Cities need more trees

01 Jul, 2024

I grew up just outside of Johannesburg, which is touted to be "the greenest city in the world". During the summer, when seen from above, it's a verdant, green landscape, with the tops of buildings popping through the canopy.

Yes, there's a full city beneath those trees.

However, this is not what the land originally looked like.

To cut a long story short, Johannesburg was established following the discovery of gold in 1886. It's known colloquially as eGoli, or "The City of Gold". Gold extraction here is vastly different to other parts of the world. We don't have gold veins running through the rock like in old Western movies. Instead it's mineralised into the rock itself which requires significant processing to extract.

This form of extraction leads to the crushing of millions of tons of rock, which is then dumped creating large, dusty hills around the city. These hills are (especially to the communities surrounding them) toxic and radioactive¹, and when the wind picks up yellow dust clouds used to envelop the city.

One of the proposed solutions to suppressing this omnipresent dust was the planting of trees (1.2 millions as of 2024) on sidewalks and pavement, and millions more planted privately in back-yards and plots. Due to Apartheid in South Africa, which economically and socially disadvantaged the majority of the population based on race, these planting efforts were not equitably distributed. You can still see a stark difference between rich and poor neighbourhoods to this day based purely on tree cover.

But I'm not here to talk about the socioeconomic problems facing South Africa. Instead I'd like to focus on how the planting of so many trees affected the city itself.

Since these trees were originally planted to manage dust, they are generally big and leafy. This has the benefit of creating a lot of shade throughout the city, mitigating a lot of the "heat island" effect which is pervasive in any city since asphalt and concrete are great at absorbing visible spectrum light and radiating it as heat². Walking down a shady street on a hot summer day is such a pleasant experience when compared to being out in the blistering sun.

Trees are not just great dust sinks and heat shields, but great sound barriers as well³. Having tree lined streets not only reduces the noise of traffic (and the dust kicked up from their tyres), but also protects pedestrians and infrastructure on the sidewalks from stray vehicles.

On top of that it just looks better. I'm certain humans have genetic biophilia⁴, which is why we love being in nature or taking walks in the forest. Having trees around us, teeming with birds and other life just feels good. Speaking of birds, trees increase the biodiversity of insects and other small critters in urban environments. It also gives birds a safe-haven from the deadliest hunter of all: the humble house cat⁵.

I've been to many cities, all over the world. Some that do trees well, and others that fail miserably. And I'm always shocked at the contrast. Given two cities with a similar climate, I'm significantly more likely to do things like walk to a local coffee shop and browse corner stores, or sit on a park bench. But only if the walk is pleasant and shady.

Planting trees as a form of environmentalism or carbon capture is generally a hit-or-miss strategy. The vast majority of planted trees never reach maturity⁶, and depending on where the trees are planted it could have a net heating instead of a net cooling effect due to the change in the environment's albedo⁷. However, planting trees in cities is pretty much all upside with almost no downside (except that birds tend to shit on my car).

So the next time you're enjoying a walk down a lovely shady street, take a look up and appreciate the trees.

Footnotes