MADISON – The shade of a single tree can bring relief from the hot summer sun. But when a single tree is part of a small forest, it creates a deep cooling effect. According to a study published today in Materials from the National Academy of Sciences, trees play a big role in maintaining the cold in our cities.
According to the study, the right amount of wood cover can reduce daytime temperatures by as much as 10 degrees Fahrenheit. And the effect is quite noticeable from the neighborhood to the neighborhood, even to the scale of a single city block.
"We knew cities are warmer than the surrounding countryside, but we've found that temperatures vary the same in cities. Keeping temperatures more comfortable on hot summer days can be important for those of us who live and work there, "says Monica Turner, professor at the University of Wisconsin-Madison at the Department of Integrative Biology and co-author of the study.
Along with climate change, which causes more and more extreme extreme heat events every summer, city planners are working on preparation. Thermal waves increase the energy and costs demand and can have a big impact on human health. One of the potentially powerful tools, say the authors of the study, are organisms that existed long before human civilizations could appreciate their leafy benefits. And these trees can be the secret to keeping living places.
Basically, says Turner, impermeable surfaces – such as roads, sidewalks and buildings – absorb the heat of the sun during the day and slowly release heat at night. Trees, on the other hand, not only shade these surfaces from the sun's rays, they also move or release water into the air through their leaves, a process that cools things down.
To get the maximum benefits from this cooling service, the study found that tree crown coverage must exceed 40 percent. In other words, an aerial photograph of one city block would have to be almost half covered with a green network of branches and leaves.
Traditionally, says Carly Ziter, the head author of the article, studies like these focus on the effects called the "urban island of heat." These studies often use satellites to read the surface temperature of the earth or measure the air temperature inside and outside the city. Studies have shown that the developed, less vegetative urban landscapes are much warmer than the rural areas around them. But these studies, according to Ziter, allowed the researchers to look at the temperatures on a much smaller scale – up to the space "where we live in our everyday life in the city."
It turns out, he says, that the effect of the "island of heat" is more similar to what some scientists call the "archipelago of heat" – with smaller heat islands in the city interspersed with cooler areas of shade.
To get data on a local scale, Ziter and her colleagues had to become creative thanks to their sampling methods.
Satellite ground surface temperature measurements do not really provide air temperature data, says Ziter, "they do not bring you closer to what people feel."
However, the deployment of a sufficient number of air temperature sensors throughout the city to get the precise resolution they wanted was far too expensive. Research at the University of Wisconsin in Madison had temperature sensors attached to 150 poles throughout the city and its surroundings, but these sensors were often one or more miles away – far too far to provide real-time data on the temperatures in the yards and individual boulevards.
Finally, Ziter decided to solve the sampling problem. She only needed one sensor and two wheels.
In the summer of 2016, it was not unusual for Ziter to ride a bike around the city of Madison with a small weather station attached to the back of the bike. In total, she repeatedly cycled through ten different transects of the city at different times of the day. The sensor on her bike meant her position and read the air temperature every second she rode, which gave the data in real time every five meters.
In total, she estimates that she cycled between 400 and 500 miles and was "in very good shape" at the end of the test. She also raised a huge amount of data that showed how instrumental trees moderate heat in cities.
"The tree's canopy can actually do more than just balance the effects of impervious surfaces," says Ziter. During the day, "an equivalent amount of canopy cover can cool the air more than the pavement will warm up."
The data shows that forty percent of the canopy coverage is the threshold required to produce the large cooling effects trees have to offer. The greatest amount of cooling occurs after exceeding this threshold by the scale of the city block or more.
"It's not enough to just go out and plant trees, we really have to think about how much we plant and where we plant them," he says. "We're not saying that planting one tree does nothing, but you will have more effect if you plant a tree, and your neighbor plant a tree, and their neighbor plant a tree."
To get the biggest bang for his money, Ziter says city planners should focus on areas that are close to the forty percent threshold above this sign, planting trees. But he warns that it should be in places where people are active and live, not just in parks. In addition, he says, "we do not want to give up the lowest canopy areas in our city," because these are areas with lower incomes and marginalized communities. "We want to avoid advocating politicians who are simply" rich to be richer, "he says.
Its results, says Ziter, indicate the importance of urban landscape and development in increasing the vitality of districts in the future. It is also a call for stakeholders to work together on their trees. It often happens that "different people are responsible for different spaces" – he says. For example, a city may be responsible for planting trees along streets, while the park department oversees planting in parks, and homeowners make decisions regarding their own private plots.
It is important that we start to enter the same page, says Ziter, because "the trees that we plant now or the areas that we are now hardening will determine the temperatures of our cities in the next century".
From the city square to Times Square, if we want the places we live in to be more comfortable and resistant to future climate scenarios, say researchers from Wisconsin, someone will have to talk about trees.
– Adam Hinterthuer, Hinterthuer@wisc.edu, 608-630-5737
This study was supported by the National Science Foundation, long-term environmental research (DEB-1440297); National Science Foundation, Water sustainability and climate (DEB-1038759); University of Wisconsin-Madison Vilas Trust (for MGT); National Science and Engineering Research Council of Canada doctoral fellowship (to CZ).