It’s time that we upped the ante on the level of public discussion on the urban forest. What is the urban forest you ask? Well it’s those pleasant remnant stands of forest that speckle the urban landscape, like Vancouver’s Stanley Park – but it’s also every other tree in the city, from that old oak tree on the front lawn to those pitiable and short-lived trees sprouting from the concrete downtown. Trees have been the recipient of a growing urban celebrity over the past years. I subscribe to Google News alerts, so whenever the words “urban forest” appear in a news article, I get an update. As these alerts were chiming away in my inbox in increasing abundance, I noticed that a solid 90% of the headlines were all hailing the benefits of trees for the city dweller. And I thought to myself, the other half of the story is missing.

First – why all the fuss? A lot of this celebrity can be attributed to the growing attention given to the urban forest by academic researchers. City forests, with all their idiosyncrasies, are attracting more and more researchers, who in turn are discovering more and more benefits of having trees in cities. An early example of this was the “View through a Window” [1] study, which showed that hospital patients with a window view of trees and natural settings recovered faster from surgery. Since then, research has uncovered a slew of other benefits selflessly provided by urban trees, from reducing crime rates in Baltimore [2] to increasing the value of homes in Finland [3].

The next critical point in the urban forest timeline came with the development of a computer model. The United States Forest Service created the i-Tree [4] model (suite of models, actually), which synthesizes scientific research into an off-the-shelf piece of free software that can be used by non-experts – usually city staff or community groups – to inventory their urban trees and actually put numbers and dollar figures to their value. Toronto recently conducted an analysis of its urban forest using i-Tree in 2008 [5], and found that the bare-bones replacement value of the urban forest is a whopping seven billion dollars. Moreover, they provide Torontonians with sixty million dollars in ecosystem services.

There are also some more esoteric benefits from trees that are steeped in the realms of the social sciences that I don’t fully comprehend. Trees provide place attachment and a sense of identity for people to their streets, parks, neighbourhoods, and city [6]. In Japan, people partake in Shinrin-yoku (literally, forest bathing), where they immerse themselves in the woods to reap the spiritual benefits [7]. Even though they cannot be as easily affixed with a dollar value in computer models, these psychological benefits that arguably all city residents experience, knowingly or not, are really where the rubber hits to road in terms of ecosystem services.

Amazing, right? Absolutely. So how can we blame the media for the fluffy and abundant coverage of city trees of late?

We can’t. But ending the narrative at this point paints an incredibly unbalanced picture of trees in the city. To the passerby it seems as though we have always had trees in the city, and just recently science came along and told us that they’re tops. Urban trees and the urban forest ecosystem, however, are far more complex (and frankly a little gloomier) than the happy-go-lucky altruists we are making them out to be.

City trees are actually subject to an appalling amount of stress, disregard, injustice, and blatant abuse. First off, the urban natural environment in general is degraded – without question – because we live in it and build things and pave surfaces and throw stuff out in it. We plant plants that shouldn’t be there, which become invasive. We introduce nasty critters and blights like the Dutch elm disease, Asian longhorn beetle, and most recently the emerald ash borer. We plant trees in compacted and degraded soils or tiny little concrete planters known by arborists as tree coffins. We chop off their limbs in a brutish manner to accommodate our utility lines. We gash their boles with lawnmowers, snowplows, and bicycle locks. We even vandalize trees! It’s just a hard-up life for John Q. Maple and Joe the Plumb Tree, trying to carve out a little piece of soil to call their own, make a living at providing us with ecosystem services, and raise a family of seeds.

All of these issues that face urban trees, and urban ecosystems in general, lead to a somewhat counterintuitive conclusion: the urban forest needs us. Unlike the tried, tested, and true forms of conservation in the hinterland where we can simply leave nature alone and it will mend itself of our transgressions, the urban forest ecosystem demands a new type of conservation; a new type of management. A seminal journal publication on urban forestry stated clearly that “sustainable urban forests require human intervention” [8]. So what does this mean? Let’s examine.

Enter Toronto, my hometown and the biggest city in Canada. Take a stroll for a moment along the Lower Don River trails; down the stairs at the pedestrian bridge in Riverdale Park and into one of Toronto’s key green features. The stately silver maples and graceful magnolias that lined your walk through the Victorian neighbourhood of Cabbagetown to arrive here are transformed abruptly into a dark and tangled wood, with ragged stands of gnarly old willows and bulging Manitoba maples, dripping with equal parts of invasive Virginia creeper and torn-up plastic bags.

Another key feature along the trail is a dense canopy of Ailanthus altissima, known somewhat ironically as the tree of heaven. This tree is also referred to as “Ghetto Palm” [9] because of its prevalence in unmanaged – and frequently poor – urban neighbourhoods. Before your arrival in Caggabetown, walk through some contrasting high-density neighbourhoods and look along building foundations, fencelines, and alleyway potholes and you will see the tree of heaven growing from mere cracks in the asphalt.

This species originally calls from China, and was brought first to Europe then to North America as an alluring ornamental wrought in the mysteries of the Orient. Its insatiable capacity for growth earned it the title tree of heaven, since it grew with such vigour towards the skies. This made it an ideal candidate for the harsh urban environment, where it showed utter disdain for the aforementioned assaults on city trees, and indeed flourished. However, the tree’s vigour soon became a problem. Its amazing capacity for growth meant that it could establish within naturalized, unmanaged forest ecosystems, like the Don Valley, and outcompete the region’s native tree species with ease. Moreover, the tree is allelopathic, meaning that it produces biochemicals that stop other (i.e., native) species from growing anywhere near it. The tree of heaven may be the poster child of urban forests without intervention.

Go a little farther north along the Don trail, underneath the majesty of the Bloor Viaduct, and you enter a sunny meadow. But in this seemingly appealing meadow can be found an unnavigable bramble of some of Ontario’s most notorious invasive plants: Japanese knotweed, garlic mustard, and endless fields of dog-strangling vine. Perhaps not coincidentally, this ecologically-marginalized meadow is also home to some of the City’s socially-marginalized homeless populations that inhabit camps just beyond the treeline.

Please don’t misinterpret me. These Lower Don trails are my favourite spot in the City precisely because, among many other benefits, they are so fascinating for the urban ecologist. And to be fair, incredible work on ecological restoration and management is indeed already in progress in the Don River Watershed at the hands of the Toronto and Region Conservation Authority [10]. What the Don does in the context of this story is provide a case and point that urban trees and forests are incredibly vulnerable, and left to their own devices to grow in the city will likely suffer. They demand varying approaches to management – human intervention.

The form of that human intervention is critical, however. There exist hidden vulnerabilities of the urban forest. These vulnerabilities are dangerous because of their relative invisibility, but perhaps even more so because they can manifest while we are trying to help the urban forest. In response to the aforementioned stream of news stories about the benefits of trees (which, to be fair are legitimately amazing), a natural reaction is to plant even more trees. But what would happen if all those planted trees were the same species? It would make the urban forest highly vulnerable to both existing and yet-to-be-introduced insects and diseases that eat and kill trees. These forest pests often target one species or one genus, so when the urban forest is composed largely of those species and genera, we lose them all at once! The most notable disaster of this kind was in the mid-1900s, when North American cities often planted nothing but elm trees along streets. Then the Dutch elm disease arrived, and it transformed these green-canopied, sun-dappled boulevards into barren and exposed tree-less streets. The unexpected horror that was felt by society in this instantaneous loss of their trees helped to birth the concept and practice of urban forestry in North America [11].

Now, what would happen if we planted all those trees in just one happy summer, and we planted so many trees that we didn’t need to plant more for a hundred years? Well, when all of those trees simultaneously enter their winter years and eventually succumb to the ravages of time, we again lose them all at once! Moreover, forests and urban forests alike are far more susceptible to falling over in storms with an over-abundance of old trees of the same age. And we will certainly be seeing a lot more storms in the coming decades. This latter scenario happens more frequently than logic would dictate. Everyone loves scotch. But imagine if distilleries only waited to start the next batch of twelve-year old single malt when the last batch ran out? That would be a pretty bleak eleven years.

As you are starting to guess, managing an urban forest is hard. Because of the long life of trees, the urban forest ecosystem, like all forests, is shaped by very slow processes, so management must have considerable foresight to avoid potentially delayed and disastrous effects. Management also needs to be intensive and continuous to combat all those tree injustices we talked about earlier.

Management needs to be inclusive – often over half of a city’s urban forest is situated on private residential property and subject to the whims of the homeowner, for better or for worse. This also means that city foresters can’t get their hands a notable chunk of the trees in their jurisdiction. Consequently, municipal governments can’t be expected to shoulder the whole burden of management. Outreach, educational material, by-laws, and incentive programs to engage homeowners and businesses in urban forest management are a must.

The complex ownership regimes of cities really put a stick in the spokes of sustainable management of the urban forest too. In those endless swaths of Canadian boreal forest that we typically associate with forestry, a single forest products company will manage hundreds of thousands of hectares of Crown forest. In the city, a single hectare of urban forest might be under the management (or mismanagement) of dozens of homeowners, small business, institutions, random pedestrians, and city arborists and foresters, all with differing values and objectives. This is complicated even further by the fact that the product being harvested is not timber, but psychological benefits and other such intangible amenities that care little for property boundaries. Lastly, urban forest management needs to transcend the various socio-political quandaries that we create in cities, like policy gaps and a lack of enforcement, development pressure and densification, insufficient municipal urban forestry budgets, and a flat-out lack of awareness.

These are my concerns, if we tell just one side of the story.

So remember, the next time you take a leisurely stroll through your neighbourhood, enjoy the view of the park out of your office window, dabble in some Shinrin-yoku, or enjoy any number of the ecosystem services city trees provide, please be sure to ask what you can do for your urban forest, because it needs you.


1. Ulrich, R. S. (1984). View through a window may influence recovery from surgery. Science, 224, 420-421.

2. Troy, A., Grove, J. M., & O’Neil-Dunne, J. (2012). The relationship between tree canopy and crime rates across an urban-rural gradient in the greater Baltimore region. Landscape and Urban Planning, 106, 262-270.

3. Tyrväinen, L. (2000). Property prices and urban forest amenities. Journal of Environmental Economics and Management, 39, 205-223.


5. City of Toronto. (2008). Every tree counts: A portrait of Toronto’s urban forest. Retrieved from

6. Jones, O., & Cloke, P. (2002). Tree cultures: The place of trees and trees in their place. London, UK: Berg.


8. Clark, J. R., Matheny, N. P., Cross, G., & Wake, V. (1997). A model of urban forest sustainability. Journal of Arboriculture, 23, 17-30.

9. Burkholder, S. (2012). The new ecology of vacancy: Rethinking land use in shrinking cities. Sustainability, 4, 1154-1172.

10. Toronto and Region Conservation Authority. (2009). Don River watershed plan: Beyond forty steps. Retrieved from–

11. Jorgensen, E. (1986). Urban forestry in the rearview mirror. Arboriculture Journal, 10, 177-190.