In this study we propose a methodology for understory characterization based on the isolation of the forest reflectance that is exclusively due to the background. This is achieved by extracting the contribution of the forest canopy from the overall forest reflectance using forward modeling based on forest reflectance models and existing data on forest structure. Then, the understory can be characterized by applying any methodology developed for open-sky shrub lands, such as vegetation indices or linear mixture models. The developed methodology was successfully applied to simulated data of maritime pine forests with different biophysical stand characteristics and with different understory characteristics.