Seasonal Reflectance Course of Some Forest Types in Estonia from Multi-Year Landsat TM and SPOT Images and via Simulation

A time series of Landsat TM and SPOT images over Jarvselja forest region, south east Estonia, was compiled from the set of 23 images acquired in years 1986-2003 to study the seasonal changes in reflectance among different forest types. The images were transformed into ground-level reflectance factor units and the seasonal courses of reflectance were constructed for several forest types in all six reflective spectral bands of Landsat TM. For better comparability between different years, the seasonal time course was presented as a function of temperature (phenology) time. A smoothing of the time series was applied. The smoothed seasonal series were compared with the model simulations for the same forest types by Kuusk-Nilson forest reflectance model. The input for the model was formed using the data from the respective forestry database. An expert guess was used to obtain the values for the biochemical and biophysical parameters lacking in the forestry database and of their change in course of the season. The simulations showed that as main driving factors in the seasonal course, the course of LAI of overstorey and understorey, chlorophyll content in leaves and ground wetness could be pointed out. The course of Sun elevation during the season appeared also to be important in forming the image-based seasonal course. The validity of the Monteith hypothesis was tested by establishing a linear relation between the yearly trunk mass increment and seasonal average NDVI.