The directional spectral reflectance of deciduous forests changes markedly throughout the year as a consequence of the phenology of the canopy and understorey and changes in the spatial arrangement and density of scene components. This paper presents the results of a 1-year experiment using a tower-mounted spectroradiometer to measure the seasonal changes in the nadir reflectance properties of ash and beech canopies. Seasonal variations in reflectance in red wavelengths were found to be inversely related to percentage cover, whilst variations in near-infrared reflectance were directly related to percentage cover although for both wavelength regions changes in reflectance were not always exactly synchronous with changes in percentage cover. A strong rectilinear relation was observed between NDVI and percentage cover when data from both species were combined. The shape of the seasonal NDVI profile revealed differences between the two species that was ascribed to phenological differences between ash and beech canopies. The position of the wavelength of maximum slope in the region 680-750 nm, was found to shift between two domains depending on the particular association of scene elements presented to the sensor as canopy cover changed.