Urban Forests

If you were a lucky kid in one of America’s urban forests, your first friend was a primordial beast. Its knotty arms would cradle you when you were overheated or sad or alone, and keep you safe when you picnicked and plucked grass at its feet. And if you were really, really lucky, you owned a piece of real estate in its canopy. You reigned from your pirate ship in the sky, and you had the tactical advantage in a snowball fight.

You had a tree.

Urban forests create an archway of trees at Jefferson National Expansion Memorial.
Ash trees at the Jefferson National Expansion Memorial, courtesy of NPS.

For some of us, the only nature we get to know is on our street– and our little landscape is threatened. Many cities, including Boulder, Colorado are experiencing the demise of their urban vegetation due to the emerald ash borer (EAB). The beetle feeds and lays destructive colonies in ash trees, the cornerstone of most American urban forests.

Dangerous Combinations: Elm, Ash, and a Beetle

A quick google search of the term “ash tree” summons a frenzy of dramatic reports of this plague and its pending catastrophe. Experts point to the irresponsible practices of urban developers for the proliferation of EAB. Planters have traditionally picked a limited diversity of vegetation, or have taken advantage of the relaxed international trade of plants, which has helped spread the diseases and pests that come along the ride.[1] Despite this contemporary consensus, it is worth exploring why we turn to this pattern again and again to build urban forests in the first place. According to the city of Fort Collins, nearly twenty percent of city-owned trees are ash. [2] Why are there so many ash trees? Where did they come from? This angle is often either completely forgotten or glazed over by those reporting the epidemic. The answer weaves an intricate path through a countless of historical, cultural, and environmental factors.

Stately yet Delicate Urban Forests

Here’s what we know: this crisis has happened before. In the early 1930s, American streets, parks, and college campuses began to lose their precious elms. An 1932 excerpt from Scientific Monthly urgently cried: “Estimates made last summer indicate that approximately 100,000 elms in southern Connecticut were seriously damaged by the elm-leaf beetle. As many more were seriously injured in other New England states and some 250,000 in New York State;” the report claims the epidemic had stretched as far south as Virginia, and as far west as Oregon and California.[3] And none of this even accounted for the damage done by the , the cause of yet another illness. Dutch elm disease snuck into North America in 1930s but peaked in the 1970s.[5] By the 1980s, landscapers had mostly given up on reviving the species. The race was on to find the next great tree.

By 1982, the botanists at the Arnold Arboretum of Harvard University reminisced about the unbroken canopy the elegant and grand American elm offered America’s cityscapes. According to their quarterly bulletin, developers chose the elm to grace our streets because of their “relatively rapid growth, longevity, structural strength, and adaptability to a wide range of environmental conditions, soil types, and soil conditions.” And yet, their ubiquity spelled destruction. The botanists at the Arboretum frantically seized on twelve alternatives, most of which were environmentally impractical. Sure, the silver maple may have been a stunning goliath, but its “shallow roots [wreaked] havoc with the pavement.” And the Japanese zelkova might have offered a lush and continuous canopy, but their “susceptibility to branch damage from ice and snow” posed a risk to urban structures and pedestrians alike.[6] Some alternative tree species needed constant and laborious pruning, while others posed the problem of excessive nut and fruit litter, while others still suffered from the inability to thrive in dry soil.[7] The exception to the rule is the white (and green) ash.[8]  Today, the ash is still lauded as a climate chameleon; both white and green ash thrive in a wide range of soil types, and green ash can even thrive in dry climates like that of Colorado.[9]

The penchant for elm and ash species, however, can also be explained by the phenomenon of the “stately tree,” or the idea that some trees are monuments. While other trees may also flourish environmentally, landscape architects and urban planners dismissed them for their lack of stature.

Organic Architecture

But the earth did not just auto-populate our landscape with tidy rows of elms and ashes. The history of monocultured landscaping runs deep. An excerpt from the Scientific American published in 1920 accentuates the health benefits of publicly tended foliage, but stresses that “mixed plantings of different kinds of trees are not as pleasing and effective as the use of a single species for considerable distances.”[10] This attitude runs clear through the 20th century and was only exacerbated by the monotony of preassembled suburban architectural styling, and mass-produced landscaping. By the post-war era, Americans had come to expect their neighborhoods to look a certain way; the greatest generation clamored for white picket fences and manicured lawns of Kentucky bluegrass, regardless of their local environment.[11] And a neighborhood just wasn’t right without a runway of stately elms.[12]

Urban Forests for the Future?

America’s taste for homogeneous scenery even bled into “natural spaces.” National Parks, monuments, and memorials are painstakingly curated landscapes. Generally, cultural resource managers vie for the opportunity to preserve and restore natural landscapes to match their historical character, which may differ from the land’s ecology.[13] The renovation of Jefferson National Expansion Memorial in Missouri, for instance, offers a case that illustrates the negotiation between ecological and cultural priorities. When landscape architect Dan Kiley designed the runway leading up to the Gateway Arch in the 1960s, his blueprints called for a 1,200 count cascade of tightly knit tulip poplars. By 2010, the park’s trees were widely infected by EAB.[16] And yet, despite the cautionary tales of the elm and ash, the cultural pervasiveness of the stately tree persisted; the memorial’s keepers decided to substitute the sickly vegetation with new, repetitive channels of London plane trees. It’s worth noting, however, that the monoculture on the grounds were limited to the land directly bordering the sidewalk runway of trees.[17] Memorial landscapers have otherwise made a considerable effort at diversifying the surrounding vegetation with native plants, and a new variety of soils.[18]

Several popular parks on the east coast, including Harper’s Ferry National Historic Park, Antietam National Battlefield, and the C&O Canal National Historic Park, have been floundering against the onslaught of EAB.[19]Closer to home, the city of Boulder has already been actively fighting the plague– and although the city of Fort Collins has begun by employing pesticide treatment, for instance, many argue that efforts are likely too little too late.[20] The problem will continue unless land managers and planners make changes in parks and public spaces. Only time will tell.


-Gentrice Petrie, PLHC Intern

Published 02/06/2019



[1] Local Governments are Paying the Price for Global Trade’s Effects on Trees, CityLab, Julian Spector, April 25, 2016 The Ash Tree, 8

[2]“Invasive Insect Alert: Emerald Ash Borer (EAB)” City of Fort Collins, https://www.fcgov.com/forestry/

[3] Shade Trees Threatened by Insect Pests, 59

[4] Republic of Shade, 149, 163-5

[5] Replacing the American Elm: Twelve Stately Trees pg. 91

[6] Replacing the American Elm: Twelve Stately Trees pg. 98

[7] Ibid. 92, 95-97

[8] Ibid. 94

[9] Selecting Landscape Plants, Virginia Cooperative Extension, 2

[10] Shade Trees for City Streets, Scientific American, 157

[11] Down to Earth, 22

[12] Republic of Shade, 165

[13] Cultural Landscape Preservation in Context: Responding to a Changing Environment, 59

[14] Jefferson National Expansion Memorial, The Landscape Architecture Legacy of Dan Kiley

[15] Ibid., Cultural Landscape Preservation in Context: Responding to a Changing Environment, 67

[16] Cultural Landscape Preservation in Context: Responding to a Changing Environment, 66

[17] Cultural Landscape Preservation in Context: Responding to a Changing Environment, 67

[18] Cultural Landscape Preservation in Context: Responding to a Changing Environment, 67

[19] Ash Tree Update 2017, National Parks Service

[20] “Emerald Ash Borer: Fort Collins Ash Trees are in Trouble. Just Ask Boulder,” Coloroadan, https://www.coloradoan.com/story/news/2017/09/18/emerald-ash-borer-fort-collins-boulder-ash-trees/644376001/