Comparison of measured boreal forest characteristics with estimates from TM data and limited ancillary information using reflectance model inversion

Boreal forest cover and tree crown transparency (a function of crown leaf area index [LAI]) were estimated from Landsat TM data and a limited amount of ancillary information by inverting a hybrid geometric–radiative transfer model. Inversion performed better than a number of empirical spectral features tested, including single TM bands and the NDVI. Inversion allowed both cover and transparency variations in the TM data to be simultaneously taken into account, while the presence of variations in both factors impaired the empirical relationships. However, the method could not provide accurate information on a per-stand basis. Consideration of a number of sources of uncertainty in the inversion process highlighted some important limitations in the method; these included the adequacy of the model, noise in the TM data, and errors in the ancillary parameters. The results indicated that a lack of stand-specific ancillary information on the needle and background reflectances was a serious limitation in the utility of the approach. Improved methods for obtaining ancillary information seem necessary. In this context, using existing forest inventory information to help estimate some of the key ancillary parameters is considered a possible way forward.