Seasonal reflectance course of some forest types in Estonia from a series of Landsat TM and SPOT images and via simulation

A time series of Landsat TM and SPOT images over the Jarvselja forest region of southeast Estonia was compiled from a set of 23 images acquired in the years 1986 to 2003 in order to study the seasonal changes in reflectance among different forest types. The images were transformed into ground-level reflectance factor units and 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 also applied. The smoothed seasonal series were compared with model simulations for the same forest types using the Kuusk-Nilson forest reflectance model. The input for the model was formed using the data from the respective forestry database. An expert estimate was used to obtain the values for the biochemical and biophysical parameters lacking in the forestry database and of their change in course in the season. The simulations showed that, as main driving factors in the seasonal course, the course of leaf area index (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. For the studied forest types, the seasonal average normalised difference vegetation index (NDVI) was calculated from the smoothed seasonal courses of reflectance in the red and near infrared bands. The validity of the Monteith hypothesis was tested by establishing a linear relation between the yearly trunk mass increment and seasonal average NDVI.