CityReads│Ode to Urban Trees

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Ode to Urban Trees

Why cities should plant more trees?

Brad Plumer, 2016. Why planting more trees is one of the smartest things a city can do

Sources: https://www.vox.com/energy-and-environment/2016/11/4/13510352/planting-trees-pollution-heat-waves

https://thought-leadership-production.s3.amazonaws.com/2016/10/28/17/17/50/0615788b-8eaf-4b4f-a02a-8819c68278ef/20160825_PHA_Report_FINAL.pdf

Picture sources:  Jacaranda trees in South Africa, sourced from FB/SA Tourism UK and taken by @unclescrooge (Instagram)

Urban trees are not only good for individual mental and physical health, but also beneficial for the public health.  One study from Toronto shows that after controlling for income, education, and age, an additional ten trees on a given block corresponded to a one-per-cent increase in how healthy nearby residents felt. To get an equivalent increase with money, you’d have to give each household in that neighborhood ten thousand dollars—or make people seven years younger.

Urban trees do at least two important things for public health:

1) They can soak up fine particle pollution from cars, power plants, and factories — an important job, given that particulates wreak havoc on human lungs and kill some 3.2 million people worldwide each year. The precise effect varies from city to city, but generally trees do improve air quality.

2) Urban trees can also cool down neighborhoods anywhere from 0.5 degrees Celsius to 2 degrees Celsius on the hottest summer days, which is vital during deadly heat waves. (Studies have found that every additional 1 degree Celsius in a heat wave leads to a 3 percent or more increase in mortality.) The amount and type of vegetation plays a big role in how much the urbanization changes the temperature. Trees and other vegetation are essential for keeping our cities cool. Trees naturally cool the air by a process called evapotranspiration. It’s a process plants undergo that’s a little similar to sweating, in that released water vapor carries off heat.

The new Nature Conservancy report sifts through all this research and lays out some global scenarios. At the high end, a massive new tree-planting campaign in the world’s 245 largest cities, costing around $3.2 billion in all, could save between 11,000 and 36,000 lives per year worldwide from lower pollution. Those trees would also prevent between 200 and 700 heat-wave deaths per year — with that number presumably going up over time as global warming unfolds.

That’s not all: Because the trees would cool neighborhoods significantly, many households could end up using less energy for air-conditioning — reducing electricity use by between 0.9 and 4.8 percent in some cities and helping slow the pace of global warming. They can also retain storm water, boost real estate values, and may even have important mental-health benefits. Plus, they’d just look nice.

But the trees have to be planted in the right spots

So is that worth the price tag? Trees are often a bargain in this regard. On average, a well-targeted tree-planting campaign is roughly as cost-effective as other strategies for cutting pollution, like getting dirty diesel buses off the road. (That said, the precise cost-benefit ratio varies across cities, and most governments will want to employ lots of different strategies for cutting pollution. Trees alone are never sufficient.)

The Nature Conservancy

There is one catch, though: The tree-planting campaign has to be well-targeted. And that gets a bit complex.

Trees only improve air quality in their immediate vicinity, about 100 feet or so. That means cities need to figure out which neighborhoods benefit most from new trees (typically the densest areas, but also areas around hospitals and schools). They also have to plant species that are most effective at trapping pollution (typically those with large leaves).

Officials also need to account for things like wind patterns and tree spacing and figure out whether they’ll be able to maintain their trees. Plus, if water is scarce, they’ll want to consider drought-tolerant varieties. And they may want to steer clear of trees that increase pollen and allergies.

The report itself offers broad guidelines on planting and tree selection, and the researchers may release more fine-tuned data in the future to help urban planners figure this out. This map, for instance, shows the streets in Washington, DC, where trees have the highest and lowest “return on investment” for pollution reduction. In some cities, the difference can vary by a factor of 100:

Neighborhood-level patterns in the return on investment (ROI) of tree planting to reduce particulate matter for one city, Washington, DC. Streets that are darker green have higher return on investment. (The Nature Conservancy)

At a global level, the return on tree-planting is higher in places like Southeast Asia or Mexico — dense cities with considerable air pollution.

The Nature Conservancy

Finally, here are the 10 cities where the return on investment is absolutely highest for both cutting pollution and reducing heat. Note that many of these cities are likely to be some of the most vulnerable to climate change in the future:

The Nature Conservancy

So what’s stopping many cities from going on a tree-planting binge? Space is sometimes a major obstacle, as is water availability. Thirdly, many cities just don’t think of trees as a public-health measure — they often fall under the purview of parks and recreation departments. That can lead to an underinvestment in urban planting. Finally, there’s maintenance. For many cities, it’s easier to plant trees in the first place — developers may be required to plant them, say. But once they’re planted, they have to be maintained continually, by pruning branches, protecting against disease, and so forth. And that requires proper staffing, higher operating budgets, and so on. Those maintenance costs can be a hurdle for many strapped local governments, especially if trees are mostly seen as an aesthetic luxury.

Source: https://iamaileen.com/cherry-blossom-2018-japan-forecast-

Hence the argument for thinking of trees as more than just an aesthetic luxury. Our conception of urban trees has already changed dramatically in the past. Before the 1600s, street trees were rare, with the Dutch pioneering the practice to help stabilize their canals. It wasn’t until the 19th century that tree-lined boulevards became common in European cities. Today, they’re ubiquitous.

Source: https://twitter.com/i/moments/991528251435638784

As our cities grow, and green spaces are replaced with more impervious surfaces, their temperatures rise too. The answer is pretty straightforward: more trees and plants means cooler cities. And more trees also mean a more pleasant city environment for those living there. We just have to figure out how to build cities and plant trees at the same time.

Click to watch the video, Why cities should plant more trees?

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