Summary of Condition of the Trees
Data was collected for each of the 550 elm trees
found growing on the National Mall. Thirty-one
potential sites for future plantings were also noted,
although at least a few of these have underground
utility conflicts that could preclude growing trees.
Over 80% of the 550 elm trees were rated as
being "excellent" or "good" in condition based on
the visual assessment of foliage, branching struc-
ture, and insect and disease infestation. The rating
of "poor" condition was given to only 3% (18) of
trees, for which removal and replanting are rec-
ommended. Symptoms of Dutch Elm Disease (DED)
were observed in at least four trees, and four
additional trees may also have been infected.
Summary of Condition of the Soils
Sampling and testing of soils was performed at 49
locations across the Mall in order to study key soil
components. Soil compaction, a physical character-
istic, was found to be the major threat to tree health
and will impact any future plantings if not addressed.
The high bulk density of the soil severely limits root
growth. In over 70% of the soil sampling locations
across the Mall, bulk density exceeded 1.3 g/cm3,
which is above the ideal range for a loamy soil.
Additionally, none of the sample locations had a
usable soil depth of more than 12 inches (30 cm),
and most had much less, resulting in a restricted
useable soil volume that is considered inade-
quate for healthy tree growth. The observed soil
compaction had also altered the soil structure in
nearly 60% of the sampling locations. These sam-
ples showed an aggregate stability below 50%,
which reduces drainage, increases soil density,
and makes those soils susceptible to erosion.
Unlike the issues of soil compaction, the chem-
ical, biological, and other physical properties
of the soil were found suitable for tree growth.
Among the macro – and micronutrients ana-
lyzed, none were deficient. These nutrients can
>>
The Mall elms in contemporary
times. Worn turf grass hints at
the impacts of soil compaction
on the landscape. Photo
National Park Service
www.urban-forestry.com 29
