Synthetic-aperture radar images of a forest site near Manley Hot Springs (64°N, 151°W), Alaska, were collected between August 1991 and December 1991, day and night, every 3 days, at C-band frequency (λ = 5.7 cm), vertical receive and transmit polarization, by the European Space Agency first Remote Sensing Satellite, ERS-1. During the same period, air and soil temperatures and dielectric and gravimetric moisture properties of the forest canopy and forest floor were monitored in three forest stands dominated, respectively, by black spruce (Picea mariana), white spruce (Picea glauca), and balsam poplar (Populus balsamifera). The calibrated ERS-1 radar backscatter values, σ°, of the forest stands are shown to exhibit a pronounced temporal pattern, with little separability between tree species. The largest change in σ°, α 3-dB decrease almost independent of tree species, is observed in early winter when the soil and vegetation freeze. In the summer, temporal fluctuations in σ° are about 1-2 dB in magnitude, depending on tree species. Diurnal variations in σ° are as large as 2 dB during fall freeze-up, and less than 1 dB in summer and winter. These temporal variations in radar backscatter from the forest are interpreted using the MIMICS radar backscatter model and the in situ surface observations as due to changes in the dielectric properties of the forest floor and forest canopy induced by precipitation (summer), drought (fall), and freezing (fall-winter) events. In winter, σ° increases across the entire landscape, probably because of volume scattering from large depth hoar ice crystals forming in the snow pack. © 1994.