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Trees in northern cities are feeling the impacts of climate change. Heat and water stress might be hindering tree growth more severely in Boston’s and New York City’s forests than in surrounding rural areas—although experts say more work is needed to know for sure. That’s according to a new study published in Ecological Applications.
“There is a lot of research in natural or near-natural environments that are dwindling on our planet, and there is not enough research in managed and urban areas.”
If trees struggle to grow in a warmer future, they may not offer cities the same benefits that trees today provide, said environmental ecologist Andrew Reinmann of the City University of New York, who supervised the work.
“There is a lot of research in natural or near-natural environments that are dwindling on our planet, and there is not enough research in managed and urban areas,” said forest ecologist Flurin Babst of the University of Arizona, who was not involved with the research. The new study begins to fill that gap.
A Tale of Three Cities
Urban trees bring a plethora of benefits, from providing shade to removing pollutants from the air to improving mental health. But urban trees also experience unique stresses. Cities tend to be hotter than surrounding areas, their soil tends to be more compacted, and city air tends to be drier and contain more particulate matter.
These stresses are compounded by heat waves and droughts brought on by climate change, according to the new study. In his previous research as a postdoc at Boston University, Reinmann and his colleagues found that unusually hot weather affected the growth of trees in Boston’s urban forests more strongly than that of trees in nearby rural forests. He wondered whether the same was true in other cities.
In 2020, Barnard College undergraduate student Kayla Warner used her time during COVID lockdown to dig into this question by analyzing tree cores that had been collected for other research prior to the pandemic. The cores came from several dozen trees in each of three cities—Boston, New York, and Baltimore—as well as from nearby rural forests. The researchers selected these locales because they have similarly humid, temperate climates, but a wide geographic span.
Warner correlated the growth rings of three species that are common in these cities—red oak, white oak, and red maple—with climate data going back to the 1990s.
Distinct Behavior
During much of the year, the growth rates of oak trees in Boston and New York suffered more from hot, dry weather than the growth rates of trees in surrounding areas, the researchers found. For all trees in Massachusetts and New York, the distance between rings grown during hot, dry years was smaller than between those grown during wetter, cooler years. But that distance was reduced more significantly for city trees than for their rural counterparts.
Baltimore’s urban forests also suffered from water stress, but unlike in the other two cities, they were not strongly affected by heat stress.
“I don’t have a clear answer for why Maryland behaves a little bit differently than New York and Boston,” Reinmann said.
“It reminds me of some research on people’s experiences of heat stress.”
“It reminds me of some research on people’s experiences of heat stress,” said quantitative ecologist Ailene Ettinger of The Nature Conservancy. “People in northern latitude cities also tend to be much more sensitive to high heat events because we’re not as well acclimated.”
It’s possible that each city contains genetically distinct subspecies of maples and oaks that have adapted to different weather patterns, Reinmann said. Given that hypothesis, Babst said it could make sense to compare the growth of trees derived from all three cities side by side under a range of climate conditions—a “common garden experiment,” as it’s called in the field. “That would be a really, really cool project,” he added.
The study is a nice start, but scientists have a way to go before they fully understand how urban trees are responding to climate. “Sample size in this research is relatively small,” said forest ecologist and postdoc Jiejie Wang of Université Laval. Analyzing more trees will help researchers draw robust conclusions. Babst suggested analyzing longer time series.
Measuring water stress also can be tricky because it can be difficult to say exactly how much of the water in the environment is actually available to trees. Wang said she thinks analyzing soil moisture would be the most appropriate way to determine how much water is available to trees. Instead, the researchers used a combined measure of heat and water. “I would have liked to see a water-only metric, like simply precipitation, for example,” Babst said.
The sooner follow-up work can happen, the better, Ettinger said. “Urban trees are critical for bolstering our climate change resilience.”
—Saima May Sidik (@saimamaysidik), Science Writer
Citation: Sidik, S. M. (2024), Some urban trees suffer under climate stress, Eos, 105, https://doi.org/10.1029/2024EO240415. Published on 17 September 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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