent of current or former grasslands in the United States have experienced shrub invasion. Because of the areas being remote and vast, we need a method to remotely map these shrublands and monitor them with time. 275m resolution Multiangle Imaging SpectroRadiometer data in nine viewing angles from NASA's Terra satellite were used with a simplified geometric-optical model in parts of the Chihuahuan Desert rangelands, particularly the Jornada Experimental Range and the Sevilleta National Wildlife Refuge. Estimates of the woody shrubs showed a high correlation with ground measurements. Outside the shrub area, the relationshop was not good, e.g., in the woodland areas of high elevation, because the geometric-optical model parameters were not derived for trees. Changing parameters in the future would be required. Eventually woody shrubs can be monitored by land resource agencies.
Technical Abstract: A simplified geometric-optical model (SGM) was inverted using red band reflectance data acquired at 275 m in nine viewing angles from the Multiangle Imaging SpectroRadiometer (MISR) flown on NASA’s Terra satellite, to provide estimates of fractional woody plant cover for large areas (over 3519 km2) in parts of the Chihuahuan Desert in New Mexico, USA. The use of the model in these semi-arid environments was enabled by the derivation of a priori estimates of the soil/understory background reflectance response. This was made possible by determining relationships between the kernel weights from a LiSparse-RossThin model adjusted against the same MISR data – together with spectral reflectance data derived from MISR’s nadir-viewing camera – and the parameters of the Walthall model used to represent the background. Spatial distributions of retrieved fractional woody plant cover match those of % tree cover in the global MODIS Vegetation Continuous Fields product but also include shrubs. Good relationships were obtained with fractional shrub cover measured in pastures in the USDA, ARS Jornada Experimental Range but tree cover in higher elevation and riparian zones was dramatically overestimated as a result of the fixing of crown height and shape parameters.