MICROWAVE TRANSMISSIVITY OF A FOREST CANOPY - EXPERIMENTS MADE WITH A BEECH

The microwave brightness temperatures over a frequency range from 1 GHz to 100 GHz were monitored below a large beech tree (Fagus silvatica L.) from August 1987 to August 1988, and several observations were added in the following 2 years until the beech was felled. The data were used to compute microwave transmissivities and opacities of the vegetation canopy. The results show that the transmissivities are influenced by wind, temperature, and liquid water, leaves play a dominant role, especially at frequencies above 5 GHz. With increasing frequency, also, the inhomogeneity of branches and leaves becomes more and more pronounced. An interesting result is the anticorrelation found between the microwave opacity of the test beech and a regional forest-damage parameter. These results are based on the full 4-year observation period. Potential applications of microwave radiometric tree observations are the study of the water status in vegetation, the estimation of forest-canopy parameters, and the development and the validation of microwave propagation models.