THE EFFECTS OF CHANGES IN LOBLOLLY-PINE BIOMASS AND SOIL-MOISTURE ON ERS-1 SAR BACKSCATTER

For young (< 15 years old) loblolly pine stands at Duke Forest (North Carolina, USA), when the ground was wet, the observed ERS-1 SAR backscatter from short-grass fields of 0.05 kg/m2 biomass was greater-than-or-equal-to the backscatter from the stands, and there was no significant correlation between the backscatter and biomass (r2 = 0.19). Under dry soil conditions, the backscatter increased about 2-3 dB as the biomass increased from 0.05 kg/m2 to about 0.5-1.5 kg/m2, and the backscatter may be saturated near a 0.5-1.5 kg/m2 biomass level. The correlation coefficient between the backscatter and biomass was r2 = 0.46. When the Santa Barbara microwave canopy backscatter model was applied to simulate the ERS-1 SAR backscatter from the stands over dry ground, modeled and observed backscatter had similar trends with increasing biomass. For these stands, sensitivity analyses using the model showed that as the surface-soil moisture increased, the major contributor to the total backscatter was changed from canopy volume scattering to surface backscatter between 0.4 kg/m2 and about 1 kg/m2. Signal saturating at low standing biomass and high sensitivity to soil moisture conditions limit the value of a short-wave (C-band) and steep local incidence angle (23-degrees) microwave sensor such as the ERS-1 SAR for forest monitoring.