National Mall and Memorial Parks Wood-Chip Project

The weight of the cherry blossoms caused a limb to break and fall into the Tidal Basin last spring. “This photo was to document the crowds walking under broken portions until we arrived. A proper work zone was immediately set up to protect pedestrian traffic. The wood chips are from our work the year before,” says Matthew Morrison. Photos courtesy of Matthew Morrison.

A team from the National Park Service, under the leadership of Matthew Morrison, an arborist for the National Park Service who is responsible for managing the trees of the National Mall and Memorial Parks in Washington, D.C., recently completed a project of replacing the sod with a bed of wood chips for the entire urban forest on the grounds of the Washington Monument, including the famous cherry trees.

Matthew Morrison

The project was aimed at addressing multiple issues the trees were having that Morrison, an ISA Certified Arborist, ISA qualified tree risk assessor (TRAQ) and Maryland DNR Roadside Tree Care Expert, attributed to years of the sod and soil being compacted by an estimated 36 million visitors every year.

“Urban forestry is associated with the care of individual and populations of trees in the urban environment. Our groves of trees in such proximity to one another, such as on the Washington Monument grounds, around the tidal basin, on the National Mall or flanking the sides of the Reflecting Pool, are true urban forests,” says Morrison. “The canopies overlap one another, tree roots have grafted with neighboring root systems, there is a consortium of mycorrhizae fungi in the soil that the trees access to communicate with surrounding trees and they respond physiologically to cues from their neighbors. There is a hormonal response within each tree influencing its cooperation and competition, all in the name of survival.

“The urban forester manages individual trees and tree populations in the urban environment,” continues Morrison. “In our case, here at the National Mall and Memorial Parks (NAMA), the urban forester has an opportunity to both develop and manage urban forests in Washington, D.C. This challenging undertaking makes much of NAMA’s urban forester’s daily responsibilities unique in the world of urban forestry and the federal government.

“Observe one of our groves of closely related trees, like the grove of mature cherry trees on the Washington Monument grounds shown in the photo (above). Now observe the grove in a multitude of lateral horizons. Consider this: the grove’s collective canopy as one horizon and the ground-cover vegetation as another horizon.

Workers begin spreading wood chips in one of the groves of mature cherry trees on the Washington Monument grounds. Consider its multiple lateral horizons.

“The third horizon is the vast, collective root system of the trees of the population, and another would be the horizon of mycorrhizae fungi and other microbes that comprise the forest soil microbiota,” says Morrison. “What has been missing from the NAMA urban forests for more than 100 years, in most cases, is the horizon of forest litter.”

Forest litter results from natural debris from trees in the form of leaves, bark, flowers, fruit, branches and trunk wood that would remain on the forest floor, according to Morrison. The biodegradation of these materials amends and insulates the soil, creating a water-holding, nutrient-rich environment.

Crews introduced nearly 300 cubic yards of wood chips to this grove of mature cherry trees on July 19, 2021.

“The introduction of wood chips to the grove, beneath the canopy, at an average depth of 6 inches, acts as a substitute for the forest’s otherwise-absent horizon of necessary litter, facilitating biodegradation and soil amendment.

“Moisture is trapped in this wood-chip layer, aiding fungi that decompose the chip’s lignin and cellulose. This natural process is beneficial to the surrounding ecosystem, including insects, pollinators, soil microbes and, in turn, the trees,” says Morrison.

Moisture is trapped in this wood-chip layer, aiding fungi that decompose the chip’s lignin and cellulose. This natural process is beneficial to the surrounding ecosystem, including insects, pollinators, soil microbes and, in turn, the trees.

According to Morrison, some of the benefits of the introduced wood chips include, but are not limited to:

  • Providing a satisfying aesthetic. “A positive byproduct of the mission.”
  • The mycorrhizal fungi thriving in the rejuvenated soil, which perpetuates the cycle of information and nutrients being transported and exchanged among the trees. “This is an essential element of the forest.”
  • Reducing carbon footprint by utilizing the in-house tree crew’s daily generated green waste, negating the necessity of its removal from the park and subsequent disposal. “This is contributing to complying with Executive Order 13693 Planning for Federal Sustainability in the Next Decade, regarding requirements for NAMA’s waste.”
  • Providing amended soil with rich organic nutrients and minerals.
  • Creating aerated soil.
  • Eliminating the parasitic impact of mowers, string trimmers and herbicides.
  • Reducing soil compaction around tree roots.
  • Retaining moisture for uptake through roots.

“These last five benefits will add to the longevity of the existing trees. Future trees will have an opportunity for a healthier life, leaving a legacy for future generations to enjoy and expand upon. Wildlife will be afforded opportunities for reprieve in natural settings, and the soils will be moist and fertile again. The NAMA natural environments will heal, forever,” says Morrison.

“In response to this forest-management practice, the trees provide food sources, living-and-breeding habitat and protection for the incredibly diverse wildlife inhabiting and migrating through the urban complex of Washington, D.C. As a good steward, I have an obligation to manage our trees, not as a grouping of individual trees, but as one superorganism called the urban forest.”

The introduced wood-chip beds require a border for chip containment, says Morrison. “The border created for this project was a hand-cut edge 3 inches deep and 3 inches wide.

Wood-chip migration and retention: Choosing your border

The introduced wood-chip beds require a border for chip containment, says Morrison. “The border created for this project was a hand-cut edge 3 inches deep and 3 inches wide, cut on July 19, 2021. This type of edging requires ongoing, labor-intensive maintenance.”

The healing begins

In the aftermath of the project, nature abounds in the urban forest, according to Morrison. Fungi advances biodegradation in the dark, moist places where wood chips cover the soil. Above grade, birds and squirrels hunt and forage naturally in the new forest litter, rather than on human litter.

“Disturbances to the bed of wood chips made by wildlife is integral to keeping this layer aerobic in order to aid in decomposition. In turn, the urban forest provides park wildlife with a natural food supply,” Morrison says. “Many symbiotic relationships form because of the urban forest on the Washington Monument grounds, and will rely on this ecosystem to maintain them.”

As this article was going to print in mid-March, Matthew Morrison and his crews at the National Mall and Memorial Parks were welcoming visitors back to the National Mall as the cherry trees were beginning to blossom in time for the 2022 Cherry Blossom Festival, March 20 through April 17.

4 Comments

    1. Tom Prosser, as you know, it takes a village. I consider myself most fortunate to be able to rely on the guidance of outstanding professionals such as yourself!

  1. Thank you Matthew Morrison for taking such good care of the National Parks and your dedication to preserving the beauty of the Mall in Washington D.C.

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