KATHLEEN ROSS

Urban street trees simultaneously occur at various boundaries and blur them. The boundaries are physical, as between the street and the sidewalk; conceptual, as between the human and the urban, or the technological and the natural; and practical, regarding governance, management, care, and responsibility. Our understanding of street trees has evolved to alternately obscure and reveal the tensions between boundaries. The  popular understanding (in urban planning and policy) of street trees as “green infrastructure” has accentuated these tensions, particularly around  governance. Often, the idea of street trees as green infrastructure is associated with an ideal of civic stewardship. It is increasingly unclear, however, if this ideal can manage apparent infrastructure, “green” or otherwise.

When considering the “edginess” of street trees, their physical position immediately comes to mind. They stand at the edge of the sidewalk and the street—an evocative place, one that implies a neat boundary, a clear division of space and function. It is a placement that inherently regulates human movement, belying the ostensibly “apolitical” nature of a street tree and rendering it “a symbol of order.” Simultaneously, street trees are “a symbol of disorder.” They are disruptive and messy, associated with crime and pests, and can damage  infrastructure and property. Moreover, this messiness is both literal and conceptual. They confuse any tidy boundary between the city and nature, taking root at the edge of each. This is perhaps true of all urban trees, but none more than street trees, which occur outside the bounds of carefully delineated parks. Rather, they “bring nature… at a very fine geographic scale where people live, work, and trade.” 

Further, the term“street trees” emphasizes their conceptual blurriness, similar to “urban forest”  or “green infrastructure.”  Each term is a collision of the natural and the man-made, the urban, or the technological. The collision creates a conceptual fogginess, a sense of shifting boundaries—an urban tree is, in one moment, a “living being planted and cared for,” and in another, a “nonhuman thing,” an “urban object,” a bit of “street furniture.”

This fogginess is exacerbated because the character of street trees, as perceived by urban decision makers, has changed dramatically over time. Lucie Laurian identifies a range of perspectives—from “commodification” to “biophilia” and “stewardship”—that have guided urban tree planting throughout history.  As early as the 14th century, French kings oversaw a series of large-scale street tree plantings in Paris, motivated by practical considerations as well as beautification. The  aesthetic interest dominated urban tree planting for several centuries thereafter. In19th century North America, street trees were considered “ornamental” and associated with the beautification of residential streets, culminating with the City Beautiful Movement at the turn of the century. Newspapers from this era exhibit a fawning association of street trees with beauty, but hesitate to credit them with much else. An 1889 article entitled “Trees in Our Streets” exemplifies this sentiment with its declaration that Washington D.C.’s street trees are single-handedly responsible for “[making] the city… one of the most beautiful on earth.” 

The same article, on the other hand, insists that those “who have made a careful and scientific study” of street trees reject all claims that they perform the practical function of moderating urban climates. This is not to say that urban trees were never associated with ecological functions. Multiple 19th century urban park systems, for example, were linked to the mitigation of flooding and pollution. As late as 1968, however, a Planning Advisory Service report asserted that urban trees may provide “ecological benefits,” but their “most significant benefits… are aesthetic.” Today, this view is neatly reversed. Urban trees are overwhelmingly associated with a range of practical and vital benefits and these features drive the growing number of massive tree planting initiatives in cities around the world. 

The now ubiquitous association of trees with such ecosystem services has spurred a new conception of urban trees. They are no longer the purely ornamental trees of the 19th and early-20th century. Nor are they the mid-20th century’s infrastructure-destroying nuisances. Rather,  in the 21st century, urban trees are themselves essential infrastructure,  as valuable as any sewer line or water pipe. Since the 1980s, a growing body of literature has highlighted their services and quantified their value. A 2018 study of urban forests nationwide, for example, calculated that urban trees produce $18.3 billion in value annually, including $5.4 billion in air pollution removal and $4.8 billion in carbon sequestration. New York state is among the greatest beneficiaries of urban trees, receiving an estimated $1 billion in value annually, including $346 million in energy savings caused by the shading of buildings and cooling of air temperatures.

These valuations justify the price tags of urban tree-planting initiatives, which can be substantial. When former Mayor Michael Bloomberg announced the Million Trees NYC Initiative in 2007, initial plans suggested the Parks Department would be granted $400 million to plant 600,000 trees over ten years. Tree planting costs have grown in the interim—by 2020, the cost of planting a single tree in New York City averaged $2,700. Importantly, in order for trees to deliver the services that justify the costs, a sufficient number of planted trees must survive. Large and mature urban trees benefit the ecosystem most —a state that may not be reached for decades after planting. The difference in benefits provided by a long-established versus newly-planted tree cannot be overstated . Larger trees, for example, can remove as much as seventy times more air pollution than smaller ones.

Unfortunately, urban trees face a variety of unique stressors that impact their survival,  including urban heat island and dry island effects, air pollution, construction damage, vandalism, and the compaction, contamination, and degradation of soil. These stress factors shorten lifespans and increase the mortality rate of urban trees compared to rural trees. Street trees experience these stress factors at an even greater intensity than the broader category of urban trees. Street trees also experience a number of challenges from which urban trees in other locations may be shielded, such as vehicular traffic and problematic sidewalk and building design. The resultant mortality rates can be  startling and raise serious questions about the efficacy of urban tree planting initiatives. Studies have found survival rates as low as 34.7 percent for newly-planted street trees. Worldwide, street tree populations have a half life of thirteen to twenty years, meaning half of the street trees planted in a given year will die in that time span.

Research on how street tree mortality impacts ecosystem benefits of tree planting is limited. In fact, many of the largest urban tree-planting initiatives have utilized overly optimistic mortality estimates based on insubstantial or unclear research, while failing to track actual survival rates. The research raises concerns. A study in Boston, for example, found that the mortality rate of  street trees was twice the rate than in nearby rural forests. This high mortality rate indicates that even if tree planting increased 50 percent, street tree carbon density in Boston would decline 20 percent over the next several years. In other words, absent a dramatically increased investment in tree maintenance, street tree mortality would outweigh planting, meaning urban tree biomass, and the carbon storage benefit they  provide, would actually decline. 

While these projections are specific to Boston, the same basic pattern might hold true more broadly. For example, urban areas across the United States lost approximately 28.5 million trees per year between 2009 and 2014, despite massive tree planting investments in many of the nation’s largest cities during the same period. These trends and projections are troubling given that tree-planting is intended to increase carbon storage and climate mitigation. However, planting and maintaining street trees has a significant carbon footprint. The Boston researchers calculate that the carbon costs of street trees—including producing, planting, irrigating, and eventually removing them—are so substantial that trees must live twenty-six to thirty-three years to merely achieve carbon neutrality.

An analysis of the  literature suggests that the average life expectancy of street trees is nineteen to twenty-eight years—a lifespan that, most optimistically, barely makes street trees carbon neutral. Fortunately, research suggests that greater maintenance and stewardship might dramatically increase street trees’ survival rate, bolstering the otherwise questionable sustainability promises of tree-planting initiatives. One  New York City study found that street trees with designated stewards have a mortality rate three times lower than unstewarded trees, meaning that maintenance can preserve urban canopy cover better than planting.Early-life maintenance can prevent more costly maintenance later, and late-life maintenance can reduce potential liability costs from property damage, as well as tree failure and resultant removal costs.

Thus, the cost savings and enhanced benefits and services brought by tree maintenance highlights the cost of not maintaining them. Unfortunately, cities’ growing enthusiasm for tree planting has not been matched by a commensurate enthusiasm for maintenance. Tree expenses make up a fraction of a percent of city budgets nationwide, and those expenses include both park trees and street trees. In many cities, the acquisition of trees is institutionalized in the capital budget, but their maintenance is not. Budgets for tree maintenance are “frequently considered non-essential” and often “on the chopping block” during economic downturns. In New York City, for example, plans to supplement the Million Trees Initiative with increased tree pruning fell by the wayside in budget cuts, leading one Parks employee to remark that tree maintenance “in effect… doesn’t exist.” 

Often, not only tree maintenance itself but the entire city agency that handles it is treated as non-essential and therefore prone to budget cuts in times of austerity. In Gary, Indiana, for example, the entire department of urban forestry was eliminated during the Great Recession. In New York City, the COVID-19 crisis prompted eighty-four million dollars in cuts to the Parks Department and a 1,700-person staff reduction. Some of this funding was restored in a subsequent city budget, only to be cut again in the most recent budget proposal under Mayor Eric Adams. Despite Adams’ campaign promise—reiterated within weeks of the new budget—to give the Parks Department one percent of the overall budget, the proposal leaves Parks funding at an intractable half a percent. Such limited and precarious funding calls into question cities’ supposed commitment to urban trees as vital infrastructure. 

While “green infrastructure” branding appears convincing when politically popular tree-planting initiatives are rolled out, the lack of subsequent maintenance suggests a stubbornly persistent view of street trees as largely ornamental. This perpetual dichotomy is reflected in the confusion around the governance of street trees. Generally, cities rely on an array of public-private and public-civic partnerships to plant and maintain street trees during periods of austerity. Civic groups, nonprofits, and private citizens thus end up in the strange vacuum of responsibility for ostensibly public infrastructure. Los Angeles’ Million Tree Initiative, for example, depended on a convoluted array of funding from foundation and  businesses, as well as nonprofit and citizen volunteers. In the end, the city only managed to plant about 400,000 trees over seven years.

In contrast, New York City’s Million Trees Initiative, which has been lauded as “‘the Cadillac’ of tree-planting programs” for its superior funding, managed to plant one million trees two years ahead of its original schedule. Rather than relying on volunteers for planting, New York City uses experienced contractors, which likely contributes to the slightly above-average survival rates of New York City street trees. Contractors’ responsibility for street tree survival, however, expires after two years; thereafter, New York City relies on the same messy arrangement of civic volunteerism of other cities. As part of the Million Trees Initiative, both then-mayor Bloomberg and the Parks Department explicitly encouraged city residents to care for street trees, launching stewardship events, tree care workshops, and adopt-a-tree programs. Many forms of tree care, such as the installation of tree guards, continue to be overwhelmingly carried out by city residents at personal cost.

One of the most interesting features of citizen intervention is how frequently it is portrayed romantically, rather than as a failure of state capacity. Despite frequent mistrust of public-private partnerships created during austerity, the civic stewardship of street trees and other green infrastructure can elicit heady excitement in urban policy and planning literature. Civic stewardship has been described as a form of governance “innovation,” capable of “enhancing the flexible problem-solving capacity needed to meet the demands of complex social–ecological systems in cities.” Civic stewardship is also seen as an end in itself. Interviews with stakeholders in the green infrastructure of various cities, for instance, identified “establishing a new civic relationship to the urban environment” and “[getting] city residents to understand that as [urban citizens], ‘you live in an urban forest’” as common goals of tree-planting initiatives.

There are several reasons why this romanticization is problematic. Civic stewardship improves street tree survival, but it may not be  a scalable means of managing large investments in green infrastructure. Can citizen stewards who improve tree survival in a particular neighborhood maintain an entire urban forest? Also,  civic stewardship may be a fictitious ideal. One scholar believes there is  an “attribution of morality to tree practices” and a “normative discourse whereby trees are not only considered good but are also represented as if they are, or should be, loved by everybody.” However, educational attainment, class, and race and ethnicity all potentially influence interest in and affection for trees, meaning trees are not necessarily “loved by everybody.”

Thus, the distribution of urban canopy becomes inconsistent, depending on citizens’ varying attitudes and ability to dedicate time and resources to tree maintenance. This may be true even where planting choices explicitly aim to reduce these inequities. The MillionTrees NYC initiative, for example, specifically targeted areas with high population density and limited canopy cover, leading to reductions in neighborhood-based inequalities when considering tree count alone. However, inequalities arose when considering tree health and tree biomass. These trends may continue depending on links between educational level and neighborhood socioeconomic status,  on one hand,  and interest in tree stewardship and tree survival rates, on the other, potentially reversing any gains in tree count equality achieved through planting.

To rely on civic stewardship, then, is to insist “that low-income communities should volunteer their participation to green their own environment.” Often, this somewhat problematic demand is portrayed positively, as an opportunity for greater “public participation” or as a way of becoming an ideal “urban citizen.” These demands seem to absolve the state of their basic duty of governing urban infrastructure. Such modernist infrastructure was, as Karen Bakker writes, a public good, universally provided as a “material emblem of citizenship.” In contrast, the emerging norms of civic stewardship governing our new green infrastructure gesture toward a stranger arrangement. Rather than infrastructure constituting a basic right of citizenship, citizenship is itself constituted by the act of personal care for infrastructure. Citizen stewardship as a new mode of governance has evolved alongside our shifting conception of street trees. It is unclear, however, if citizen stewardship evolved as an ideal form of management for “green infrastructure,” or as a method of patching holes in crumbling municipal capacity.

In the most cynical view, perhaps green infrastructure itself is only a band-aid on the declining ability of the state to maintain traditional infrastructure and a shiny distraction from the state’s general disinterest in meaningfully confronting issues of sustainability, resilience, and equity. This is the impression given from one perpetually flooded Brooklyn neighborhood known as the Hole, which is not connected to the city sewer system. After decades of sewer system promises to residents of this neighborhood, the city is now considering whether “less costly and disruptive” green infrastructure options might suffice. Whether green infrastructure will suffice—in this and other contexts—may depend on our ability to treat it as the vital infrastructure we claim it is.

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Kathleen Ross is a recent graduate of the Hunter College Urban Policy & Planning Department’s undergraduate program in Urban Studies. In the fall, she will attend the University of Michigan Law School and is hoping to pursue a career in environmental law.