Barb Neal (left), Bryan Denig, and Nina Bassuk on the National Mall.

In hot and steamy June of 2017, a team of researchers and arborists from Cornell University’s Urban Horticulture Institute (UHI), headed up by UHI Director Nina Bassuk, worked dawn to dusk evaluating the condition of the American elms and soils on the National Mall in Washington DC. This iconic landscape is often referred to as “America’s Front Lawn,” and the National Mall turf grass was fully renovated between 2010 and 2016, involving infrastructure upgrades, at a cost of $40 million dollars. Now, UHI hopes the Mall trees will get the same level of attention.

Bassuk and then-graduate student Yoshiki Harada worked together on soil evaluation, taking 108 soil samples back to Cornell, while ISA Board Certified Master Arborist Barbara Neal and UHI Visiting Fellow Bryan Denig performed an ISA Level 2 evaluation of the National Mall’s 550 trees. Bassuk and team also used ground penetration radar on a sample of 16 of the trees to find out precisely where the roots are.

A report on the findings of that work was completed in April of 2018 and presented to the National Park Service (NPS). “The Mall’s sandy soils are fine in terms of nutrient availability,” Bassuk says, “but the big issue is soil compaction, which has the cascading effect of destroying soil structure, reducing micro-organism populations, and decreasing aeration. We couldn’t get a penetrometer more than 3 inches in the ground!”

It’s perhaps not surprising, then, that the ground penetrating radar confirmed that the Mall trees’ roots are shallow at best. Indeed, a simple visual assessment shows roots are on the surface and being damaged by mowers. “There’s very little grass under many of the largest trees,” Bassuk says, “but instead of routine mulching, mowers have been running over the roots.” Because the roots are surface to shallow in depth, the UHI team recommends that the NPS not attempt to remediate the soil of existing trees, because that would be too disruptive to the roots. This goes for soil aeration techniques as well as soil amendment.

So how should the NPS approach remediation 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 remediation 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 a dedicated arborist for the trees and soils of the National Mall,” Bassuk says. “However, since our 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 implement 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 recommendations 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 recommendations, 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 otherwise) for this disease. The NPS must diversify the Mall’s canopy, but “how do you start changing an iconic monoculture without reducing the cultural significance of the landscape?” Bassuk asks. That process is detailed later in this article.

There is no treatment for elm yellows/elm phloem necrosis. Photo Pennsylvania Department of Conservation

The twelve overarching recommendations in the Operations and Maintenance Guide are as follows:

  1. Have an on-staff arborist or contract with a consulting arborist.
  2. Use a tree management software system.
  3. Develop and fund a budget so that work can be scheduled and completed in a timely manner.
  4. Follow a management calendar for timely pest scouting and treatment, planning for tree replacement and pruning, and other tree-related tasks.
  5. Conduct regular tree risk assessments of the trees on the National Mall.
  6. Build tree and soil protection into permits for large-scale events.
  7. Consider increasing the tree species diversity on the National Mall.
  8. Undertake a robust annual planting effort to replace dead and declining trees.
  9. Soil compaction mitigation and mulching should be a regular part of tree maintenance at the National Mall.
  10. Consider prophylactic treatment of the “signature” elms with a fungicide for Dutch elm disease.
  11. National Park Service tree care personnel should participate in ongoing technical tree care training.
  12. The National Park Service and its subcontractors should follow industry best practices in all aspects of tree care.

Recommendation 7-Resistance: The Way Forward  

“Consider increasing the tree species diversity on the National Mall”—may seem obvious and basic to city foresters. However, the goal of National Mall landscape sustainability is coming against an attachment to the cultural idea of the American elms. We say “the idea” because in truth, the elm monoculture is not homogeneously healthy and with 3 to 5 trees dying each year, it’s not sustainable.

Furthermore, in the places where NPS has replanted sites with trees other than straight American elm, visual coherence has, despite best intentions, been disrupted. For instance, the elm cultivar ‘Augustine Ascending’ is dramatically different in habit than the straight Ulmus americana and draws attention to itself with this upright habit. “It’s a beautiful tree, but looks nothing like a straight-species American elm,” Bassuk says.

Which one of these is most unlike the others? It’s an attractive American elm cultivar, but ‘Augustine Ascending’ (far left) ascends rather than cascading in a vase shape. Photo by Nina Bassuk

The Operations and Maintenance Guide further explains: “It is important to note that the original design intent of having perfectly matched trees with high canopies creating a gothic arch effect on 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 documented in a 1945 report on the young landscape.”

Comparison of mature tree forms of elm cultivars and other species from the Operations and Maintenance Guide

The UHI team provided NPS with specific schematic 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 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 coffeetree (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,” Bassuk says. 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 illustration of how the tree population of the National Mall could be diversified over the next 20 years by planting trees other than straight-species 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 recommended 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 distinguish 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 recommended 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 plantings in the less prominent inner row locations.

Dark green circles represent the “outer,” highly visible trees. These must look like the classic vase-shaped American elm. Light green circles represent the “inner,” less visible trees. These may deviate slightly from the classic vase-shaped American elm.

When the UHI team inventoried the trees in June 2017, essentially 100% of the trees were American elms. They also recorded approximately 30 potential planting sites (empty spaces in the grid). Planting trees in the 30 open spaces would start adding diversity in the first year: 95% American elm, 3% Group A trees, 2% Group B trees.

After the initial planting to fill the gaps, if we assume that 10 trees are replaced each year, the population slowly continues to diversify. At year 5 the Mall inventory would be 88% American elm, 7% Group A trees, 5% Group B trees.

Continuing to assume that 10 trees are being replaced each year, at year 10 the Mall inventory would be 79% American elm, 11% Group A trees, 9% Group B trees.

Continuing to assume that 10 trees are being replaced each year, at year 20 the Mall inventory would be 62% American elm, 20% Group A trees, 18% Group B trees.

“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 devastating 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.” ♦