Shrinking tree canopy at California schools could put kids at risk of extreme heat

The tree canopy is shrinking across thousands of California schoolyards, which may leave children more vulnerable to the harmful effects of extreme heat, according to a recent study.

In the research, published in the journal Urban Forestry & Urban Greening, scientists looked at snapshots of more than 7,000 public schools in urban areas across the Golden State. The analysis included schoolyards and the areas immediately surrounding them. One image, taken in 2018, provided baseline information about each school's existing tree canopy. The next, taken in 2022, showed whether that canopy had increased, decreased or stayed stable.

Across the schools evaluated, the majority saw some amount of tree-canopy decline between the two time points. "A major important takeaway is that across the state, most schools (85%) experienced tree canopy loss during the study period," Dr. Lindsey Burghardt, chief science officer for the Center on the Developing Child at Harvard University, who was not involved in the work, told Live Science in an email.

Some schools lost more than 40% of their canopy cover over the four-year period, according to the report. This percentage change was relative to each school's baseline tree cover, so these declines were "even more concerning" to see in areas that had poor canopy cover to begin with, the authors wrote.

Past research had already shown that tree cover on California's school campuses tends to be lower than the average canopy cover across the cities in which they're situated. "The urban tree canopy cover in California is pretty low, but in school grounds, it's even lower than the average, on the city level," study co-author Alessandro Ossola, an associate professor in the Department of Plant Sciences at the University of California, Davis (UC Davis), told Live Science.

Given the amount of time children spend at school, these campuses are an important venue for kids to exercise and play outdoors, the authors argue, and Burghardt agrees.

"The existing literature confirms that increasing temperatures in schools affect children's ability to learn and play, and this can affect their physical and mental health in childhood and beyond," she said. "Young children's bodies heat up more quickly than ours, and they are less able to cool themselves by sweating." This overheating can affect multiple bodily systems, undermining cognitive function and concentration, activating the body's stress response, and driving dehydration that can injure organs, particularly in children with existing chronic conditions.

But as average temperatures rise due to climate change, droughts and extreme heat events are expected to become more frequent, prolonged and severe across California. In urban centers, those effects can be amplified in "heat islands," where buildings and roads emit more heat than would landscapes with more natural features, like trees and bodies of water.

"The impact is exacerbated because there's so much built infrastructure that actually accumulates the energy and then radiates it out again," said study co-author Luisa Velásquez Camacho, a postdoctoral scholar at UC Davis. Trees cool their surroundings, both by providing shade and by releasing water into the air from their leaves, she told Live Science.

"A tree is like an air conditioning unit because it's actually providing evaporative cooling," Ossola said. By comparison, artificial shade structures, which are notably expensive to install, can end up radiating heat across a playground rather than cooling the air as a tree would, he said.

"It's [like] the broiling function in the oven, when it comes from the top," added study co-author Moreen Willaredt, another UC Davis postdoc.

In summary, trees can be powerful mitigators of heat, so the researchers wanted to take stock of the state of trees located around California schools. The dataset they used enabled them to examine changes in the trees' canopies with a "fine degree of resolution," Burghardt said.

Areas of canopy loss were dispersed across the state, but there were pronounced clusters in the Central Valley and certain parts of Southern California. Hard-hit counties included Tulare, Fresno and San Bernardino, which generally had relatively poor canopy cover at baseline. In the Central Valley, the study authors suspect that prolonged drought conditions and frequent extreme heat events may have contributed to this decline, but their study wasn't designed to pinpoint the exact drivers of the trend.

About 15% of the schools included in the study saw their tree canopies increase, generally by more than 20%, "highlighting areas of remarkable canopy recovery or growth," the authors wrote. Imperial County and San Joaquin County stood out as having marked increases, and some pockets of growth were also spotted in the Central Valley, suggesting that there's a "mosaic" of trends across that region.

The study authors hope their work informs future greening initiatives to help create and restore canopy cover at schools. "What we are trying to do is to essentially see where the gaps are, and [find] opportunities to prioritize the limited amount of trees going into the ground," Ossola said.

"I agree that this is an important next step," Burghardt said. "All children deserve to learn and play in an environment that supports their healthy development, and access to safe greenspace is a key element of a healthy developmental environment."