In the first edition of Walkable City, I briefly railed against the indiscriminate use of shadow studies and how they can impede the creation of spaces with good edges. Now, as our cities get hotter, good edges become even more essential. A recent study of urban heat islands found a 16-degree difference between the hottest and coolest parts of Washington, DC, on a blistering summer day. No surprise that the coolest areas were in forested places like Rock Creek Park; the surprise was that, street by street, it was the presence of tall buildings that had the greatest impact on local temperatures.
This outcome mirrored earlier studies in Richmond, Virginia. Computer modelling in Australia – where the sun is upside down but no less intense – has determined that the classic 1:1 height-width ratio is an important threshold. It is precisely the “street canyon” effect that most cities fear that creates the shadows necessary to cool our urban spaces. A similar 2016 study in Portland found that no factor was more powerful than building heights in determining local temperature variations.
This conversation reminds me of a favourite photo of mine, tweeted by Jake Schmidt, who witnessed a throng of activists along a street on New York’s Upper East Side, protesting a proposed medical tower, with signs proclaiming “SAVE OUR SUNLIGHT” and “FIGHT FOR LIGHT.” It’s a sunny day, and about 50 participants wander in the bright foreground, squinting. A bit harder to see are the remaining protestors – perhaps 100 – who huddle with their signs across the street in the welcome shade of a medical building.
You want to see something funny?— Jake Schmidt (@TheJakeSchmidt) May 23, 2021
Some Upper East Side NIMBYs are holding a #StopTheTower rally against the New York Blood Center project, right now. In 88° weather, in the sun, with “Save Our Light” signs.
You’ll never guess where fully half the rally attendees are. pic.twitter.com/7TzMhPtAIS
Also noticeable in the photo is the complete lack of trees on the sunny side of the street, the precise thing that would have made it comfortable. I am pleased that I dedicated a full chapter of Walkable City to urban trees but, in retrospect, it could have been a lot more. We are only beginning to understand all the ways that trees are essential to cities.
In fact, were it not for trees, the vast majority of the continental US would be a mostly lifeless desert. Evaporation from oceans only directly moistens land within a few hundred miles of the coasts. Over the millennia, trees have absorbed coastal rainfall and passed it inland, creating the ecosystems that make most of the US habitable.
I can tell you this because I have spent a good part of the past 18 months focused on trees. On 10 August 2020, Cedar Rapids, Iowa, was hit by a record-breaking derecho (straight-wind) storm, with gusts hitting 140mph. In just one hour, the city lost fully two-thirds of its canopy. In a place historically called the “Emerald City” for its lush tree coverage, whose very city seal is a picture of a tree, more than 660,000 trees came down. It is doubtful that any other storm in modern history has killed so many urban trees.
Within a week of the storm, the city manager was on the phone. I had previously redesigned their street network – turning a downtown of half four-lane one-ways into mostly two-lane two-ways – with the expected results. Would I help them get their trees back? The resulting plan, ReLeaf Cedar Rapids, was my first urban forestry plan, and I did it with a ton of help. Working on it taught me more about trees, and reinforced some of the suspicions already voiced in Step 8 about where the practice of urban forestry has lost its way.
I also found a bunch more information on all the good that trees do. Not only do they improve our health, raise our property values, provide UV protection, eliminate urban heat islands, reduce car crashes, and absorb greenhouse gases and stormwater. They also provide vital wildlife habitat – if chosen properly – as they improve student performance and fight crime.
The educational and crime data is astounding. One University of Illinois study of more than 50,000 Washington State middle-schoolers found that “the more tree cover around a school, the better its standardised test scores in both math and reading [are].” This study controlled for 17 other factors including neighbourhood demographics. Remarkably, what mattered was the tree cover close to the school. Neither neighbourhood greenery nor schoolyard grass had a similar impact. “Even if the larger neighbourhood was leafy, students were no better off if the schoolyard wasn’t.” As your child’s school district struggles to improve its test scores, how much is it investing in trees?
Meanwhile, the removal of 646 trees devastated by the emerald ash borer in Cincinnati provided a rare opportunity for a longitudinal study in which other factors were held constant. In this case, the loss of trees was associated with “an uptick in property crimes, assaults, and violent crimes.” Happily, the reverse also proved true. A 38-month randomised trial of 500 properties in Philadelphia found that compared with others left unchanged, those planted with trees experienced a significant reduction in crime, including a 29% drop in gun violence in neighbourhoods below the poverty line.
The fact that trees reduce crime is better understood than the reasons behind that fact. But we need not understand why trees make places safer to act upon our confidence that they do.
Return of the natives
My biggest recent education around trees surrounds their contribution to the food web, something I did not think about much until I met the entomologist Doug Tallamy. Before I read his recent book, I was as likely to recommend a non-native tree as a native one for the streets I designed. I know better now.
In Nature’s Best Hope, Tallamy describes how climate change and unchecked development have launched the Earth’s sixth mass extinction event. Currently, 52% of insects, 13% of birds, and 25% of mammals face extinction risk due to habitat loss. Key to human survival is the pollinators, which include butterflies, bats, hummingbirds and especially bees. We rely on pollinators for 30% of our food crops and 87% of all plant life. In many places, bees are experiencing ‘colony collapse disorder’. In the past 50 years, half of the US Midwest’s native bee species have disappeared.
Just as crops, plants, and trees need bees and other pollinators, pollinators need trees, both as habitat and for food. And only native trees are food because native insects can’t eat non-native plants.
For many years, urban foresters have debated the importance of planting native trees in streets and yards rather than the wide variety of attractive imported species that have proliferated across the continent. What the foresters have questioned, the biologists have now confirmed: non-native trees can provide many useful ecosystem services, but one thing they do not do is support the food web. In fact, they actively undermine it.
The reason for this is simple: plants and animals have evolved in tandem across the millennia. They change very slowly. It literally takes hundreds of thousands of generations for insects to alter their diets. Moreover, most creatures are diet specialists; 90% of insect herbivores are restricted to eating just a few plants. Monarch butterflies, for example, have evolved to eat milkweed, and milkweed is the only thing they can eat.
In this way, native plants support native wildlife, while importing foreign plants does not cause foreign wildlife to populate our shores. Case in point, Tallamy notes that “after nearly 500 years in residence in North America, Phragmites australis, the common reed, supports only 3% of the insects it supports in its European homeland [5 species vs. 170].”
But it gets worse. Imported trees and plants arrive without their collection of natural enemies, “the insects, mammals, and diseases that keep them in check in their homeland”. This gives them an unfair advantage over native species, which they can quickly overrun. Planting an imported tree is not merely a missed opportunity to plant a native tree; rather, it usually leads to a net loss in native trees overall. And fewer native trees means less food for native creatures.
The tree list we created for Cedar Rapids does not rule out all imported tree species. Rather, they are “contingent”: the city is instructed to never plant a non-native tree in a location where a native tree will thrive. Meanwhile, local nurseries are ramping up production of native seedlings as the city plans to spend close to $40m in public and private funds rebuilding its canopy over the next decade.
Streets of character
Google the phrase “beautiful tree-lined street” and tell me what you see: it is likely image after image of streets flanked on both sides by rows of nearly identical trees, something that many cities no longer allow. When I argued ten years ago against the typical urban forester’s tutti-frutti approach to diversity, I thought that I was offering readers a reasonable trade-off: place-making versus resilience. Both have their value, and it seemed worth it to trade a little of the latter for a lot of the former.
It turns out I was wrong, but not in the way you may suspect. Like many people who are paid to know better, I was suffering from a superficial understanding of diversity. For decades, urban foresters have regarded these street-by-street “tree monocultures” as unnatural and unhealthy. That was a mistake. As Peter Wohlleben teaches in The Hidden Life of Trees, the most resilient groups of trees are family members that help one another. Same-species trees are most likely to intertwine their roots underground, where they share nutrients and information, and support each other against dangerous winds.
It is essential that cities maintain an overall diversity of tree stock to protect against future blights. In 1999, Frank Santamour of the National Arboretum suggested what has become known as the 10-20-30 Rule: cities should plant no more than 10% of any one species (like white oak), no more than 20% of any one genus (like oak), and no more than 30% of any one family (like beech, to which oak belongs). Since then, this advice has become standard practice. But the mandate for overall diversity should not be confused with demanding diversity on every block, where an individual species can be used to establish a continuous canopy and make resilient places of unique character.
Santamour put it this way: “Strips or blocks of uniformity should be scattered through the city to achieve spatial as well as biological diversity. Twenty to fifty trees of a single species, or even a single clone, do not constitute a “dangerous” monoculture… Genetic diversity is achieved by mixtures of uniformity.”
Many streets in our cities currently enjoy a pleasant potpourri of tree species, and that should not change. But when the beauty of local uniformity is possible, that is a goal that should drive our plans. This would seem to be a lesson that most contemporary urban foresters need to learn.
[Read more: Unlocking the power of green spaces in cities]