In our sample, roots were detected down to
36 inches (91 cm) from the soil surface, but the
greatest root density occurred between 8 to
16 inches (20 to 41 cm) from the surface. Given
that the soil was consistently compacted, we
can only conclude that these roots grew at this
depth soon after the trees were transplanted
when the soil density was temporarily reduced.
It is important to recognize that GPR only detects
roots that are greater than 1 cm in diameter. Roots
detected under the elms represent the older struc-
tural roots, not fine feeder roots. Concentric scans of
roots were measured within a radius of 24 feet (7.3 m)
from the trunk. In most cases, the greatest root den-
sity occurred in the first 8 feet (2.4 m) from the trunk.
Roots were significantly curtailed when they encoun-
tered a paved sidewalk, although the gravel paths
lining the outer paths of the Mall did not seem to
hinder root growth. Interestingly, surface roots
were not detected when scanned with the GPR.
These roots were mostly woody in nature and had
little intact water-filled xylem (plant plumbing),
making them undetectable by GPR. However, their
visual prominence will figure into the management
plan that is to follow in late 2018 or early 2019.
There was no apparent relationship between the
diameter and condition of the tree and the number
of detected roots. It was notable that several large
trees in good condition between 20 to 40 inches
(51 to 102 cm) DBH had the greatest root density;
however, this relationship was not consistent.
The elms present on the Mall today display many growth forms.
Photo by Nina Bassuk
What Next?
The UHI team's management rec-
ommendations for the National
Mall elms are underway in a sep-
arate report to be released in late
2018 or early 2019. That report will
also be excerpted in City Trees.
Read the full
report on the
National Mall
Elms here.
www.urban-forestry.com 31
