Table 3 Final sets of image predictors (left side) and model coefficients for respective total fuel density (TFD; kg m−3) and canopy bulk density models (CBD; kg m−3) calibrated using dependent ground data collected in 2015 and 2016 in the Superior National Forest, Minnesota, USA. Model subscript notations 1, 12, and 123 reference the zenith angle range used for canopy gap fraction (CGF) measurements (0 to 7°, 0 to 23°, and 0 to 38°, respectively), while FC represents FuelCalc-derived field estimates of CBD. Dashes (–) indicate image predictors removed during model calibration. Image predictors include synthetic aperture radar (SAR) backscatter amplitudes and ratios, optical sensor bands and ratios, and previously derived estimates of forest basal area (BA) by species or group: Abies balsamea (ABBA), conifer (CON), broadleaf (BRD), Larix laricina (LALA), Picea glauca (PIGL), Picea mariana (PIMA), Pinus resinosa (PIRE), Pinus strobus (PIST), and total basal area (Total). Where necessary, image month is indicated. Combinations of SAR send and receive polarization are indicated with H (horizontal) and V (vertical). Spatially filtered (low-pass) versions of original resolution SAR variables have notations 3 × 3, 5 × 5, and 7 × 7. Optical sensor indices include the normalized difference vegetation index (NDVI), shortwave infrared to near-infrared ratio (SWIR:NIR), and SWIR to visible ratio (SVR). Sensors include Phased Array L-band Synthetic Aperture Radar (PALSAR-1); Sentinel-1 C-band SAR; Sentinel-2 optical MultiSpectral Instrument (MSI); Satellite Pour l’Observation de la Terre (SPOT-5) multispectral (XS) optical sensor; and Landsat-8’s optical Operational Land Imager (OLI)
| Â | TFD1 | TFD12 | TFD123 | CBD1 | CBD12 | CBD123 | CBDFC |
|---|---|---|---|---|---|---|---|
Sentinel-2A (MSI) | |||||||
 MSI 2 | 0.0014 | 0.0104 | – | – | 0.0070 | 0.0085 | – |
 MSI 3 | – | −0.0058 | −0.0083 | – | −0.0041 | – | – |
 MSI 4 | – | – | – | −0.0019 | – | −0.0109 | −0.0044 |
 MSI 8 | – | – | – | 0.0005 | −0.0021 | −0.0018 | – |
 MSI 11 | 0.0000 | – | 0.0055 | – | – | – | – |
 MSI 12 | 0.0000 | – | – | – | 0.0068 | 0.0118 | 0.0084 |
 MSI NDVI | – | – | – | −0.0247 | 0.0585 | – | – |
 MSI SVR | −0.0419 | – | −5.2946 | – | −3.1050 | −5.5613 | −7.2481 |
 MSI SWIR:NIR | – | – | – | – | – | – | 2.9540 |
Palsar-1 (P1) | |||||||
 HH, 5 × 5, Dec | – | – | – | – | 17.7236 | 25.2843 | −12.0514 |
 HH, 7 × 7, Dec | – | – | −12.5204 | −8.0620 | −33.3317 | −47.0995 | – |
 HH, 3 × 3, Nov | – | – | – | −2.8668 | – | – | – |
 HH, 5 × 5, Nov | −0.7608 | – | – | – | −12.8692 | −24.8212 | – |
 HH, 7 × 7, Nov | – | – | – | 3.5551 | 14.6509 | 27.6294 | – |
 HV, Nov | – | – | – | −2.5963 | – | – | – |
 HV, 5 × 5, Nov | – | – | – | – | – | −19.4630 | −48.3685 |
 HV, 7 × 7, Nov | – | – | – | 11.7498 | 15.6451 | 44.7208 | 80.4010 |
Landsat-8 (OLI) Feb | |||||||
 OLI 2 | – | – | – | −0.0002 | – | – | – |
 OLI 3 | – | – | 0.0002 | – | – | – | −0.0032 |
 OLI 5 | – | – | – | −0.0001 | 0.0004 | 0.0013 | 0.0030 |
 OLI 6 | – | – | – | 0.0010 | −0.0007 | −0.0023 | −0.0033 |
 OLI NDVI | – | – | – | – | −0.0265 | −0.0223 | – |
 OLI SVR | – | – | – | −5.9211 | 0.0121 | 0.0053 | −0.0314 |
 OLI SWIR:NIR | – | – | – | – | – | 24.7883 | 50.4194 |
Sentinel-1 (S1) | |||||||
 VH | – | – | – | 0.0066 | 0.0141 | 0.0332 | 0.0259 |
 VH, 3 × 3 | – | 0.0630 | 0.0625 | – | – | – | – |
 VH, 5 × 5 | – | −0.1781 | −0.1152 | −0.0349 | −0.1061 | −0.2172 | −0.1708 |
 VH, 7 × 7 | – | 0.0642 | – | 0.0291 | 0.0645 | 0.0961 | – |
 VV | – | −0.0106 | −0.0251 | −0.0103 | −0.0061 | −0.0142 | −0.0095 |
 VV, 3 × 3 | – | – | 0.0205 | 0.0147 | – | – | – |
 VV, 5 × 5 | – | 0.0452 | – | – | 0.0488 | 0.1091 | 0.0981 |
 VV, 7 × 7 | – | – | 0.0488 | – | −0.0182 | −0.0350 | – |
 VH:VV | – | −0.0034 | −0.0030 | −0.0005 | – | −0.0013 | −0.0028 |
 VH:VV, 5 × 5 | – | 0.0146 | – | 0.0027 | 0.0108 | 0.0278 | 0.0319 |
 VH:VV, 7 × 7 | – | 0.0038 | 0.0185 | – | – | – | – |
S1:P1 (Nov) ratios | |||||||
 VH:HV | −0.0051 | – | – | – | – | – | −0.1553 |
 VH:HV, 3 × 3 | – | – | 0.3947 | – | – | – | – |
 VH:HV, 5× 5 | −0.0051 | – | −0.4542 | −0.0950 | – | – | −0.1553 |
 VH:HV, 7 × 7 | −0.0025 | −0.0706 | – | 0.0770 | – | – | 0.4612 |
 VV:HH, 5 × 5 | – | 1.2505 | – | −0.8196 | −1.6456 | −3.7624 | – |
 VV:HH, 7 × 7 | – | 1.9131 | – | 0.9522 | 2.1599 | 4.6099 | – |
SPOT-5 (XS) | |||||||
 XS 1 | – | – | – | −0.0341 | −0.0851 | −0.1435 | – |
 XS 2 | – | – | 0.1409 | 0.0368 | 0.0488 | 0.1022 | 0.1278 |
 XS 3 | – | −0.0172 | −0.1854 | – | – | −0.0781 | – |
 XS 4 | −0.0028 | – | – | −0.0201 | – | 0.0618 | −0.1030 |
 XS NDVI | – | – | – | – | 0.0367 | 0.0770 | 0.0228 |
 XS SVR | – | −1.2025 | – | −2.5112 | −10.5606 | −17.6685 | 4.6225 |
 SWIR:NIR | – | – | – | 2.1590 | 6.5851 | 9.2176 | – |
BA estimates | |||||||
 ABBA | – | – | −0.6006 | – | 1.8371 | 2.3587 | – |
 CON | – | – | 0.3828 | −0.1695 | −0.3521 | – | – |
 BRD | – | – | – | – | −0.3770 | −0.5943 | – |
 LALA | – | – | 2.8078 | 1.6604 | – | – | – |
 PIGL | 0.4334 | 1.2487 | 1.6620 | 1.9919 | 2.2904 | 2.8725 | −2.5577 |
 PIMA | – | – | −0.5171 | – | 0.2985 | – | – |
 PIRE | – | – | – | 0.4379 | 1.2880 | 1.3795 | −0.9524 |
 PIST | 1.3065 | – | 4.5840 | – | −2.8129 | −4.6389 | −1.1636 |
 Total | – | 0.2282 | – | – | – | – | 1.7470 |