The composition of understory vegetation plays a central role in satellite-based estimation of biophysical properties of the tree layer in managed boreal forests. In this letter, we assess the contribution of understory vegetation to the reflectance of a managed boreal forest area throughout a growing season both at stand and landscape levels. We use a time series of SPOT and Hyperion, and MODIS satellite images, and a concurrent set of ground reference. Our results show that the contribution of understory to stand or landscape reflectance depends on tree canopy gap fractions and understory reflectance spectra, and their seasonal development. The reflected signal from the understory vegetation can account for more than 40% of boreal forest reflectance at stand level and for 20% at landscape level.