Wednesday, March 26, 2025

How changing L.A.’s tree rules could cool more neighborhoods

A new USC Dornsife study finds that outdated guidelines are limiting tree growth — especially in lower-income neighborhoods — and offers a path forward.

University of Southern California

Expansive tree canopies are crucial for healthy ecosystems and livable cities. Yet, Los Angeles’ strict tree planting rules, originally meant to protect infrastructure and public safety, are now widening shade disparities, particularly in lower-income neighborhoods. A new study published in Landscape and Urban Planning, led by the Spatial Sciences Institute and Public Exchange, both based at the USC Dornsife College of Letters, Arts and Sciences, suggests that easing these decades-old restrictions could significantly grow the city’s urban tree canopy — without compromising safety.

This research builds on an earlier USC study comparing L.A.’s municipal codes, engineering standards and urban forestry guidelines to those of 25 other cities — 17 outside California and eight within the state. That study found L.A.’s tree placement rules are among the nation’s strictest, often limiting new planting. The team concluded that substantial changes could be made to existing tree-spacing guidelines without re-writing laws, but better coordination between city departments is essential.

“Fixing L.A.’s rules is a step toward addressing inequities and bringing the health and ecological benefits of green infrastructure to underserved communities,” said Laura Messier, PhD candidate in the population, health and place program at the Spatial Sciences Institute.

Testing looser tree spacing rules

To test how relaxing planting guidelines might increase tree coverage, Messier and her team compared two L.A. neighborhoods: Boyle Heights, a historically lower-income area east of downtown; and Studio City, a wealthier community in the San Fernando Valley. Both areas studied were similar in size, topography and parcel layout. But Studio City had about 3,020 trees per square mile, compared to 2,183 in Boyle Heights — a gap researchers linked to the neighborhood’s denser street grid, higher concentration of multi-family housing and smaller parcels, all of which limit where trees can be planted.

Using mapping software, the team identified obstacles like utility poles, gas lines and bus stops, then modeled potential new planting sites. They compared L.A.’s current planting restrictions with more flexible guidelines in other California cities — including San Francisco, Fremont, Oakland and Anaheim — and identified infrastructure changes that could further expand tree coverage.

The results were striking. Under L.A.’s current rules, Studio City could support up to 140 trees per square mile, while Boyle Heights maxed out at 121. But with looser guidelines, the gap nearly vanished — Studio City’s capacity rose to 158, while Boyle Heights jumped to 153, a 26% increase in the historically Latino neighborhood.

Still, Boyle Heights faces challenges beyond planting guidelines. Narrow sidewalks limit the ability to plant large shade trees. Even with the same number of trees, only 34.5% in Boyle Heights could be large-canopy species, compared to 61% in Studio City.

Small tweaks to tree spacing make big impact

Even modest policy changes could open up more space for tree planting in crowded areas.

The study found that easing restrictions near intersections could increase the number of trees in Boyle Heights by 7.6%, while relaxing rules around utility poles could add another 5.5%. Adjusting guidelines for gas lines (2.6%), streetlights (2.2%), driveways (1.4%) and other infrastructure could push the total canopy gain to 26%, helping expand shade in other dense, lower-income neighborhoods.

While planting trees at bus stops would add less than 1% to overall canopy coverage, it could make a big difference for transit riders exposed to extreme summer heat.

A major obstacle to planting more trees is L.A.’s 45-foot visibility rule at intersections, last updated in 1988. Studies show high-canopy trees don’t block drivers’ views, making this restriction ripe for revision.

Easier to change tree spacing rules than laws

Many of the restrictions are internal guidelines rather than laws, meaning changes could be implemented more easily. The city’s Urban Forestry Division would need to update its Tree Spacing Guidelines memo, but getting agreement from other departments — such as transportation and street lighting — could still be a challenge, Messier explained.

Ironically, the study found that half the street trees in Boyle Heights and nearly 40% in Studio City don’t comply with city guidelines. Yet, there’s little evidence that these violations create safety or liability issues.

Messier suggests that updating the guidelines is more practical than enforcing rules that are often ignored and seem to have little impact on safety.

While modernizing L.A.’s rules is an important step, closing the shade gap will require broader infrastructure changes. Messier and her team point to strategies like reducing street widths — known as “road diets” — to create more space for trees.

“To truly close the shade gap and ensure more equitable access to cooling and green spaces, the city must invest in infrastructure that makes room for more trees in underserved areas,” Messier said.

Journal

Landscape and Urban Planning

DOI

10.1016/j.landurbplan.2025.105345

Method of Research

Case study

Subject of Research

Not applicable

Article Title

Equity impacts of street tree spacing guidelines: A case study in two Los Angeles neighborhoods

Article Publication Date

25-Mar-2025

Getting hit by lightning is good for some tropical trees

Cary-led study reveals lightning benefits some trees by killing off parasitic vines and opening up the canopy

Cary Institute of Ecosystem Studies

image:
Technician Cesar Gutierrez climbs a tower to detect and locate lightning strikes in the study area. After detection, drones and on-the-ground teams monitor the strike’s impacts.
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Credit: Evan Gora / Cary Institute of Ecosystem Studies

Getting zapped with millions of volts of electricity may not sound like a healthy activity, but for some trees, it is. A new study, published in New Phytologist, reports that some tropical tree species are not only able to tolerate lightning strikes, but benefit from them. The trees may have even evolved to act as lightning rods.

The research was led by Evan Gora, a forest ecologist at Cary Institute of Ecosystem Studies. Gora studies how lightning impacts biodiversity and carbon storage in Panama’s tropical forests.

Lightning kills hundreds of millions of trees per year. But in 2015, while working in Panama, Gora and his colleagues came across a Dipteryx oleifera tree that had survived a strike with little damage — even though the jolt had been strong enough to blast a parasitic vine out of its crown and kill more than a dozen neighboring trees.

“Seeing that there are trees that get struck by lightning and they’re fine was just mind-blowing,” Gora recalled. Over time, the team encountered other Dipteryx oleifera trees thriving after getting hit, so they decided to take a closer look.

Scientists had previously suspected that some trees evolved to tolerate lightning, but evidence to back it up was lacking. In 2022, Gora and colleagues demonstrated for the first time that trees differ in their ability to survive getting hit by lightning. Their new paper, published Wednesday, is the first to show that trees can benefit from these electric jolts.

Using a unique lightning location system, the team tracked the outcomes of 93 trees that had been struck by lightning in Barro Colorado Nature Monument in central Panama. For two to six years after the strike, the team measured tree survival rates, crown and trunk condition, number of parasitic vines or lianas, and neighboring tree mortality. The study included nine directly struck Dipteryx oleifera trees, and compared them with 84 other trees that had been struck.

All nine Dipteryx trees survived direct lightning strikes with only minor damages. In contrast, directly struck trees of other species were badly damaged, losing 5.7 times more leaves from their crowns, and 64% died within two years.

When each Dipteryx tree was zapped, an average of 9.2 neighboring trees were killed as the electricity traveled between adjoining vines and touching branches, or jumped across small gaps between trees. Lightning strikes also reduced Dipteryx liana infestations by 78%, freeing trees from some of the pressure these parasitic vines have on light and nutrient availability.

These patterns also bore out across the broader population. The team found that Dipteryx trees tend to have fewer lianas. Analyzing trends in tree death over the past 40 years, the researchers found that the trees neighboring Dipteryx trees were 48% more apt to die than other trees in the forest, likely because of lightning.

Using drones, Gora and colleagues created 3D models of canopy height, which showed that Dipteryx trees tend to be about four meters taller than their nearest neighbors, likely because lightning killed their taller neighbors, giving them an advantage in competing for light and space.

“These data provide the first evidence that some trees benefit from being struck by lightning,” the authors write. Or, as Gora puts it, “It's better off for a Dipteryx oleifera tree to be struck than not.”

Because of all these benefits, Dipteryx oleifera trees may be specially adapted to attract lightning. With their distinctive height and unusually wide crowns, they may be up to 68% more likely to get electrocuted than other trees with average height and crowns, according to the team’s calculations.

Estimates suggest individual Dipteryx oleifera trees are directly hit by lightning every 56 years, on average. And since the trees can live for hundreds or possibly more than a thousand years, they are expected to survive these blasts many times throughout their lives. During the study, one of the Dipteryx trees was struck twice in just five years.

The remarkable ability to survive lightning strikes and benefit from the removal of lianas and competitors gives Dipteryx trees a major advantage over other trees. According to the scientists' calculations, lightning tolerance boosts the species’ ability to produce offspring by 14 times.

Next, the team aims to investigate what electrical or structural traits allow these trees to survive lightning strikes. They would also like to explore whether other species show lightning tolerance, to better understand how common this phenomenon is.

What is clear is that lightning plays an underappreciated role in tree competition. And with lightning on the rise in many regions due to climate change, its influence may increase, potentially favoring lightning-tolerant species like Dipteryx oleifera. Understanding lightning and its role in shaping forests may be important for predicting changes in biodiversity and carbon storage, and for informing tropical reforestation efforts.

A Dipteryx oleifera tree just after being struck by lightning in 2019 (left) versus two years later (right). The tree survived the strike with minimal damage, and benefited from having its parasitic vines and competing neighbors removed by the strike.

Also known as the eboe, choibá, tonka bean or almendro, Dipteryx oleifera is native to Honduras, Nicaragua, Costa Rica, Panama, Colombia, and Ecuador. Its hard wood is used in construction, and it produces almond-flavored seeds that are edible and sold in local markets. A keystone species of Panamanian forests, D. oleifera fruits and seeds are a crucial food source for rainforest mammals such as agouti during the dry season.

Credit

Evan Gora / Cary Institute of Ecosystem Studies

Study co-authors include: Helene Muller-Landau and Pablo Narváez of the Smithsonian Tropical Research Institute; KC Cushman of Oak Ridge National Laboratory; Jeannine Richards of Florida Gulf Coast University; Phillip Bitzer and Jeffrey Burchfield of the University of Alabama in Huntsville; and Stephen Yanoviak of the University of Louisville.

This work was supported by grants from the National Science Foundation (DEB-1354060, DEB-1655346, and DEB-2213246 to SPY, DEB-1354510, DEB-1655554, and DEB-2213247 to PMB, and DEB-2213245, DEB-2241507, and GRF-2015188266 to EMG), the National Geographic Society (9703-15 to EMG), and a Smithsonian Tropical Research Institute Tupper Postdoctoral Fellowship to EMG. KCC was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Cary Institute of Ecosystem Studies is an independent nonprofit center for environmental research. Since 1983, our scientists have been investigating the complex interactions that govern the natural world and the impacts of climate change on these systems. Our findings lead to more effective resource management, policy actions, and environmental literacy. Staff are global experts in the ecology of: cities, disease, forests, soils, and freshwater.

New Phytologist is a leading international journal focusing on high quality, original research across the broad spectrum of plant sciences, from intracellular processes through to global environmental change. The journal is owned by the New Phytologist Foundation, a not-for-profit organisation dedicated to the promotion of plant science.