Quantification of large vertical tree roots with borehole radar

Ground-penetrating radar can be used to detect tree roots provided there is sufficient electromagnetic contrast to separate roots from soil. Forest researchers need root biomass, distribution and architecture data to assess the effects of forest management practices on productivity and resource allocation in trees. Ground-penetrating radar is a non-destructive alternative to laborious excavations that are commonly employed. Tree roots are not ideal subjects for radar studies; clutter from non-target materials can degrade the utility of GPR profiles. On amenable soils, rapid root biomass surveys provide valuable information in a short period time, though some destructive ground-truthing may be required. Surface-based GPR can provide excellent resolution of lateral roots. However, some forest trees have significant allocation to large vertical taproots roots (i.e. loblolly pine, Pinus taeda L., longleaf pine, Pinus palustris Mill.), which cannot be accurately assessed by surface measures. A collaborative project between the USDA Forest Service, Southern Research Station, Radarteam AB and the Swedish Experimental Forest system was undertaken in 2003 to assess the potential of high-frequency borehole radar to detect vertical near surface reflectors (0-2 m). A variety of borehole methods were assessed to identify the most promising technique to image large vertical roots. We used a 1000 mhz transducer (Radarteam tubewave-1000) along with a GSSI ground-penetrating radar unit (Sir-20) to collect reflective data in boreholes adjacent to trees as well as cross-hole travel time measurements. This research was conducted near Vindeln in northern Sweden in August 2003. Six trees (Pinus sylvestris) whose DBH ranged from approximately 20-60 cm were intensively measured to provide information on a variety of size classes. On either side of each tree a 5 cm diameter hole was excavated to a depth of 2 m with a soil auger. One antenna was configured as a transmitter (Tx), the other as a receiver (Rx) and they were lowered into the holes opposite each other. The Tx was operated in single shot mode, where an electromagnetic pulse was propagated and the time it took to penetrate the soil matrix and be detected by the Rx was measured. To allow for tomographic reconstruction of the vertical roots, a series of vectors were created by raising and lowering the antennas at intervals of 5 cm. Then the antennas were moved to opposite holes and the process was repeated creating 3200 unique travel-paths per tree. Borehole to surface measures were collected in a similar fashion, though the Rx was moved across the soil surface (10 cm interval) and the Tx was manipulated below ground (5 cm interval), generating 2400 unique travel-paths per tree. This is the first report of using borehole radar to study vertical tree roots. Cross-hole tomography provided excellent information on the depth of tree roots, but was less useful for imaging near surface features. Borehole to surface measures provided the best information on the near surface, where the bulk of roots are found (0-0.3 m). Cross-hole and borehole to surface data may be combined to further define vertical roots systems. Analysis of root mass and projected root mass is ongoing.