Relationships between leaf area index and Landsat TM spectral vegetation indices across three temperate zone sites

Mapping and monitoring of leaf area index (LAI) isimportant for spatially distributed modeling of vegetationproductivity, evapotranspiration, and surface energy bal-ance. Global LAI surfaces will be an early product of theMODIS Land Science Team, and the requirements forLAI validation at selected sites have prompted interest inaccurate LAI mapping at a more local scale. While spec-tral vegetation indices (SVIs) derived from satellite remotesensing have been used to map LAI, vegetation type, andrelated optical properties, and effects of Sun—surface-sensor geometry, background reflectance, and atmosphericquality can limit the strength and generality of empiricalLAI—SVI relationships. In the interest of a preliminary as-sessment of the variability in LAI—SVI relationships acrossvegetation types, we compared Landsat 5 Thematic Map-per imagery from three temperate zone sites with on-siteLAI measurements. The sites differed widely in location,vegetation physiognomy (grass, shrubs, hardwood forest,and conifer forest), and topographic complexity. Compar-isons were made using three different red and near-infra-red-based SVIs (NDVI, SR, SAVI). Several derivations ofthe SVIs were examined, including those based on rawdigital numbers (DN), radiance, top of the atmosphere re-flectance, and atmospherically corrected reflectance. Forone of the sites, which had extreme topographic complexity, additional corrections were made for Sun—surface-sensor geometry. Across all sites, a strong general rela-tionship was preserved, with SVIs increasing up to LAIvalues of 3 to 5. For all but the coniferous forest site,sensitivity of the SVIs was low at LAI values above 5. Inconiferous forests, the SVIs decreased at the highest LAIvalues because of decreasing near-infrared reflectance as-sociated with the complex canopy in these mature to old-growth stands. The cross-site LAI—SVI relationships basedon atmospherically corrected imagery were stronger thanthose based on DN, radiance, or top of atmosphere reflec-tance. Topographic corrections at the conifer site alteredthe SVIs in some cases but had little effect on the LAI—SVIrelationships. Significant effects of vegetation properties onSVIs, which were independent of LAI, were evident. Thevariability between and around the best fit LAI—SVI re-lationships for this dataset suggests that for local accu-racy in development of LAI surfaces it will he desirableto stratify by land cover classes (e.g., physiognomic typeand successional stage) and to vary the SVI. ©ElsevierScience Inc., 1999