Five ways to increase tree cover in cities (commentary)

As cities in the U.S. and other nations suffer from current heat waves, one proven way to cool urban areas and clean the air is by planting trees.
The solution sounds simple but there are numerous barriers to increasing tree cover in urban areas, from high mortality rates to capacity limitations within municipal forestry, parks, and recreation departments.
"Trees are as integral to city infrastructure as sidewalks and power lines,” a new op-ed that shares useful resources says: people need improved information and tools to advocate for, plan, and implement urban tree conservation, maintenance, and planting activities to support cities' future livability, equity, and public health.
This article is a commentary. The views expressed are those of the author, not necessarily of Mongabay.

From increases in severe storms to warming average temperatures, the climate crisis presents pressing challenges to environmental and social conditions around the world. Forests have been recognized as a key to mitigating the impacts of global climate change, with an emphasis on their intake and storage of carbon. Additionally, urban forests and built green infrastructure (e.g., rain gardens, green roofs, vegetated buffer zones) provide myriad localized ecosystem services to surrounding areas, such as improvements in mental health and water quality, lower levels of localized air pollution, and reductions to the urban heat island effect.

These advantages make urban forests critical to the environmental and social well-being of cities and their communities. City planning must prioritize, budget for, and create policies to support the health and resilience of urban forests as part of a green infrastructure framework.

Forest management is a significant factor contributing to the amount of carbon stored in an area, and management falls upon many different actors from multiple levels of government to private landowners. Informing decision-makers at all scales is crucial to maintaining and creating forests that are resilient in the face of a changing climate.

An avenue of urban trees in San Francisco. Image courtesy of Photo by Vladimir Kudinov on Unsplash. — Street trees in San Francisco. Image courtesy of Photo by Vladimir Kudinov on Unsplash.

Forests across the world play an integral role in climate change mitigation—from the expansive Amazon rainforest down to small, half-acre parks in cities. Awareness of the role urban forests play in climate change mitigation and social equity is gaining traction in the U.S. (see this September 2021 White House press release and an August 2021 National Geographic podcast episode). Greenspaces and urban forests present cities with an opportunity to adapt to a growing climate emergency, and help create more equitable, healthy, and climate resilient communities. Decision-makers can access tools such as the Climate Adaptation Guide and the Vibrant Cities Lab to improve their ability to address these climate issues and create resilient, adaptable communities.

Despite their many benefits, urban trees in the United States are half as common in poorer neighborhoods as opposed to their wealthier counterparts, sometimes within the same city. This means that communities facing economic hardship often do not reap the benefits of forest cover. Instead, they pay for this deficit with negative health outcomes, increased environmental disasters, and higher crime rates. Increasing canopy cover in urban forests is a key strategy for addressing these inequities while also improving the quality and quantity of environmental services in urban areas.

There are several barriers to increasing tree cover in urban areas, including:

High rates of tree mortality
Planting, maintenance, and removal costs
Capacity limitations within municipal forestry, parks, and recreation departments
Lack of technical knowledge
Limited public support and enthusiasm

These barriers can overlap and create negative feedback loops amongst themselves, intensifying and cementing their impacts. It is critical that decision-makers have an awareness of the challenges trees face in urban settings, as well as an understanding of how to overcome these barriers.

Mortality

One overarching concern is the life expectancy of urban trees, which can vary greatly from city to city but is generally highly abbreviated. For example, two studies of urban tree lifespan found that half of the trees planted in urban areas will die before 7 and 13 years of age (Moll 1989, Skiera and Moll 1992).

An analysis of more recent estimates found annual urban tree mortality rates around only 3.5–5.1%, giving them a half-life of 19-28 years. Despite the wide range of half-lives reported, most research agrees that urban trees are expected to live shorter lives than rural trees. Rural trees are assumed to live an average of 150 years, but this can vary greatly by species. The truncated lifespan of urban trees is due to many factors, such as poor soil structure (as shown in Figure 1), soil and air pollution, and water deficits. An effective way to combat these stressors is to plant climate smart species. Despite the challenges they face, urban trees can have an outsized impact, as populations of people are concentrated in urban centers, making the direct impact of a small number of trees relatively great.

The pure replacement cost of urban trees increases when trees live shorter lives and, on the other end of their life cycle, trees in decline are expensive to maintain and remove post mortality. This leads to higher costs in both the short and long run. As shown in Figure 2, costs outweigh benefits at the beginning and end of an urban tree’s life, but when properly cared for it produces the most value at semi- and full maturity. Without proper maintenance, the full urban tree benefit is not realized yet planting and removal costs persist. With finite funds, cities are forced to allocate money where they believe it will be most impactful, and forestry-related efforts are frequently seen as peripheral benefits rather than the critical social, economic, and environmental infrastructure that they are. Correcting this misperception is crucial in educating current and future leaders in effective, climate-adapted urban planning.

Figure 2: A theoretical model of the cost and benefit of an urban tree over its lifetime. Source: Hauer et al. 2015

Capacity

According to Funding Trees for Health, a paper published by The Nature Conservancy, one other barrier to increasing canopy cover is the existence of municipal “silos,” when lone city forestry, parks, or recreation departments are tasked with the large job of planting and maintaining tree stocks. Urban forestry is a cross-cutting issue with broad ecological and social implications that are often not covered by the scope of any single department and can be difficult for one department to manage independently. Increased capacity and funding can help alleviate the stressors put on local forestry and parks departments, many of which face drastic cuts when budgets run dry. This often leads to limited arboriculture expertise and reduced capacity to advocate for tree planting. In the face of the disproportionate impacts climate change will have on urban populations, properly valuing and training a workforce equipped to support healthy urban forests is a step that city governments cannot afford to overlook.

Knowledge and public support

Two other major barriers to increasing urban canopy cover are a lack of technical knowledge and public awareness. These two constraints are intertwined, as one can lead to the other. McDonald et al. (2017), argue that “decision-makers and the public may lack knowledge of the benefits trees provide.” This sentiment is echoed throughout other academic papers (e.g., Driscoll et al. 2015; Wolf & Kruger 2010). Elected officials who understand the roles forests play in public health and wellbeing, and hire staff with technical expertise on planting the right tree in the right place can support the growth of urban canopies. Public attitudes and knowledge are key players in increasing urban forest cover, as they both can dictate policy agendas and priority funding areas for municipalities. Without public—and governmental—support, urban forests are unlikely to take root any time soon.

Washington Square Park in New York City. Photo by Eden, Janine and Jim, licensed under CC BY 2.0.

Increasing canopy cover and improving green infrastructure in cities is critical to improving environmental and social equity, combating global climate change, and minimizing negative local impacts.

To achieve this, municipal decision-makers must:

Increase spending on urban tree maintenance to maximize tree health and the indirect benefits they provide
Include forest canopy targets in city-level strategies and plans
Link local policy to state-level policies and objectives on climate mitigation
Build new partnerships to leverage capacity (such as national funding and local planting groups)
Engage with educational programming, information sources, and funding opportunities from actors such as:

Another significant way these challenges are being addressed in the U.S. is through the passage of the Inflation Reduction Act, which has earmarked roughly $1.5 billion for the Urban and Community Forestry Assistance Program. This program provides technical, financial, and educational assistance to municipalities working to increase tree canopy cover. Important funding sources can be generated by establishing systems of fees and regulations. For example, the Rain to Recreation system implemented in Lenexa, Kansas, imposes a set of fees for development, erosion, and stormwater utilities that are used to fund green infrastructure solutions. Lenexa’s commitment to green infrastructure and creative planning solutions garners national recognition for its impacts on water quality improvement. While an influx of funding may help ease the barriers urban forestry faces, ample engagement and outreach needs remain.

Under a climate change and social justice framing, urban forestry is becoming a hot topic that crosses multiple disciplines. However, a substantial knowledge gap persists in how to link urban forests to climate mitigation and adaptation. Rita Hite, CEO of American Forest Foundation, says “We need to do work to make people know: Trees are a need to have, not a want to have.”

Providing diverse actors with the information and tools they need to advocate for, plan, and implement urban tree conservation, maintenance, and planting activities will support the future livability, equity, and public health of urban populations. This will lead to the best possible outcomes for society and our changing climate. Trees are as integral to city infrastructure as sidewalks and power lines.

Evan Beresford is a Program Associate with the Michigan State University Forest Carbon and Climate Program, where he focuses on urban forestry and its relationship to climate change.

Related audio from Mongabay’s podcast: A conversation with urban forester Georgia Silvera Seamans about efforts to increase green spaces in New York City, listen here:

Citations:

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