If trees had a resume, the trees in Los Angeles would need a serious resume update.
A recent University of Southern California study shows that the city’s urban forest isn’t just providing shade and aesthetics, it’s quietly pulling more pollution out of the air than scientists realized, especially when the summer heat kicks in.
In the Los Angeles community of Mid City, researchers found that trees are removing about 60% of the fossil fuel emissions released during the day time in spring and summer—mostly from cars, trucks, buildings and industrial activities. And over the course of a year, the trees absorb around 30% of emissions.
“What’s surprising is No.1, that’s a really big number, and No. 2, this is not what you and I would consider to be the most green parts of L.A.,” said William Berelson, a USC Dornsife professor of earth sciences, environmental studies and spatial sciences, who led the study.
One reason trees are absorbing so much CO₂ may be their unexpected resilience. Despite L.A.’s famously dry summers, the study found that trees absorb the most CO₂ during the hot season.
Satellite imagery shows that L.A.’s greenery remains remarkably verdant in summer, likely thanks to irrigation, groundwater from leaky pipes, residential watering and drought-tolerant species that continue photosynthesizing even in extreme heat.
To track how trees absorb pollution, Berelson and his team installed a network of 12 carbon sensors across a 15-by-6-mile section of the city stretching from La Brea Avenue and Olympic Boulevard to the USC Hospital in East Los Angeles.
Since Los Angeles has a fairly consistent west to east wind pattern, these sensors continuously measure how much CO₂ and other pollutants the wind picks up, recording data every few seconds as the wind moves the pollution through the city.
The process works much like passengers boarding and exiting a moving train, Berelson said.
“A train is moving down the track and more and more people are getting on the train. So as you get further down, there’s more people on the train and that’s telling you how fast people are coming on the train,” he said. “Or maybe sometimes the train is moving and there’s less people on the train as it goes down. That means people are leaving the train.”
Berelson’s team applies the principle to map CO₂ movement across Los Angeles. As air flows through the urban landscape, sensors detect whether CO₂ levels are rising or falling, taking into account factors such as wind speed, direction and urban density. The data helps researchers determine how effectively local greenery including trees offsets emissions.
Trees play a crucial role in this process through photosynthesis, the natural mechanism that allows them to absorb CO₂ from the air and convert it into oxygen. This ability makes urban forests a key tool in reducing pollution levels, particularly in dense city environments.
Informing the city’s tree-planting strategies
The USC study ran from July 2021 to December 2022. It helped establish baseline CO₂ emission levels—essentially, how much pollution is naturally present in Mid City and other neighborhoods in L.A., before factoring in the role of trees in absorbing emissions.
The findings provide a framework for tracking long-term emissions trends, which could help city officials and researchers measure progress toward L.A.’s goal of reaching zero emissions by 2050, Berelson said.
By increasing the number of CO₂ sensors across L.A., and continuously monitoring emissions over time, researchers can map where emissions are highest, track whether they are decreasing, and evaluate the effectiveness of climate policies.
Their findings could also help guide tree planting efforts, ensuring that greenery is placed where it can have the greatest impact.
That’s one of the main goals of Urban Trees Initiative, a partnership between USC, the City of Los Angeles and community-based organizations working to expand the tree canopies in neighborhoods that need it the most.
Amy Schulenberg, tree planting project coordinator at the city’s L.A. Sanitation & Environment division, said, “Our goal in L.A. Sanitation is because we do have this amazing opportunity to bring extra resources to the city that supplements the actions being taken by our urban forestry division and others,” and she added, “We like to try to get the most bang for the buck.”
Schulenberg oversees a grant-funded urban forestry program under the Los Angeles Department of Public Works.
A major focus of Schulenberg’s team is to increase the number of trees on key corridors—the key north-south and east-west streets lined with bus stops, laundromats, small markets, and multifamily housing—where many residents rely on public transit.
They are also planting trees around schools that are near highways, where children play just yards away from major roadways like the 5 and 110 freeways.
“That’s why it helps to have more information, because there are so many places that need trees,” she said about the USC study. “It’s just a matter of trying to figure out where we can make the best use of them, right now, with what we have.”
Turning research into action
For advocates who have long pushed the city to invest more in greening efforts, the study offers compelling proof of what they’ve known all along: L.A.’s urban forest is more resilient than many assume.
The study shows, “Wow, there’s a really strong living ecosystem right in the heart of L.A. that’s active through the summer,” said Dustin Herrmann, principal scientist at TreePeople, a Beverly Hills-based nonprofit that focuses on urban forestry and environmental advocacy.
Yet the study’s findings arrive at a time when many experts and advocates remain frustrated by the city’s slow response to tree canopy expansion and environmental investment.
Joanne D’Antonio, chair of the Community Forest Advisory Committee, or CFAC—a volunteer group of community representatives nominated by City Council members and appointed by the mayor—said she hopes the study will help shape the city’s tree-planting decisions. But progress has been slow, she noted.
“One would hope,” she said, “but (the USC study) has to be read, and it has to be read by somebody who has the ability to change.”
For years, respected environmental organizations have urged Los Angeles to plant more trees, prioritize trees in city planning, and follow the advice of its own tree advisory committee.
The Community Forest Advisory Committee, led by D’Antonio, previously recommended that the city implement findings from the 2018 Dudek study of urban Los Angeles trees, which involved elected city officials, neighborhood councils, community stakeholders, and city staff. The 2018 study called for stronger and more sustainable urban forest management.
While the city is now developing an Urban Forest Management Plan—one of Dudek’s key recommendations—many concerns remain.
The Dudek study warned that trees remain undervalued in L.A. city budgets and planning, with urban forestry funding falling far below necessary levels. To manage L.A.’s urban forest sustainably, experts estimated the city would need a $40 to $50 million budget increase—a gap that still exists today.
“This has been going on for a long time,” D’Antonio said. “The percentage of the city budget that goes for urban forestry is minuscule compared to the percentage in other cities, and that is critical. I mean, we can’t ask the urban forestry division to do more if they don’t have the resources to do it.”
Some City Hall staff support the effort to expand the city’s tree canopy but face political and budgetary roadblocks, she said. While the city forest officer is leading efforts to develop the Urban Forest Management Plan, concerns persist over how it will be funded.
“It’s going to take a lot of money to implement and (with an) annual budget much higher than it is now,” she said, “maybe by five times, maybe six times as much as what we spend now to implement this.”
The plan might need to roll out in stages, but D’Antonio anticipates that it will be in place by summer. The real challenge will be figuring out how to fund it, she said.
“But the city really needs it,” D’Antonio added. “We need to find every place we can that we can plant (trees), because if we want to make this goal by 2050, we have to be planting those trees now.”
While the USC research highlights the role of urban greenery in reducing emissions, L.A. officials continue to prioritize development and infrastructure—often at the expense of trees.
Herrmann at TreePeople noted that trees are frequently overlooked in city planning, treated as an afterthought rather than an essential part of urban design. They’re often squeezed into leftover spaces instead of being intentionally planned for.
If not properly accounted for, large trees can conflict with infrastructure, such as city sidewalks, power lines, and underground utilities, he said. To avoid this, trees must be integrated into redevelopment projects from the start—rather than forced into limited spaces later.
“You have to create the space for the forest you want,” Herrmann said. “You have to be intentional about growing urban forests, because the urban space can be highly contested for how we want to use it.”
But larger systemic issues also contribute to L.A.’s lack of trees.
Aaron Thomas, urban forestry director at North East Trees, a nonprofit that aims to increase the tree canopy and green spaces in underinvested communities, pointed to a deeper cultural issue–L.A.’s undervaluing of public space.
“We live in a culture here of extreme individualism where people are supposed to just enjoy the outdoors in the privacy of their home, their own home or yard,” said Thomas, who is also the CFAC’s former chair. “But then of course, that overlooks the thousands and thousands of people who live in apartments, or who don’t own a single family home.”
Beyond the lack of green space, Thomas noted that even when opportunities arise to plant more trees, some communities are hesitant—not because they oppose trees, but due to a history of neglect by the city and other agencies responsible for maintaining the trees.
“We have to take their concerns and do our best to reassure them that we’re making every effort to avoid those same problems that have happened from generations in the past where people weren’t thinking nearly about it,” Thomas said. “But that takes a lot of education, outreach and engagement.”
While urban trees help, the majority of L.A.’s carbon sequestration occurs in surrounding natural ecosystems including the Santa Monica Mountains, San Gabriel Mountains, Verdugos, and Griffith Park, as well as chaparral and riparian areas, Thomas said.
He sees the increasing frequency of wildfires as a wake up call for city officials, a reminder that trees–both in urban areas and surrounding wildlands–are not a luxury but a crucial part of climate resilience.
Thomas said that today many of L.A.’s trees, planted 50 or 60 years ago, are aging — which means it’s important to start planting new trees now so they can grow and replace the old ones over time.
“We know that their lifespan is pretty short in an urban area,” he said. “So we need to be planting trees now that will eventually replace that canopy as those trees go away.”