So how should the NPS approach remedia-
tion of these highly compacted soils? "They lose
3 to 5 American elms each year to Dutch elm
disease," Bassuk says. "When they replace the
trees, that's an opportunity to do soil remedia-
tion over the area before a new tree is planted."
She recommended using the "Scoop and Dump"
technique, a method which has been validated
over a twelve-year study at Cornell University. A
video about the technique can be seen here.
"We were surprised to learn that the NPS didn't
heretofore have an arborist dedicated to the
trees and soils of the National Mall," Bassuk says.
"However, since the report came out, they've hired
an arborist." That person will have the benefit
of the second document produced by the team
in March 2019, an Operations and Maintenance
Guide for the Trees of the National Mall. "We
included a timeline and calendar so that a skilled
arborist will be able to take this guide and imple-
ment the recommendations," Bassuk says.
The UHI team advised the National Park service to
have a qualified consulting arborist scout the trees
on the National Mall at least three times per year.
The first scouting survey would be done in the
winter in order to plan and schedule late winter
pruning of the trees. The second scouting would
be done in the late spring; the purpose of this visit
is two-fold: to scout for flagging branches that
indicate Dutch elm disease and to make specific
recommendations for tree protection fencing and
tree protection measures to protect those trees
at risk of damage due to public events such as the
Smithsonian Folklife Festival. The final visit would
be in the early fall, when the arborist would assess
those trees identified as higher risk and make rec-
ommendations for pruning, cabling, or removal. The
Cornell team estimates that each visit would require
two to three days on-site and one day for reporting/
creating work orders/managing the database.
The Operations and Maintenance Guide has 12 rec-
ommendations, each with detailed action items and
some with accompanying illustrations. The most
involved section is for recommendation #7, having
to do with increasing species diversity. The American
elms on the National Mall, most of which were
planted in the 1940s, are dying off, and if elm yellows
(also known as elm phloem necrosis) were to fly in
on an insect vector (primarily the whitebanded elm
leafhopper), all of the elm trees could be dead within
a year, as there are no treatments (prophylactic or >>
There is no treatment for elm yellows/elm
phloem necrosis. Photo by Pennsylvania DCNR, Bugwood.org
Watch the video about the "Scoop
and Dump" technique here.
www.urban-forestry.com 33
