1. Microwave satellite data from AMSR-E and SMOS were analyzed over three forest sites with different climates to investigate seasonal trends in brightness temperature and microwave indexes. 2. The polarization index at X- and Ku-bands showed sensitivity to leaf biomass cycles while the frequency index between Ku- and Ka-bands responded to snow cover cycles. 3. SMOS emissivity normalized to surface temperature decreased clearly between days 80-120, associated with snow melt increasing soil moisture, though forests masked further soil moisture variation.