Retrieval of above-ground biomass and detection of forest disturbance using SIR-C/X-SAR

A three-step process is presented for estimation of forest biophysical properties from orbital polarimetric SAR data. Simple direct retrieval of total above ground biomass is shown to be ill-posed unless the effects of forest structure are explicitly taken into account. The process involves classification by using SAR data to classify terrain on the basis of structural categories. Next, polarimetric SAR data at L- and C-bands are used to estimate basal area, height and dry crown biomass for forested areas. The estimation algorithms are empirically determined and are specific to each structural class. The last step uses a simple biophysical model to combine the estimates of basal area and height with ancillary information on trunk taper factor and wood density to estimate trunk biomass. Total biomass is estimated as the sum of crown and trunk biomass. The methodology is tested using SIR-C data obtained from the Raco Supersite in Northern Michigan on April 15, 1994. This site is located at the ecotone between the boreal forest and northern temperate forests, and includes forest communities common to both. Estimates of basal area, average height and biomass are highly correlated with in situ observations at 70 forest stands of 4-ha size; linear correlations (r²) are 0.92, 0.59 and 0.95. Biophysical attributes are estimated with relatively small rms errors (over the range examined): (1) 2.4 m for height (h<23 m), (2) 3.5 m²/ha for basal area (b_a<72 m²/ha), (3) 1.1 kg/m² for trunk biomass (B_t<19 kg/m²), (4) 0.5 kg/m² for crown biomass (B_c <6 kg/m²) and (5) 1.4 kg/m² for total aboveground biomass (B<25 kg/m²)