3-D Imaging of Root Architecture Using Multichannel GPR

Root biomass accounts for about 25% of the carbon storage in mid-latitude forests. Estimation of root biomass for carbon cycling studies requires either direct measurement by excavation of root systems, or remote measurement using ground penetrating radar (GPR) or other geophysical methods. This study evaluated the ability of a 2-GHz multi-channel GPR system (IDS Hi-BrigHT) to detect and map white pine roots in managed forest near Turkey Point, southern Ontario. The GPR system employed eight dual-polarized antenna pairs separated at 10 cm intervals. GPR data were acquired as overlapping swaths (2 cm line spacing, 0.4 cm inline) across a 25-m2 test site (TP74-R) containing a juvenile white pine tree. Radargrams were processed to full 3-D radar volumes for time slicing and interpretation of root architecture and comparison with the excavated root network.
\nRadargram signal processing was successful in suppressing airwave and other background noise and improved the detection of root diffractions on radargrams. The majority of roots were found in the rooting zone at a depth of 5-40 cm. Roots as small as 0.5 cm were detected with the 2-GHz frequency, but many roots <1.5 cm diameter could not be detected as continuous root structures. Root detection was strongly dependent on root orientation; large, coarse roots (>3-5 cm) were imaged as continuous root segments when oriented perpendicular to GPR profiles. Roots intersecting GPR profiles at angles <30-45 degrees were either imaged incompletely or not detected on radargrams. The highest rate of root detection was achieved with horizontally polarized (HH) antennas (dipole axis parallel with the root structures). Isosurface root models constructed from the Hilbert-transformed radargrams allowed mapping of the 3-D dimensional root architecture for large (> 3-5 cm diameter) roots. Isosurface models provide a means for estimating the coarse root volume for large roots and could be employed in future work to monitor temporal changes in root biomass by repeat survey at the same measurement site.
\nRadargram signal processing was successful in suppressing airwave and other background noise and improved the detection of root diffractions on radargrams. The majority of roots were found in the rooting zone at a depth of 5-40 cm. Roots as small as 0.5 cm were detected with the 2-GHz frequency, but many roots <1.5 cm diameter could not be detected as continuous root structures. Many roots were not detected due to dependence of root reflection amplitude on root orientation. Roots oriented at >30-45 degrees to the survey swaths were imaged incompletely or not detected. Most large coarse roots (>5 cm diameter) were mapped as continuous structures when the root orientation was either parallel to, or perpendicular to the GPR transects. The highest rate of root detection was achieved with the horizontally polarized (HH) antennas, with the dipole axis perpendicular to the root structures. Isosurface root models constructed from the Hilbert-transformed radargrams allowed mapping of the 3-D dimensional root architecture for large (> 3-5 cm diameter) roots. The isosurface models provide a means for estimating the coarse volume and belowground biomass but further work is required to improve 3-D image resolution to allow detection of the entire root network. The method could be employed to measure the temporal changes in root biomass by conducting repeat surveys at the same measurement site.
\nRadargram signal processing was successful in suppressing airwave and other background noise and improved the detection of root diffractions on radargrams. The majority of roots were found above a depth of 40 cm with the root zone being detected at a depth of10-15 cm. Roots as small as 0.5 cm were detected with the 2-GHz frequency, but many roots <1.5 cm diameter could not be detected as continuous root structures. Many roots were not detected due to dependence of root reflection amplitude on root orientation. Roots oriented at >30-45 degrees to the survey swaths were imaged incompletely or not detected. Most large coarse roots (>5 cm diameter) were mapped as continuous structures when the root orientation was either parallel to, or perpendicular to the GPR transects. The highest rate of root detection was achieved with the horizontally polarized (HH) antennas, with the dipole axis perpendicular to the root structures. Isosurface root models constructed from the Hilbert-transformed radargrams allowed mapping of the 3-D dimensional root architecture for large (> 3-5 cm diameter) roots. The isosurface models provide a means for estimating the coarse volume and belowground biomass but further work is required to improve 3-D image resolution to allow detection of the entire root network.

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