the Mall was never fully realized. While originally
intended to be a landscape composed solely of vase-
shaped American elms, this landscape has historically
always been composed of American elms with diverse
growth habits. Variations in tree habit and form
were noted soon after the initial plantings, as docu-
mented in a 1945 report on the young landscape."
The UHI team provided NPS with specific sche-
matic maps of how visually coherent species
diversity can be achieved over time while still
maintaining the visual coherence of the landscape
as a whole. Half of the secret lies in using trees
that are visually similar in habit to the American
elms, such as 'Accolade' elm (a hybrid of Ulmus
japonica and Ulmus wilsoniana), hardy rubber tree
(Eucommia ulmoides), non-fruiting Kentucky coffee-
tree (Gymnocladus dioicus 'Espresso'), and 'Green
Vase' Zelkova. The other half of the secret is using
trees that have a similar height-to-first-branch.
"Height-to-first-branch is as critical as tree shape
from the perspective of the pedestrian. When
branching begins at a similar height, pedestrians
at the ground level perceive a visually coherent
planting, a sense of uniformity, even though the
species are quite diverse," Bassuk says. Strategic,
gradual crown-lifting ensures that trees of a similar
shape and height also have a similar height to first
branch, which for elms and their analogs will be
at around 12 to 14 feet. This will take a number of
years, depending on the growth rate of the species.
The following diagrams provide a theoretical illustra-
tion of how the tree population of the National Mall
could be diversified over the next 20 years by planting
trees other than American elms. These graphics are
only meant to illustrate a concept; they are not meant
to suggest which individual trees need replacing.
Group A trees are extremely similar to the classic
vase-shaped American elm and are recom-
mended for the highly visible "outer" locations.
This group is composed of other elm species and
hybrids that have the desired growth habit and
are highly resistant to Dutch elm disease and Elm
Yellows. Most people would be unable to dis-
tinguish these trees from Ulmus americana.
Group B includes non-elm trees that are similar
to American elms visually, but less so than the
trees in Group A. These trees are not recom-
mended for the highly visible outer row locations,
where their differences from the American elm
ideal may be noticeable. However, these trees
would be good choices for replacement plant-
ings in the less prominent inner row locations.
"While this theoretical year 20 tree population does
not meet the common urban forestry guideline of
no more than 10% of one species, 20% of one genus,
and 30% of one family, its increased diversity means
that the landscape has increased resilience to dev-
astating pests and pathogens," Bassuk says. "For
example, if elm yellows were to decimate all the
American elms in this theoretical year 20 population,
at least 38% of the trees would be left standing."
Read the
Operations and
Maintenance
Guide here.
Read the Tree and
Soil Conditions
Report here.
36 CityTREES
