Urban areas are very hot. Does this affect plant evolution?

While we might think that urban environments are mostly well suited for their occupants, our collections of brick, stone, asphalt and concrete make things much hotter.

How does this heat affect the plants that share our cities with us? Have they evolved to better tolerate our urban heat islands?

A new study published in Science Advances looks into this question with a plant called creeping woodsorrel (Oxalis corniculata), finding that their leaves changed colour in the urban landscape.

“Urban heat islands are precursors to global warming. Understanding the rapid adaptive evolution of urban organisms to high temperatures will provide valuable insights on ecosystem dynamics and sustainable crop production,” said first author Dr. Yuya Fukano, from Chiba University in Japan.

“We noticed that the red-leaved variants of the creeping woodsorrel commonly grew near impervious surfaces in urban areas but rarely grew in farmlands or green spaces in and around the city.”

You’ve probably seen woodsorrel in your neighbourhood. They look similar to clover, but with heart shaped leaves and sour yellow flowers.

But when the team went out into the field to study how the woodsorrel changed in urban and non-urban regions around Tokyo, they found that red leaves were dominant in urban areas, but green leaves were dominant in green habitats.

“Growth and photosynthesis experiments demonstrated that red-leaved individuals performed better under heat stress, while green-leaved individuals performed better under nonstressful conditions,” the researchers wrote in their paper.

When the researchers compared the two in laboratory conditions, red-leaf sorrel had higher growth rates and higher levels of photosynthesis in high temperature conditions, but green-leaf variants thrived in lower temperatures.

They even set up a fake brick pathway and seeded sorrel in between – to replicate the conditions of a hardy plant growing out of a crack in the sidewalk.

This suggests that yes, the sorrel had evolved to thrive in hotter, less habitable conditions – and part of that meant turning red. 

This wasn’t just an isolated Tokyo phenomenon.  When the researchers looked at the citizen scientist website iNaturalist, they found this unfolded in other countries around the globe too.

 “Adaptive traits to high temperature stress are likely not limited to leaf colour, and future studies should focus on a variety of traits to understand plant adaptation to urban heat islands,” the researchers concluded.

“Understanding the adaptive impact of urban heat stress will also contribute to understanding adaptations required in response to predicted global warming.”