Boreal forests exhibit strong seasonal dynamics in their reflectance spectra during the short, snow-free growing period. This short communication paper reports an analysis of the seasonality of boreal forest spectra from the end of snowmelt until the time of maximal leaf area. We apply a forest reflectance model (FRT) to estimate the seasonal contribution of understory vegetation to forest reflectance froma time series of three Earth Observing 1 (EO-1) Hyperion images acquired in May, June and July. The reflectance simulations are based on detailed seasonal series of leaf area index and understory spectra measurements carried out in ten stands at the Hyytiälä Forestry Field Station in Finland. Our results showthat the contribution of understory to boreal forest reflectance is high in the visible domain, but it drops at the red edge and stays relatively low and constant in near infrared (NIR). Throughout the growing season, the contribution of the understory remains approximately the same in the NIR domain, whereas larger changes can be observed in the visible domain.