Most California public K-12 schools offer virtually no shadowing

Many of California’s urban, public grade schools have turned into “natural deserts” that lack trees or shade — many of the state’s 5.8 million school-age children bask in the sun during breaks from class as global warming creates dangerous heat waves.

That’s the conclusion of a team of California researchers from UCLA, UC Davis and UC Berkeley who studied changes in tree cover in 7,262 urban public schools in the Golden State from 2018 to 2022.

The ongoing joint project, derived from urban tree canopy maps created by study partners the California Department of Forestry and Fire Protection and the US Forest Service, revealed that 85% of schools lost about 1.8% of their tree cover on average over that four-year period.

The team said the situation looks alarming as of today.

Researchers have also collaborated with non-profits green school america, Which in its 2024 study found that only 6.4% of California public K-12 schools had average tree cover. And more than half of them exist only as decorations at school gates, in parking lots and across campus.

“Extreme heat is becoming a major public health concern in California and across the country, and trees can play a really big role in helping us cool these schools as well as building climate resilience,” said Kirsten Schwarz, UCLA’s lead researcher.

The results of the 2018 to 2022 study, funded by the US Forest Service, were recently published in the Journal of Urban Forestry and Urban Planning..

While 15% of schools surveyed saw gains in tree cover thanks to schoolyard greening projects — particularly in the Central Valley, around Sacramento and in Imperial County — most individual schools surveyed experienced large losses in net tree cover during that time. In some cases, it is added more than 40%.

Among the state’s largest school districts, San Francisco had the highest canopy loss, at 16.3%. At the other end of the spectrum, Sacramento had the biggest gain at 7.5%, followed by Long Beach, which saw a 4% canopy increase.

Schools in Los Angeles showed a slight net loss of 0.5%. The researchers cautioned not to read too much into this mild pattern, as long-term differences in tree cover and shade across the city still make schools in poorer areas far from the ocean shine more brightly than schools with greater sunlight and extreme outdoor temperatures than schools that benefit from their proximity to cooling sea breezes and longer cover from sea breezes.

As part of the ongoing data collection, the team conducted new field research this summer at a subset of schools — some in Southern California, some in the Bay Area and some in the Central Valley. Because of research agreements with various districts, Schwarz said he could not disclose the exact locations.

UCLA researchers conducted a complete tree inventory for 16 schools in each district, counting all the trees they found on campus, mapping their exact locations, identifying the many different species they saw, measuring at the base and crown of the tree and assessing the overall health of each tree.

Along with the UCLA researchers at the school selection in each district were the UC Davis researchers who measured the temperature.

They bring portable weather stations and sensors, as well as swatches of different paving materials such as grass, mulch, turf, rubber and concrete to each site. The researchers took thermal images, measured the air temperature and measured the humidity around the surface material at different times of the day when the children were most likely to be at school. This allowed the team to examine the microclimates that are unique to these campuses over long periods of time.

Measuring outdoor temperatures on school property is important because children spend most of their time at school during the academic year and because their small statures make them vulnerable to heat radiation from the floor, said Alessandro Ossola, an urban plant scientist who led the UC Davis team.

Children also haven’t fully developed the ability to regulate their body temperature the way adults can, making them more vulnerable to extreme heat and potentially hindering their ability to learn.

In addition, Osola stressed that for children who live without grass or protected parks and playgrounds, school may be a place where they can experience a cooler outdoor environment and dirtier surfaces.

“Combining this information — looking at the complete tree inventory and taking extensive temperature measurements on individual campuses — we can better understand the cooling benefits of these trees,” Schwarz said. “We can also see what tree species are present and how well they have adapted to future climate change.”

Schwarz said the team also interviewed local people in each location to find out who cares for trees in schools, what barriers exist that prevent better tree care and what programs exist to facilitate tree care.

There are many barriers to making campuses more welcoming. Schwarz cited a 2024 policy report by her UCLA team that examined the greening of inadequately shadowed schools and policies that make it difficult to make improvements. In some cases, staffing shortages, bureaucratic hurdles, state earthquake safety standards that encourage outward rather than vertical construction and funding models that prioritize low-maintenance campuses have stood in the way of schoolyard greening, the report said.

Schwarz, the urban ecologist, said he was surprised to learn the extent to which regulations dictate the design of some schoolyards requiring non-grass surfaces for sports and outdoor physical education.

Other schools must choose between conflicting long-term priorities, the student report said: Future plans to build additional classrooms to accommodate increased student enrollment could increase the desire to create open shaded spaces.

The Tree Canopy researchers plan to provide each participating school with a tree inventory, analysis of findings, policy recommendations and recommendations to incorporate their study into classroom lessons and educate parents.

The researchers said their main motivation for starting the study was to help communities get more than $150 million in CalFire grants approved by the state legislature that schools can apply for to plant grass and trees on their campuses and reduce heat damage, such as asphalt.

“That’s a really key part of this conversation about greening the schoolyard, because cleanup is that important first step,” Schwarz said. “The overarching goal is how can we increase the investment we’re making in school greening?”

Osula said that in some ways, Californians who want to improve their children’s school yards are playing catch-up even with community intentions and funding sources. It takes decades for young trees planted today to provide the necessary cooling effects that can protect children on a warming planet.

“This is a significant investment that we should have made 20 or 50 years ago,” Osula said. “Now we’re kind of missing the bus.”