Tree canopies are shrinking in thousands of California schoolyards, potentially making children more vulnerable to the negative effects of extreme heat, according to a recent study.
In the study, published in the journal Urban Forestry & Urban Greening, scientists looked at a snapshot of more than 7,000 public schools in urban areas across the Golden State. The analysis included the schoolyard and its immediate surrounding area. One image taken in 2018 provided basic information about each school’s existing tree canopy. The following photos were taken in 2022 and showed whether that canopy had increased, decreased, or stabilized.
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Some schools lost more than 40% of their canopy over four years, the report said. Because this percentage change is relative to each school’s baseline tree cover, it is “even more alarming” to see these declines in areas with low tree canopy cover to begin with, the authors write.
Previous studies have already shown that tree cover on California school campuses tends to be lower than the average tree canopy cover for the entire city in which the school is located. “California’s urban tree canopy coverage is quite low, but on school grounds it’s even lower than the city-level average,” study co-author Alessandro Ossola, associate professor in the Department of Plant Sciences at the University of California, Davis, told Live Science.
Given the amount of time children spend in school, the authors argue, and Burkhardt agrees, that these campuses are important places for children to exercise and play outdoors.
“Existing literature confirms that rising temperatures in schools can impact children’s ability to learn and play, which can impact their physical and mental health from early childhood onwards,” she said. “Young children’s bodies heat up faster than ours and cannot sweat to cool themselves down.” This overheating can affect multiple body systems, impair cognitive function and concentration, activate the body’s stress response, and cause dehydration that can damage organs, especially in children with pre-existing chronic conditions.
But as climate change raises average temperatures, droughts and heatwaves are expected to become more frequent, longer, and more severe across California. In urban centers, that effect can be amplified by ‘heat islands’, where buildings and roads emit more heat than landscapes with more natural features such as trees and bodies of water.
“The effects are getting worse because we’re building so much infrastructure that actually stores energy and radiates it back out,” said study co-author Luisa Velazquez Camacho, a postdoctoral fellow at the University of California, Davis. Trees cool their surroundings by providing shade and releasing water from their leaves into the air, she told Live Science.
“Wood actually provides evaporative cooling, so it’s like an air conditioning unit,” Ossola says. By comparison, artificial shade structures are significantly more expensive to install and can radiate heat throughout the playground rather than cooling the air like trees, he said.
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“the [like] “It’s a sear feature when you top-bake it in the oven,” added study co-author Moreen Willaredt, another postdoctoral researcher at the University of California, Davis.
In summary, trees can be powerful heat buffers, so researchers wanted to investigate the condition of trees around schools in California. The dataset they used allowed them to examine changes in the tree canopy at “high resolution,” Burkhardt said.
Areas of canopy loss were dispersed throughout the state, but significant outbreaks were seen in certain areas of the Central Valley and 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 heatwaves may have contributed to this decline, but their study was not designed to pinpoint the exact drivers of this trend.
About 15% of the schools studied had tree canopy increases typically of 20% or more, “highlighting areas with significant canopy recovery or growth,” the authors write. Imperial and San Joaquin counties saw the most notable increases, and the Central Valley also saw some increases, suggesting a “mosaic” of trends across the region.
The study authors hope their research will inform future greening efforts and help create and restore canopy covers in schools. “What we’re trying to do is basically see where the gaps are. [find] We have the opportunity to preferentially plant a limited amount of trees in the ground,” Ossola said.
“I agree that this is an important next step,” Burkhardt said. “All children have the right to learn and play in an environment that supports healthy development, and access to safe green spaces is a key component of a healthy developmental environment.”
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