MONITORING OF ENVIRONMENTAL-CONDITIONS IN TAIGA FORESTS USING ERS-1 SAR

Synthetic-aperture radar images of a forest site near Manley Hot Sprin gs (64-degrees-N, 151-degrees-W), Alaska, were collected between Augus t 1991 and December 1991, day and night, every 3 days, at C-band frequ ency (lambda = 5.7 cm), vertical receive and transmit polarization, by the European Space Agency first Remote Sensing Satellite, ERS-1. Duri ng the same period, air and soil temperatures and dielectric and gravi metric moisture properties of the forest canopy and forest floor were monitored in three forest stands dominated, respectively, by black spr uce (Picea mariana), white spruce (Picea glauca), and balsam poplar (P opulus balsamifera). The calibrated ERS-1 radar backscatter values, si gma-degree, of the forest stands are shown to exhibit a pronounced tem poral pattern, with little separability between tree species. The larg est change in sigma-degree, a 3-dB decrease almost independent of tree species, is observed in early winter when the soil and vegetation fre eze. In the summer, temporal fluctuations in sigma-degree are about 1- 2 dB in magnitude, depending on tree species. Diurnal variations in si gma-degree are as large as 2 dB during fall freeze-up, and less than 1 dB in summer and winter. These temporal variations in radar backscatt er from the forest are interpreted using the MIMICS radar backscatter model and the in situ surface observations as due to changes in the di electric properties of the forest floor and forest canopy induced by p recipitation (summer), drought (fall), and freezing (fall-winter) even ts. In winter, sigma-degree increases across the entire landscape, pro bably because of volume scattering from large depth hoar ice crystals forming in the snow pack.