Geoscience Reference
In-Depth Information
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Transect Length (m)
(a)
(b)
fIGURe 30.4
Example of antenna defocusing near the trunk of a loblolly pine caused by thick, dry pine
straw, 20 cm deep on pine root discrimination (a) compared with enhanced discrimination when the pine straw
is raked away (b).
1500 MHz antenna, it created an offset between the antenna and the soil surface which was largely
air filled and degraded the ability to detect roots (Figure 30.4). If litter depth was uniform across
all treatments, the defocusing effect could have been mitigated. However, because the experimental
manipulations of tree genetics and fertilizer application resulted in differential litter depth (5 cm
versus nearly 20 cm of dry newly deposited pine straw), the differences in root mass could not be
assessed.
The Sanderson site was expected to be ideal for radar studies, but several unforeseen compli-
cations negated the quantitative value of the GPR-derived estimates of root mass. The study was
important to highlight potential sources of data clutter in forests. The soil core calibrations at KSC
became very useful for analyzing the effect of elevated atmospheric CO
2
concentrations on below-
ground biomass accumulation in the scrub system. Elevated atmospheric CO
2
concentration has
been experimentally increased at the KSC site using open-top chambers (OTC) since 1996 (Day
et al., 2006). CO
2
enters the chambers through air blowers and circulates throughout the chamber
and exits at the open chamber top. During the ninth year of the CO
2
study, sixteen OTCs (eight
elevated and eight ambient CO
2
) were surveyed with a 1500 MHz antenna. The soil core calibra-
tions at KSC (described earlier) were used to convert GPR index values from the OTC scans to mass
of belowground biomass per unit area g m
2
. Significantly greater coarse root biomass was present in
plots treated with elevated compared to ambient CO
2
(
p
= 0.049) (Stover et al., 2007).
The scrub ecosystem is dominated by large belowground structures such as stems, lignotu-
bers, and burls that serve as carbohydrate reserves that are critical during regrowth following a
disturbance in this fire-controlled system. Approximately 86 percent of total biomass was allocated
belowground in the scrub system, suggesting greater carbon sequestration in larger belowground
structures under elevated CO
2
conditions. Similar to Sanderson, the KSC site was unable to ade-
quately distinguish and separate live and dead root mass within the data.
The spatial distribution of root systems is important when examining their role in resource
acquisition, storage, and structural support. Multiple plots (0.25 m
2
) were intensively scanned (2 cm
scan width) in an intersection grid pattern with a 1500 MHz antenna and later excavated at the KSC
