Cornell UHI team Barbara Neal, Bryan Denig, and Nina Bassuk assess the health of one of the iconic elms ringing the National Mall. Photo by Yoshiki Harada

In April 2018, the Cornell Urban Horticulture Institute team of Nina Bassuk, Bryan Denig, Yoshiki Harada, and Barbara Neal released an extensive report on the elms (including American elms) of the National Mall in Washington, D.C. The report details a study conducted at the request of the National Park Service to examine the current conditions of the trees and soils on the tree panels of the National Mall, and it includes a fascinating history of the landscape. Here are some highlights from the report.

The National Mall elm trees are an important planting in the monumental core, yet they face several challenges. The soils of the tree panels are very compacted, most likely due to the constant pedestrian use and the numerous large organized events that take place on the Mall. Unevenness in the size and distribution of the tree canopy has resulted from decades of mortality (often due to Dutch Elm Disease) and the planting of certain elm varieties with growth forms that are seen as incompatible with the planting as a whole.

From June 17–20, 2017, the research team conducted a tree inventory and collected soil data and samples for later analysis. In addition, in November 2017, ground penetrating radar done by Council member Gary Raffel was used to document root growth for seventeen of the trees. This report deals with the current tree and soil conditions, while management recommendations are in a separate report to be released in late 2018 or early 2019. 

The Mall elms in contemporary times. Worn turf grass hints at the impacts of soil compaction on the landscape. Photo National Park Service

The National Mall circa 1936: newly planted American elms can be seen in panels at lower left and lower right of photo. Photo in public domain

Study Objectives

  • To examine each elm tree on the National Mall for health and risk assessment according to the International Society of Arboriculture Level 2 protocol.
  • To sample soils associated with the elms and analyze their physical, chemical, and biological characteristics
  • To summarize opportunities and challenges to elm management based on tree assessment and soil analysis

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 structure, and insect and disease infestation. The rating of “poor” condition was given to only 3% (18) of trees, for which removal and replanting are recommended. 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 characteristic, 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 inadequate for healthy tree growth. The observed soil compaction had also altered the soil structure in nearly 60% of the sampling locations. These samples 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 chemical, biological, and other physical properties of the soil were found suitable for tree growth. Among the macro- and micronutrients analyzed, none were deficient. These nutrients can be available to plants at adequate soil moisture levels because the soil pH was within the range between 6.0 and 7.3. It is relatively easy to maintain adequate soil moisture levels in the sampled areas because plant available water was generally above 20% of gravimetric (dry) soil water content.

Also, active carbon content, organic matter, and soil respiration rates generally received “fair” or “good” ratings, which are likely to indicate adequate nutrient cycling and sufficient pools of nutrients in the soils at the Mall. The less-than-excellent ratings for active carbon (the fraction of organic matter used by soil microorganisms) and soil respiration are likely exacerbated by soil compaction. All these soil properties are adequate yet have been underutilized by the trees due to the high levels of soil compaction across the Mall.

The elms present on the Mall today display many growth forms. Photo by Nina Bassuk

Species Distribution

Maintaining a diverse species composition is a good way to maintain a healthy, resilient tree population. A general guideline in urban forestry suggests that an urban tree population should include no more than 10% of any one species, 20% of any one genus, or 30% of any one family. While it is debatable if this guideline is appropriate for a monumental landscape such as the National Mall, it underscores the importance of tree diversity to reduce the risk of catastrophic loss.

All 550 trees inventoried on the National Mall are elm trees. This number excludes the five bald cypress trees (Taxodium distichum) found growing in front of the National Museum of Natural History, as this study is focused specifically on the elm planting. It is a monoculture landscape, and the vast majority of the trees are just one species, the American elm (Ulmus americana). While it is possible that a handful of trees are Ulmus × hollandica or newer hybrid elms developed for their DED resistance, U. americana dominates. Younger trees tend to consist of American elm cultivars planted because of their resistance to DED, especially ‘Princeton’ and ‘Jefferson’. Another prominent cultivar on the mall is ‘Augustine Ascending’, which stands out visually from the other trees due to its markedly upright form (photo below).

The tall, narrow form of the cultivar ‘Augustine Ascending’ makes it easy to spot among the other elms. Photo by Nina Bassuk

Ground Penetrating Radar Sampling of Elm Roots

In November of 2017, 17 elm trees in twelve panels were selected for deeper, more intense investigation by Council member Gary Raffel of Dynamic Tree Systems using Ground Penetrating Radar (GPR). These represented some of the oldest and largest trees at the east end of the Mall with the greatest observed soil disturbance as well as trees from the west end, which has the best turf and showed the least observed disturbance.

GPR has been shown to successfully locate tree roots non-invasively and in three dimensions in forest soils. Although this technology has a long history in archaeology and engineering to locate antiquities and utilities, the practice of using it to map roots in urban soils, which can be compacted, layered and discontinuous, is comparatively new.

Root detection is possible in principle because of the moisture content within the woody root that provides an excellent contrast with the surrounding soil. Although roots are detectable, it may only be possible to estimate a bulk property of the roots’ structure, such as root frequency at different distances from the trunk and different depths in the soil.

In this 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, one 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 structural 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 density occurred in the first 8 feet (2.4 m) from the trunk.

Roots were significantly curtailed when they encountered 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; that report will be shared here on the blog.

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.

Read the full report on the National Mall Elms here.