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 backsc atter from short-grass fields of 0.05 kg/m2 biomass was greater-than-o r-equal-to the backscatter from the stands, and there was no significa nt correlation between the backscatter and biomass (r2 = 0.19). Under dry soil conditions, the backscatter increased about 2-3 dB as the bio mass increased from 0.05 kg/m2 to about 0.5-1.5 kg/m2, and the backsca tter may be saturated near a 0.5-1.5 kg/m2 biomass level. The correlat ion coefficient between the backscatter and biomass was r2 = 0.46. Whe n the Santa Barbara microwave canopy backscatter model was applied to simulate the ERS-1 SAR backscatter from the stands over dry ground, mo deled and observed backscatter had similar trends with increasing biom ass. For these stands, sensitivity analyses using the model showed tha t as the surface-soil moisture increased, the major contributor to the total backscatter was changed from canopy volume scattering to surfac e backscatter between 0.4 kg/m2 and about 1 kg/m2. Signal saturating a t low standing biomass and high sensitivity to soil moisture condition s limit the value of a short-wave (C-band) and steep local incidence a ngle (23-degrees) microwave sensor such as the ERS-1 SAR for forest mo nitoring.