So, what’s the difference and why is that important for us to know?

Most intelligent people understand that urban trees and urban forests provide important greenspace, accounting for diverse economic, ecological and social benefits to us all. But many of us fail to understand the difference between the concept of a ‘city tree’ and an intact ‘urban forest’; many use these terms interchangeably, which undermines our understanding of city management and planning. This is partly because the term “urban forestry” is currently in use to encompass activities pertaining to both single tree in the city and forested natural parkland under an overarching sense of tree presence. But, as they say, “you cannot see the forest for the trees…”

While many of the world’s cities have implemented tree planting programs based on suggestions of environmental and social benefits of urban forests, most of these programs involve planting trees, not planting (or keeping) forests. There’s a major difference between urban trees and urban forests and some of the most significant differences have to do with what a tree vs. a forest accomplishes, socially and environmentally.

In Environmental Research, Pregitzer et al. provide the following distinction: “street trees occur in highly developed environments, and are planted and cared for on an individual basis, whereas forested natural areas often occur in parkland, are managed at the stand level, and are primarily sustained by natural processes such as regeneration.” Essentially, an urban forest is a densely wooded area located in the city, supports a biodiverse community and a functional ecosystem (with associated ecosystem services).

What Trees Can Do for the Urban Environment

City trees provide significant benefits to any urban environment. They: 1) help regulate the water cycle, particularly with stormwater absorption and flood reduction; 2) provide shade and reduce heat islands; 3) and they provide immense health benefits through connection.

If planted well, trees and their associated permeable surroundings reduce runoff, erosion and flooding. Their root systems help against erosion and help consolidate the soil to more effectively retain water and prevent slopes from slumping.

Large trees reduce ambient temperatures through shade and evapotranspiration to cool the air by as much as 1-5 degrees Celsius.

Trees, whether a single tree or a grove along a street, provide a wealth of well-being for humans, whether through habitat for birds, through chemical aerosols, or simple aesthetics. Many studies have shown the benefits of trees and associated green space and birds to human mental health. Scientists have proven that conservation of natural areas in urban settings is just as important for human well-being as financial security. Trees and forests provide habitat for birds; and birds give us joy. “The happiest Europeans are those who can experience numerous different bird species in their daily life, or who live in near-natural surroundings that are home to many species,” argued lead author Joel Methorst, a doctoral researcher at the Senckenberg Biodiversity and Climate Research Center, the iDiv and the Goethe University in Frankfurt. Research clearly demonstrates that bird diversity is predicated on a healthy natural ecosystem; these two are bound together.

In Part 2 of “Why I Love Trees” I write about a study by Lancaster University that showed the birch tree’s detoxifying capabilities when they planted silver birch trees between a street of houses and a busy road. According to Professor Barbara Maher, the trees absorbed more than 50 per cent of the particulate dust — linked to respiratory problems — from passing traffic. The tree “is a chemical factory,” says botanist and biochemist Diana Beresford-Kroeger. They broadcast a host of chemicals into the environment that may travel for hundreds of kilometres, as well as affect the immediate area. Beresford-Kroeger argues that trees help maintain the health of the natural world, given that they constantly shower healing chemical mists into the air. “These substances are at the heart of connectivity in nature,” says Beresford-Kroeger. for instance, during a walk through a pine forest on a warm day, the sharp pungent smell of pinene (a monoterpene), helps to relieve asthma. Another monoterpene aerosol, limonene, has an ability to fight cancer, demonstrated by Dr. Michael Gould at the University of Wisconsin.

However, these benefits may not be fulfilled, given the notable challenges to tree wellbeing posed by the urban environment. The Canadian Institute for Climate Choices writes that “obtaining benefits is about more than the quantity of trees…Quality also matters. The location of trees, the type of trees, planting conditions, and work done to maintain and protect the health of trees will determine the magnitude of benefits.”

Challenges:

City trees must deal with lack of water, compacted soil, not enough food, physical injuries, imported pests and diseases, road salt and air pollution, and restricted root space. Other considerations include suitability of tree species planted, when and where it was planted and how the individual tree is looked after.

What Trees Don’t do But Forests Do for the Urban Environment

Trees don’t behave the same as a forest.

Street trees are not part of a functional ecosystem and are therefore unable to perform certain ecosystem functions. Street trees lack coherence with other trees and other components of a natural community that includes lichen, fungi, other vegetation, insects and wildlife. Street trees remain isolated and are more vulnerable than their counterparts in a forest community to weather, disease, and other factors. Han et al. (2020) showed that urban morphology such as building and street design factor significantly in how urban trees provide ecosystem services and reduce atmospheric pollution.

In their April 2021 paper in Frontiers in Ecology and Evolution, Pataki et al.  argue that while “current evidence supports local cooling, stormwater absorption, and health benefits of urban trees for local residents, the potential for urban trees to appreciably mitigate greenhouse gas emissions and air pollution over a wide array of sites and environmental conditions is limited.” Essentially, urban trees provide adaptation strategies (helping residents cope with climate change and pollution) rather than mitigation strategies (reducing climate change and pollution), which are the domain of the forest.

Only a functional urban forest ecosystem (intact with functional natural components) will provide the ecosystem services that can help combat climate change and protect the urban community. In addition to affecting climate and heating, urban forests help filter air and water, control storm water and flooding, conserve energy and provide animal habitat. They also add beauty, form and structure to urban design.

Many of us feel a sense of peace in a forest. I have no doubt that this is the result of several factors including sounds and frequencies (e.g., infrasound), increased negative charge, scents, wood essential oils, genetic heritage and memory, and simple aesthetic appreciation and beauty. I explore this more in Part 3 of my series—Why I Love Trees: Part 3, Feeling Trees.

Challenges:

Silviculturist Jean-Claude Ruel writes in The Conversation that urban encroachment in an existing forest can compromise the woodland through indirect and direct means. “Construction in a wooded environment brings about general changes in the tree’s growing environment. By opening the canopy, trees become exposed to stronger winds, which increases their need for water and can compromise their stability.” Ruel adds that construction near and in a wooded area creates impervious surfaces that reduces water flow into the soil and promotes urban runoff. These lead to water stress and trees dying a few years after construction. Direct damage to the aerial part of the tree and its root system through excavation also occur through nearby construction. With root damage, the trees no longer maintain their functionality. Root damage will promote the closure of stoma (the pores that control gas exchange during photosynthesis) leading to growth reduction and eventual mortality.

Benefits of Urban Forests

Tree Canada lists the social, environmental, and economic benefits of functional urban forests:

Social Benefits:

• Urban forests promote physical activity by providing space for recreation and creating an appealing outdoor environment (Mytton, Townsend, Rutter, & Foster, 2012).
• Urban forests promote mental well-being and reduce stress, heart rate, blood pressure (Kardan et al., 2015) and incidence of obesity, asthma, and diabetes (Ulmer et al., 2016).
• Urban forests can reduce crime rates and violence (Parker, 2018; Troy, Grove, and O’Neill-Dunne, 2012).
• Urban forests can promote healing – people in hospital rooms with views of trees heal faster (Ulrich, 1992; Cooper Marcus, 2007).
• Urban forests make cities more beautiful (Price, 2003; Tyrväinen, Pauleit, Seeland, & de Vries, 2005) and can hide unattractive features like walls, freeways, and parking lots.
• Urban forests increase road safety by slowing traffic, reducing stress, or improving driver attention (Mok, Landphair, & Naderi, 2006; Naderi, 2003).
• Urban forests provide food for people (Colinas, Bush, & Manaugh, 2018).
• Urban forests promote social interaction and a sense of community, including stronger ties to neighbours, a greater sense of safety, and more use of outdoor public spaces (Kuo, 2003; Westphal, 2003).

Enviromental Benefits:

• Urban forests reduce air pollution and provide oxygen (Nowak, Hirabayashi, Doyle, McGovern & Pasher, 2018).
• Urban forests reduce the urban heat island effect and reduce the temperature of cities, helping cities adapt to climate change (Brandt et al., 2016; Sinnett, 2018; Rahman, Armson, & Ennos, 2014; Edmondson, Stott, Davies, Gaston, & Leake, 2016; Wang & Akbarib, 2016; Livesley, McPherson, & Calfapietra, 2016).
• Urban forests reduce buildings’ energy use, including heating costs (Nowak, Hoehn, Bodine, Greenfield, & O’Neil-Dunne, 2016; Akbari, 2002; Akbari & Taha, 1992).
• Urban forests improve water filtration, store water, and reduce stormwater runoff (Berland, 2017; Bartens, Day, Harris, Dove, & Wynne, 2008).
• Urban forests help provide habitat for wildlife and help preserve biodiversity (Aronson et al., 2017; Alvey, 2006; Mörtberg, 2001).
• Urban forests provide habitat for wildlife (Threlfall et al., 2015) and promote biodiversity (Sandström, Angelstam, & Mikusiński, 2006).

Economic Benefits:

• Urban forests provide ecosystem services evaluated at $330 million per year for Halifax, Montreal, Vancovuer, and Toronto (Alexander & DePratto, 2014).
• Urban forests add value of between $1.88 and $12.70 for every dollar spent on maintaining them, depending on the city (Alexander & McDonald, 2014).
• Urban forests increase property values (Escobedo, Adams, & Timilsina, 2015).
• Urban forests help create attractive business districts and improve visitors’ perceptions of them (Wolf, 2003).
• Urban forests have a positive influence on visitors’ perceptions of a city (Andrada & Deng, 2010).
• Urban forests provide space for recreation (Schroeder, 2009; Miller, Hauser, & Werner, 2015).

Definitions of An Urban Forest

Most definitions of urban forest adopt a corrupt version of “forest” to encompass the entire urban environment. This ranges from a single street tree to a woodlot of an urban or peri-urban area. I find this definition faulty and over-simplified, given that it does not clearly recognize the term “forest” as a functioning complex ecosystem that relies on connectivity and relationship. Most definitions regard a forest simply as a collection of trees:

“A collection of trees that grow within a city.”—Para Space Landscaping

A forest or collection of trees that grow within a city, town or a suburb. As opposed to a forest park, whose ecosystems are also inherited from wilderness leftovers, urban forests often lack amenities like public bathrooms, paved paths, or clear borders—Wikipedia.

“The trees and shrubs in an urban area, including trees in yards, along streets and utility corridors, in protected areas, and in watersheds. This includes individual trees, street trees, green spaces with trees, and even the associated vegetation and the soil beneath the trees.”—Cities4Forests

“Woodland, parks, gardens, street and square trees, and other green areas within urban agglomerations (here collectively identified as urban greenspace)”—Konijnendijk et al, 2006 

“Trees, forests, greenspace and related abiotic, biotic and cultural components in areas extending from the urban core to the urban-rural fringe.”—Canadian Urban Forest Strategy (CUFS), 2019-2024

Unfortunately, such loose definitions, which exclude ecosystem functionality in describing an “urban forest,” only help to promote mismanagement through lack of understanding. Forests must be viewed by managers as complex functioning ecosystems, not just a collection of trees.

“Forests aren’t simply collections of trees,” argues Suzanne Simard, forest ecologist at the University of British Columbia. We should not confuse ‘trees’ with ‘habitat’. Trees are NOT a forest. Trees only provide major ecosystem services through community and ‘habitat’ as a forest. Intact functioning forests include so much more than trees, such as mosses, fungi, soil and litter, decaying organic matter, undergrowth, insects and other life that together contribute and maintain a functional ecosystem. The complex processes and interactions of this community reach way beyond the forest. These include water and nutrient cycles, and climate.

Tiny Forest: More Than Just Trees

A Tiny Forest is a dense fast-growing native woodland, based on an established forest management method developed in the 1970s by Dr. Akira Miyawaki. The project run by Earthwatch Europe, identifies suitable sites in urban locations, where nature is most needed. The team engage with local communities to plant, maintain and monitor their forest over time.

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