Geoscience Reference
In-Depth Information
and buried organic debris, assess root size, map root distribution, and estimate root biomass (But-
nor et al.,
2001,
2003; Stover et al., 2007). Being non-invasive and non-destructive, GPR allows
repeated measurements that facilitate the study of root system development. Roots as small as 0.5
cm in diameter have been detected at depths of less than 50 cm with a 1500 MHz antenna in well
drained, coarse-textured soils (Butnor et al.,
2003). There has been considerable interest in mapping
tree root systems to understand root architecture and soil volume utilization (Cermak et al.,
2000;
Hruska et al.,
1999; Stokes et al.,
2002). However, without intensively detailed, methodical scanning
of small grids, it is not possible to separate roots by size class or depth (Wielopolski et al.,
2000).
Orientations of roots, geometry of root reflective surfaces, and proximity of other adjacent roots
presently confound attempts to delineate root size classes in forest soils. In addition, this methodol-
ogy cannot determine differences in species within a mixed stand.
We present the results of two case studies conducted in Florida which were aimed at determin-
ing tree root biomass with GPR in a pine plantation and a native scrub-oak shrubland ecosystem to
highlight the technical considerations, successes, and examples of where clutter confounded mean-
ingful interpretation.
30.2 MAteRIAlS And MethodS
The fundamentals of GPR are described in Chapter 7 and practical considerations are emphasized
in Case History 12.7. In the studies presented here, tree roots were the target of interest, and GPR
was applied to detect electrical discontinuities between roots and the surrounding soil. A variety of
frequencies (400 to 1500 MHz) have been used to locate coarse roots; 400 MHz antenna can resolve
discontinuities to a depth of 2 m or more, however the smallest detectable root is 4 to 5 cm in diam-
eter (Butnor et al.,
2001; Hruska et al.,
1999); 900 MHz antennas consistently resolve roots >2 to
3 cm; and under optimal conditions, while 1500 MHz center frequencies can detect roots as small
as 0.5 cm, penetration is usually limited to ~50 cm (Butnor et al.,
2001). Using the aforementioned
frequencies, it is not possible to resolve fine roots that are commonly classified as <0.2 cm diameter.
Because most tree roots are located at relatively shallow soil depths <0.5 m (Hruska et al.,
1999;
Jim, 2003) the high-frequency antenna is a good choice to maximize root detection. A trade-off
exists where low-frequency antennas penetrate deeper in the soil, but provide lower data resolu-
tion, and higher frequencies provide greater resolution with decreased depth penetration. Ground-
coupled antennas need to be in close contact with the soil surface; this is especially important with
higher-frequency antennas (e.g., 1500 MHz) where air-gaps greater than a couple of centimeters will
cause a deleterious loss in resolution. This presents problems in forested terrains where the presence
of herbaceous vegetation, fallen trees limbs, and irregular soil surfaces impedes the travel of an
antenna, requiring additional site preparation before data acquisition (Butnor et al.,
20 01).
The goals of this study were to test the feasibility of using GPR to quantify root mass and root
distribution at two sites that share similar soils but markedly different vegetation types. One site was
a scrub-oak ecosystem located on a subtropical barrier island in the northern part of the Kennedy
Space Center (KSC), Merritt Island, Brevard County, Florida. Several oak species (
Quercus
sp.)
dominate the system because of a prescribed fire ten years earlier, and they represent most of the
aboveground biomass (Stover et al.,
2007). The other was a 5-year-old loblolly pine (
Pinus taeda
)
plantation in northern Florida near Sanderson. Both sites exhibited sandy soils in the near surface,
the KSC being excessively well drained and the Sanderson site being moderately well drained.
In 2005, a SIR-2000 GPR system, manufactured by Geophysical Survey Systems, Inc. (GSSI;
Salem, NH) was fitted with a custom-designed sampling rig that steadied the high-frequency
antenna (model 5100, 1500 MHz antenna) and incorporated a survey wheel to meter electromagnetic
pulses (Figure 30.1) to make measures at Sanderson. Measures at KSC were made with a SIR-3000,
using a model 5100, 1500 MHz antenna fitted with a much smaller survey wheel than depicted in
Figure 30.1. Before spatial distribution can be assessed, it is necessary to sample test transects to
be able to correlate radar data with destructively sampled soil cores. With both systems, measures
