Is planting trees good for cities? Why this question misses an essential truth

It’s no secret that city dwellers benefit significantly from having trees line our sidewalks and fill our parks. But how does city life change trees? A pair of new studies show why when it comes to trees and urban environments, there is no such thing as a one-way street.

Scientists already knew having access to green spaces can improve mental well-being, and even prevent premature deaths. They have also touted planting trees as one of the most effective ways to store carbon and halt the climate crisis, though some studies question the feasibility of planting trees on such a large scale.

The new research supports the idea of urban trees as good for people and climate, but it also reveals the relationship is not so simple: It’s not just that trees affect cities. Cities also affect trees.

“When we plant trees in cities, we also need to consider their growing season and how that is affected by urban environments,” Lin Meng, a postdoctoral scholar at the Lawrence Berkeley National Laboratory, tells Inverse.

What’s new — Two papers published this week reinforce and also complicate the mutual ties between trees and urban environments.

The first paper, published in the journal Nature Communications, uses high-resolution satellite data to analyze, for the first time, the effectiveness of urban trees in reducing surface temperature compared to treeless green spaces.

“The most important innovation is that we compare temperatures of trees, green spaces — without trees — and urban fabric in many cities and across a range of different climates/regions,” Jonas Schwaab, lead author on the study and a researcher at ETH Zurich, tells Inverse.

The second paper, published this week in the journal Science, examines the effect of cities themselves on the changing colors of leaves in autumn as well as the spring “green-up.” It uses data gathered from 85 rural and urban areas in the largest U.S. cities between the years of 2001 and 2014.

“Spring green-up is the date when trees start to grow leaves and turn green in spring, as seen by satellites,” Meng explains.

How do cities change trees?

The Science study suggests city living has two surprising effects on trees:

1. The spring green-up occurs six days earlier in cities compared to rural areas due to hotter temperatures in cities.
2. Artificial lights in cities also extend urban trees’ growing season “by making the spring green-up occur earlier and autumn leaf-coloring occurs later, compared to rural ones.”

“Warmer urban temperatures make the spring green-up occurs earlier in cities, but it also makes spring green-up less sensitive to temperature changes,” Meng says.

The change in leaf coloration is partly due to the urban heat island effect. This is when higher temperatures — between 1 to 3 degrees Celsius — occur in urban areas due to a lack of natural tree cover, among other factors.

The finding has implications for the vitality of urban trees and the health of humans living near them.

“The changes in the growing season could also affect the timing and severity of pollen season, Meng says, “which is likely to increase the risk of pollen allergies for humans.”

Rising temperatures due to the climate crisis will only make these effects more pronounced, too.

Meng adds, “With every one degree [Celsius] warming, spring green-up shifts earlier less in urban than in rural areas.”

How do trees change cities?

The Nature Communications study reports an equally striking effect of trees on city temperatures as compared to treeless urban green areas, also known as “urban fabric.”

Trees were two to four times more effective than treeless green spaces in cooling cities — a finding that could help cool warming cities through the climate crisis.

“Our results show that trees are almost always cooler than urban fabric, but green spaces more often have about the same temperatures as urban fabric and hardly any cooling effect, particularly in hotter and drier regions of Europe,” Schwaab says.

Schwaab stresses cities — even those covered in trees — will ultimately not keep humans cool during the climate crisis for two specific reasons:

1. Limited space: City planners can only plant so many trees on so many sidewalks.
2. Limited power: The cooling effect of trees may decrease in hotter and drier climates — an important caveat as climate change makes droughts more frequent.

“Increasing the number of trees is in my perspective important,” Schwabb says, “but it is not a solution to climate change and it should not lead to what is often described as ‘moral hazard,’ meaning that we may think we can prevent the worst by planting some trees in cities.”

Why it matters — The two papers come at a pivotal time in the climate crisis, as both scientists and politicians recognize the necessity of trees as essential “carbon sinks” that store and prevent carbon from being released into the atmosphere.

World leaders at the recent COP26 climate conference pledged to end deforestation by 2030 in the Glasgow Climate Pact.

Similarly, the recent U.S. infrastructure bill, which President Biden signed into law earlier this month, allocates tens of billions of dollars toward making cities more resilient from the effects of the climate crisis, including extreme heat resulting from the urban island heat effect.

We’ve already known that green spaces can reduce temperatures in urban areas, but the new Nature Communications study shows that trees, specifically, may help keep us cool as the climate crisis literally heats up.

But these two papers also suggest something new to consider: Trees aren’t only important to cities for their ability to absorb carbon that buildings and cars emit. They’re also important because of their symbiotic relationship with cities and the people who live in them.

As urban planners design resilient cities and green urban office spaces, they need to consider how trees shape urban life, and the reciprocal effect of cities on these plants that are so essential to life on Earth.

What’s next — So, what do these studies mean for the future of planting trees in urban areas? It’s complicated.

While Schwaab acknowledges his study could be taken as “a good argument” for planting trees over simply having green spaces in cities, he also stresses there are also limitations to his study — and this argument. He says there is a need for further data “on the effect of trees and green spaces on air temperatures and human thermal comfort.”

“We need to better understand if the cooling potential of trees will change in a future climate,” Schwaab says.

Trees could have certain unanticipated negative impacts, such as absorbing longwave radiation from buildings, which could lead to slightly warmer nighttime temperatures. Another downstream effect could include trees blocking colder air flows coming from outside the city.

For her part, Meng doesn’t think we should stop planting trees but states some short-lived trees “are more susceptible to the changing urban environment such as the urban heat island effect” that put them at greater risk of spring frost, which could kill the trees.

“My study doesn’t change the need to plant trees in cities, but adds on that by saying when we plant trees in cities, we also need to consider their growing season and how that is affected by urban environments,” she concludes.

Abstract for Nature Communications study: Urban trees influence temperatures in cities. However, their effectiveness at mitigating urban heat in different climatic contexts and in comparison to treeless urban green spaces has not yet been sufficiently explored. Here, we use high-resolution satellite land surface temperatures (LSTs) and land-cover data from 293 European cities to infer the potential of urban trees to reduce LSTs. We show that urban trees exhibit lower temperatures than urban fabric across most European cities in summer and during hot extremes. Compared to continuous urban fabric, LSTs observed for urban trees are on average 0-4 K lower in Southern European regions and 8-12 K lower in Central Europe. Treeless urban green spaces are overall less effective in reducing LSTs, and their cooling effect is approximately 2-4 times lower than the cooling induced by urban trees. By revealing continental-scale patterns in the effect of trees and treeless green spaces on urban LST our results highlight the importance of considering and further investigating the climate-dependent effectiveness of heat mitigation measures in cities.

Abstract for Science study: Phenological events such as leaf out or flowering are shifted earlier under climate warming. Yet complex urban environments may affect phenology even more than climate warming and have cascading ecosystem effects. I explore how the timing of spring green-up is altered in US cities compared to in rural areas and what causes these changes. I discovered that urban warming and artificial light at night shift spring green-up earlier in these cities, suggesting an extended growing season under future warming and an ever-brightening night.