Want to find schools in satellite images? Researchers say you can spot them by looking at tree cover because schools stand out as rectangular holes in the urban canopy.
Even though access to nature offers a variety of health and social benefits for students, researchers at the University of California (UC), Davis have found that trees on school grounds are declining across California. Declining tree canopy at schools can raise temperatures to dangerous levels, forcing kids to miss out on the benefits of spending time outside.
The researchers also conducted a field study to show how much schoolyard trees influence temperature. “Our motivation is thinking about a kid of around 8 years old playing in the schoolyard with their friends,” said UC Davis urban forestry scientist Luisa Velasquez-Camacho. “It’s very nice, but when you translate this scenario to Sacramento or the Central Valley at 2:00 p.m. in the hottest months, this is a nightmare because they don’t have natural shade.”
“Shade Is King”
To track changes in tree cover at schools, the researchers examined CalFire (California Department of Forestry and Fire Protection) tree canopy maps for more than 7,200 urban schools in California between 2018 and 2022. By quantifying the tree cover, they found that 85% of the schools had experienced tree loss over that time span, and some Central Valley school districts lost 25% of their tree cover. Schools had less than half the tree cover of surrounding urban areas. The results were published in Urban Forestry and Urban Greening.
“I can’t say the results are surprising,” said Kevin Lanza, an assistant professor of environmental and occupational health science at UTHealth Houston who wasn’t involved in the study. He said the findings align with existing studies on urban forestry and noted that trees can be lost in schools to make way for building expansions or because the cost of maintaining them is prohibitive. “Schools are more stressed than ever,” he said.
Scientists collected data such as temperature, radiation, and wind at children’s height. Credit: Emily C. Dooley, UC Davis
The researchers wanted to do more than document the loss of tree cover in schools; they wanted to investigate the health cost of losing those trees. To that end, said Alessandro Ossola, an ecologist at UC Davis and a coauthor of the research, “we took to the streets” in the summer of 2025, spending long days collecting weather data at school playgrounds across California.
The researchers deployed sensors collecting data on air temperature, humidity, radiation, and wind placed at children’s height around each playground. Using these data, they were able to calculate the thermal index, which is a measure of how the environment feels to a human body.
Then, they walked a sensor-laden cart around each playground—racking up over 200 miles (322 kilometers) over the summer—to map out microclimates. The researchers also scanned thermal radiation from common playground surfaces, including dry and irrigated grass, mulch, asphalt, and rubber.
Researchers walked a sensor-laden cart over 200 miles (322 kilometers) this summer while studying California playground temperatures. Credit: Jael Mackendorf, UC Davis
Although the team hasn’t fully analyzed the data yet, early results indicate that rubberized surfaces, often found around playground equipment, are particularly dangerous for reflecting radiation. “It was ridiculous for us to stay out there in the afternoon, even as adults. A kid is much closer to the ground,” Ossola said.
They saw the heat index reach 120°F (54°C) at some schools, and a single tree could drop surface temperatures by as much as 30°F (17°C) compared to direct sunlight. But while the air temperature often wasn’t dramatically different between direct Sun and shade, the thermal index dropped considerably under the shade because of the effects of radiation.
“Shade is king.”
“Shade is king,” said Lanza, and while artificial shade is better than nothing, trees can lower temperatures even more because the water vapor produced by evaporation from the tree leaves absorbs even more heat.
Once trees are lost, planting and maintaining replacement trees until they grow big enough to offer shade are a major hurdle. The researchers suggested that after their full analysis, the results could help guide schools on where to plant new trees and what species of trees will provide the greatest benefits.
Finding a Schoolyard Shade Strategy
Finding ways to manage temperatures is vital for children’s development because if temperatures rise too high, students are forced to remain inside, and for many, recess is their only chance to be in nature. Time spent in nature increases well-being and helps build healthy physical activity habits. UC Davis researchers are also conducting studies that suggest time outside can improve academic performance.
“It’s a matter of reenvisioning trees as an asset that can be budgeted.”
Lanza also noted that “low-income and Black and Latino communities are seeing larger losses of canopy than other communities,” indicating that the impacts of losing time in nature are likely not equitable across populations.
The ongoing work by universities and Green Schoolyards America, a nonprofit partner in this research, aims to use the findings to advocate for strategic investments in trees and other plants to improve students’ time spent outside. “It’s a matter of reenvisioning trees as an asset that can be budgeted,” Ossola said. “If we are negating these opportunities to be close to nature, we are missing the bus, not just for academic outcomes but also in terms of public health in the future.”
—Andrew Chapman (@andrewgchapman.bsky.social), Science Writer
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Citation: Chapman, A. (2025), California schools are feeling the heat,
Eos, 106, https://doi.org/10.1029/2025EO250458. Published on 11 December 2025.
Text © 2025. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
