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.
Radargram 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.
Radargram 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.
Radargram 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. / Thesis / Master of Science (MSc) / Forests play an important role in the global carbon cycle by removing carbon from the Earth’s atmosphere and storing it in tree tissues as biomass. Estimation of the amount of biomass and carbon stored in forests is critical to predictive climate change models, and increasingly employs remote sensing methods to detect both the above ground biomass (e.g. leaves, tree branches) and the belowground carbon in the tree root system. Measurement of the belowground biomass is most difficult, as it cannot be directly observed without destructive excavation of the tree root system.
This study investigated the application of new technology, multi-channel ground penetrating radar (GPR), for mapping tree root systems. The GPR system (IDS Hi-BrigHT) employs ‘swath mapping’ using high frequency pulsed radio waves and multiple transmitting and receiving antennas to produce detailed maps of roots structure. The GPR capabilities were evaluated at a test site at the Turkey Point Flux Station (TPFS) in southern Ontario. The root system of a juvenile white pine tree (20-30 cm diameter) was swath mapped over a 25-m2 area with a line spacing of 2 cm. The GPR data were processed to produce a 3-dimensional radar volume, which can be ‘sliced’ in various orientations to reveal the root structure. The time slice maps show that roots as small as 1-cm can be detected and roots larger than 3 cm in diameter can be mapped as continuous root segments.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22653 |
| Date | January 2018 |
| Creators | Blomfield, Douglas |
| Contributors | Boyce, Joseph, Earth Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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