Leaves of Green: Capitol's Mature Trees Offer Big Payouts

The thousands of trees budding across the Capitol campus each spring offer more than just a picturesque sight: together, they uphold Frederick Law Olmsted's 150-year-old vision to perfectly frame our nation's foundational government buildings and provide the public a beautiful way to experience the U.S. Capitol and its surroundings.

In total, the Capitol campus is home to more than 4,000 trees. Olmsted designed the 56 acre-area surrounding the Capitol Building, and about 46 trees within the area are considered "Olmsted originals" — trees planted under the landscape architect's direction from 1874 to 1894. Beyond their historical and aesthetic significance, Olmsted's careful plantings, especially those advanced in age, bring a host of environmental benefits too.

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Today, the curated canopy provides food for pollinators, habitats and shade for local wildlife and mitigates damage from pollutants and runoff. These benefits are particularly important in urban environments, such as Washington, D.C., that face heightened levels of environmental pollutants.

According to the U.S. Department of Agriculture (USDA), every year a group of 100 trees removes 53 tons of carbon dioxide and 430 pounds of other air pollutants. Older trees have unique benefits: 100 mature trees catch nearly 150,000 gallons of rainwater annually. To reap these benefits and better understand individual tree health on the Capitol campus, the Architect of the Capitol's Capitol Grounds and Arboretum team uses a host of different technologies and valuation calculation tools. One of these is i-Tree, a free software created by the USDA Forest Service.

An AOC Capitol Grounds and Arboretum team member helps plant a young tree on the Capitol campus; This mature pecan tree, found on the West Lawn, is an “Olmsted original.” It was planted sometime between 1874-1894.

By inputting tree measurements and other information into the tree database, the team can calculate estimated benefits from an individual tree or an entire area on campus. These "eco-benefits" are instrumental in telling the story of the campus canopy. They can estimate cost savings related to storm water runoff, carbon capture — also known as sequestration — and cooling effects. With this knowledge, the team can make informed decisions about new plantings and care for older ones.

Findings show that mature trees provide the most significant benefits. Side-by-side analyses of two white oaks, one mature and one young, demonstrated that the mature tree offered nearly 40 times the greenhouse gas benefits, water, energy and air quality benefits than those of the younger tree.

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"It literally pays to preserve our mature trees and to help them thrive in the landscape for as long as possible," says Melissa Westbrook, an Urban Forester with the Capitol Grounds team. "That is no easy feat, with the number of challenges we face, and requires collaboration from all parts of the agency."