A new study shows that urbanization shifts this seasonal cue in nuanced ways, with cities in cold climates triggering earlier spring plant growth and cities in warm climates delaying it. The study also found that the urban heat island effect, the phenomenon in which cities are warmer than their surroundings, is not the only culprit behind the shift, suggesting that other aspects of urbanization, such as pollution, changes in humidity and fertilizer runoff, may also influence plants’ seasonal ...

A new study shows that urbanization shifts this seasonal cue in nuanced ways, with cities in cold climates triggering earlier spring plant growth and cities in warm climates delaying it. The study also found that the urban heat island effect, the phenomenon in which cities are warmer than their surroundings, is not the only culprit behind the shift, suggesting that other aspects of urbanization, such as pollution, changes in humidity and fertilizer runoff, may also influence plants’ seasonal patterns.

Researchers analyzed millions of observations of 136 plant species across the U.S. and Europe to study how regional temperature and the local density of people-a proxy for urbanization—affect when plants sprout leaves and blossoms. Their results revealed a complex story: Separately, warmer temperatures and higher population density each spurred earlier springs. A 3.6-degree Fahrenheit increase in temperature bumped up plants’ production of leaves and flowers by about five and six days, respectively. A fourfold increase in human population density advanced flowering and leaf production dates by about three days. But the team found that when these two factors worked together, local temperature had an outsized influence.

In cold regions—areas with an average November-to-May temperature of about 18 degrees-plants produced leaves and flowers about 20 days earlier in locations with about 26,000 people per square mile, compared with equally frigid wildland. When an area’s average November-to-May temperature jumped to 68, however, leaves and flowers appeared four and six days later, respectively, in locations with about 26,000 people per square mile, compared with equally balmy wildland. In New York, for example, plants are likely sprouting leaves about 9.5 days earlier and blossoms eight days earlier than uninhabited regions with the same temperature. Jacksonville, in contrast, is likely pushing leaf production later by about one day and flowers by about half a day, with leaves appearing two days later and flowers a day later in Houston.

Even after accounting for urban heat islands, the team’s models revealed cities significantly affect plants’ springtime growth. “Not only are there other things going on, but they actually matter quite a lot,” said study co-author Brian Stucky, Florida Museum research scientist. Plants may not be the only organisms affected by seasonal shifts, he added. “Seasons are such a big part of our lives. We define our world around seasonal things. Those rhythms are what we think of as the normal way the world works.”

(Source: https://phys.org/)

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