According to a study by Yale researchers, city park trees are experiencing a delay in seasonal change, which may lead to increased seasonal allergies.
1:49 am, Sep 15, 2025
Staff Reporter
Baala Shakya, Photography Editor
In a concrete jungle like New York City, urban trees offer refuge: they’re living cooling systems and shade for parked cars. But the seasons are changing for trees in the Big Apple.
According to a new study published by Yale researchers in Environmental Research Letters, climate change is causing urban trees in New York City to turn green later in the year, potentially shifting pollen season and affecting seasonal allergies.
“Our findings show that climate change and the urban heat island effect are shifting the timing of tree leaf-out in parks,” Juwon Kong, the lead author of the paper, said.
Kong and her team utilized satellite images taken over a 19-year period to measure the start of the growing season, or the day of the year when the trees start “leafing out in spring.”
The drastic temperature transition from winter to spring is a key trigger for springtime leaf growth. Because winters are getting warmer due to climate change, the significant temperature hike is dampened, and, thus, trees experience delayed leaf growth.
According to Kong, medium-sized parks are especially vulnerable to climate change because they have both high edge-to-area ratios and smaller interiors. This means more of the park’s trees are exposed to surrounding heat from roads and buildings, but unlike larger parks, these medium-sized forests lack the cooling capacity of many trees.
Smaller “pocket parks” are already dominated by their surroundings, so their response to climate change in time is less distinct, while larger parks can better resist heat due to their cooling capacity.
In addition to climate change, trees are under a constant barrage of invasive insects, pathogens, plants and animals due to the increase and expansion of development, said Mark Ashton ENV ’85 ’90, senior associate dean of The Forest School at the Yale School of the Environment.
“Taken together, trees and forests have never before been under such dire compounded and interacting impacts as today,” Ashton said.
In addition to affecting seasonal allergies, delayed seasons also means that trees will have less carbon intake, as a result of being less foliated. For trees, this reduces carbon uptake and growth. For people, it reduces early-spring cooling and shading benefits, and shifts pollen release, which can affect allergies.
“For ecosystems, delayed leaf-out can disrupt the timing of pollinators, migratory birds, and other species that depend on trees, altering ecological balances,” Kong said. “These shifts could reshape biodiversity and ecosystem health within urban parks.”
Kong said that successful future initiatives should include “careful consideration of park size, species selection, tree planting and long-term conservation planning.”
New Haven’s drinking water runs through 26,000 acres of natural forest.
MICHELLE SO
Michelle So is a beat reporter for the SciTech desk, covering climate change and the School of the Environment. Originally from Los Angeles, California, she is a sophomore in Timothy Dwight College majoring in Ecology and Evolutionary Biology.