Hyperspectral reflectance of boreo-nemoral forests in a dry and normal summer

The visible-near infrared (VNIR) reflectance of several mature boreo-nemoral stands in Estonia was measured in two concurrent years which differed significantly in precipitation. Custom-designed VNIR spectrometer systems, UAVSpec and UAVSpec2, mounted onboard a helicopter, were used for the measurements. We show that broadleaved and coniferous forests react to changes in water supply differently: the relative change of the NIR reflectance was equally about 10-15% in all stands, while the change of visible reflectance of coniferous stands exceeded that of broadleaved stands. The smallest change was observed for broadleaved stands in the red reflectance, i.e. chlorophyll absorption bands. Based on our results, we conclude that the change of stand reflectance caused by weather conditions, such as drought, should be taken into account in the estimation of the optical properties of the atmosphere, needed in the atmospheric correction of optical satellite images. A widely used correction method directly benefiting from our results for boreo-nemoral forests is the dark object method.