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Climate change is affecting growing season for NYC trees, Yale study finds
By
Rosemary Misdary
Published Aug 20, 2025
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By
Rosemary Misdary
Published Aug 20, 2025
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A new Yale study reveals that spring is not coming early — it’s coming later as a result of climate change and the urban heat island effect.
Warm winters are causing NYC trees to miss cues to leaf out for the spring, resulting in late leafing and shorter green periods, the researchers found. The reduced green season can result in slower growth, reduced ability to sequester carbon and effects on ecosystems that depend on leaves, such as insects.
Yale researchers said the results were surprising and it could be the beginning of a trend that park managers said could lead to tree die-outs.
“ We all thought that the expected finding was with climate change and urban heat island effect that you would actually have earlier spring,” Karen Seto, a study author, said. “But in fact, that's not what we found and we checked the results many times.”
Researchers analyzed more than two decades' worth of NASA’s satellite imagery of 346 NYC parks to pinpoint the arrival of spring, which was defined by a minimum of 15% of the leaves coming out in a single park. The analysis also involved the use of remote sensing to show the effects of the temperate winters and the heat intensification caused by manmade landscapes, or the urban heat island effect.
The most vulnerable parks are the medium-sized green spaces, such as Bryant and City Hall Parks, that have less than 45 acres. On average, these parks experienced a delay of the spring of three to five days. They have a lot of edges butting up against manmade material such as sidewalks, relative to their actual size, Seto said..
“They have a lot of exposure to heat, and at the same time, they’re not big enough to have a very large buffer,” Seto said. “In contrast, the very largest parks have a lot of cooling from being so big, so they’re not as affected.”
Large green spaces such as Central Park and Prospect Park experienced shorter delays of around two days.
Small parks, those with less than 2 acres, mimic the surrounding manmade environment; their average seasonal slowdown is about a day.
“I can see it cascading to a lot of other species,” Novem Auyeung, director of research and monitoring for environmental planning at NYC Parks, said. She was not involved in the study. ”It's going to be really disruptive for a lot of wildlife that depend on these trees for food, for shelter.”
Less green also affects the humans living in New York. Jeffrey Clark, senior manager of applied research at the Natural Areas Conservancy, said that trees will be less effective at mitigating climate change driven by carbon dioxide. The shorter period of leafy trees also means less shade and reduced cooling benefits from green spaces.
“ It's more pleasant to walk underneath trees that are lush and green than it is to walk under these kinds of skeletal structures that we see during the winter,” said Clark, who was also involved in the study. “You can no longer really spend as much time outside during these really pleasant temperatures when you also get to experience the relaxing benefits of being around leafy vegetation.”
Auyeung said that this could impact the city’s goal of having 30% coverage of tree canopy.
The warm winters are the main driver for the delayed spring. Over the last 50 years, average New York City winter temperatures have risen by more than three degrees Fahrenheit. Trees need a solid cold period to prepare for spring, said the study’s lead author, Juwon Kong.
“Trees need a certain amount of chilling period, and if they don’t get those very cold nights, they don’t know to wake up," Kong said.
A shorter green period also means a shorter window for the trees to take in the energy they need via photosynthesis to grow or fight off disease. That could give non-native species like Trees of Heaven and Ginkgo the chance to outcompete native trees like Oaks and Maples.
The impacts are felt from seed to sapling to an old-growth tree. According to the Prospect Park Alliance, which cares for roughly 30,000 trees, this year there have been outbreaks of Dutch elm disease and more vulnerability to hypoxia for oak trees, which means the leaf-out dies quickly or does not leaf out at all. Christian Zimmerman, the Alliance’s vice president of capital and landscape management, said that there has been an abnormal amount of branching, which is when leaves drop as early as June.
The shifting season will also put stress on the NYC Parks Department’s insufficient budget and staff. Auyeung said the shift in seasons is already requiring more work and investment to stay ahead of the changing temperatures. Climate change is reshaping how parks are planting trees and rethinking ways of keeping seedlings and saplings alive through water collection tools such as rain guards. Ultimately, the warmer climate could result in a focus on species that may be more tolerant rather than native plants that need seasonal changes to thrive.
“ I don't know how to prepare for the climate change in a significant way covering such a large extent of urban landscape,” Zimmerman said. “We don't have the resources and bodies and conditions to be able to do much.”
The study authors and park managers said that it’s possible to mitigate the compounded effects of the urban heat island effect and climate change. Planting more trees and creating more dense areas of forests will help combat the heating effects. But changes in the manmade landscape can go a long way to lessen the effect by covering building tops with vegetation, or green roofs, and using building materials that absorb less heat than concrete, steel and asphalt.
“If we wait for things to degrade, we wait for trees to start to die, then the amount that we'll need to invest is going to be a lot greater than if we start working right now,” Clark said.
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Health and Science
new york city
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Rosemary Misdary
Rosemary is health & science reporter. Got a tip? Email [email protected] or Signal 646-544-9524.
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