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Many plants, like ferns, prefer living in city environments

Why do some fern species adapt to urban environments while others prefer to remain rural?

Tammo Reichgelt, an expert in paleobotany at the University of Connecticut, used citizen science data to investigate. The research revealed that certain ferns can mitigate the urban heat island effect.

Abundance of ferns in urban areas

Reichgelt’s interest in this topic began as a hobby when he noticed an unusual abundance of ferns in urban areas. 

“I just happened to notice, ‘Wow, there’s a lot of ferns here. Strange,’” he said, noting the peculiarity of finding ferns in places like Rockville, surrounded by asphalt, concrete, and brick. This observation, supported by data from the iNaturalist community, indicated that specific fern species thrive in human-made environments. 

“Especially in old mill towns in Connecticut, like Rockville, Willimantic, or Norwich, that tend to have a denser and older urban core, there were rock-dwelling ferns on buildings, bridges, and other structures.”

Rock-dwelling fern species

To explore this further, Reichgelt collected over 22,000 georeferenced observations of 16 rock-dwelling fern species in the Northeast. He combined this data with land-use information from the United States Geological Survey (USGS) to differentiate between natural and developed environments. 

The findings showed a significant difference in habitat preference among fern species. “Over 50% of blunt cliff fern (Woodsia obtusa) observations were in highly developed areas,” noted Reichgelt. “The purple stem cliff brake (Pellaea atropurpurea) is more common south of Connecticut, but abundant in developed areas.”

“A surprising species is the Tennessee bulblet fern (Cystopteris tennesseensis), which is rare in its natural range but seems to thrive in the Philadelphia suburbs.”

Key factors in fern adaptation 

The study identified average summertime temperature and highest summer temperature as key climatic variables influencing fern adaptation to urban environments. Urban areas tend to be hotter due to the high heat capacity of building materials, a phenomenon known as the urban heat island effect

“That means these plants need to be adapted to hot environments to be able to survive in an urban environment,” Reichgelt said. “That these fern species are pre-adapted to hotter environments seems to predispose them to be able to thrive in urban environments. For the 11 species that don’t grow in urban environments, it seems that they cannot tolerate the Urban Heat Island effect.”

Ferns that do not grow in urban environments, such as the rock polypody (Polypodium virginianum) and Mackay’s fragile fern (Cystopteris tenuis), seem unable to tolerate the high summer temperatures of cities. 

Invaluable citizen science data 

Citizen science data from iNaturalist, which includes observations from a wide range of locations, proved invaluable for this study. 

“Very rarely would researchers go and make observations in urban environments. Citizen science data is an interesting new source of information that is rapidly crowding out other sources of information,” said Reichgelt.

The inclusion of urban habitats is an example of an advantage of using citizen science. Still, scientists have only just started to look into potential novel biases that are unique to citizen science.”

Investigating urban fern adaptations 

As a paleobotanist, Reichgelt typically studies ancient climates and plant adaptations. This background led him to investigate modern fern adaptation to urban environments. He plans to further explore the physiological differences between urban and rural ferns, including water regulation and photosynthesis. 

“I would like to compare whether urban and rural ferns have different functional traits, for example whether they differ in their water regulation or how they photosynthesize. In other words, what are the adaptations that allow certain ferns to thrive in urban environments?”

Understanding these adaptations is crucial for urban planning and mitigating issues like the Urban Heat Island Effect. Ferns, which thrive without human intervention, could be a low-maintenance solution for cooling urban areas. 

“The weird thing about these urban ferns is that they grow in places that are not tended. They’re native to the area but are weedy in these urban environments. Having something that can survive in an urban area and absorb the heat cools the city down,” Reichgelt concluded.

The study is published in the American Journal of Botany.


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