Modeling sub-boreal forest canopy bulk density in Minnesota, USA, using synthetic aperture radar and optical satellite sensor data

Fire Ecology

Table 4 The number (n) and relative proportion (% of total) of satellite predictors (optical, estimates of basal area [BA], and synthetic aperture radar [SAR]) retained by each canopy bulk density (CBD) and total fuel density (TFD) model during calibration in this Minnesota, USA, study (2015 to 2016) to quantify coniferous forest fuels. Subscript notations 1, 12, and 123 reference the zenith angle range of canopy gap fraction measurements (0 to 7°, 0 to 23°, and 0 to 38°, respectively) used to estimate field fuel density, while FC represents FuelCalc-derived estimates of CBD. A further breakdown of the SAR predictors into C-band, L-band, and C to L ratios is shown in italics. Dashes (–) indicate groups of SAR predictors not used during calibration for two fuel density models

Satellite variable	TFD₁		TFD₁₂		TFD₁₂₃		CBD₁		CBD₁₂		CBD₁₂₃		CBD_FC
Satellite variable	n	%	n	%	n	%	n	%	n	%	n	%	n	%
Optical	5	45.5	4	23.5	6	27.3	12	37.5	15	41.7	17	44.7	13	44.8
BA	2	18.2	2	11.8	6	27.3	4	12.5	7	19.4	5	13.2	4	13.8
SAR	4	36.4	11	64.7	10	45.5	16	50.0	14	38.9	16	42.1	12	41.4
C-band	–	–	8	47.1	7	31.8	7	21.9	7	19.4	8	21.1	6	20.7
L-band	1	9.1	–	–	1	4.5	5	15.6	5	13.9	6	15.8	3	10.3
C to L ratio	3	27.3	3	17.6	2	9.1	4	12.5	2	5.6	2	5.3	3	10.3
Total	11		17		22		32		36		38		29