Cities messing with spring’s signals

Urbanisation shifts nature’s seasonal cues in nuanced ways, with cities in cold climates triggering earlier spring plant growth and cities in warm climates delaying it, a new study has found.

And the “urban heat island” effect – the phenomenon in which cities are warmer than their surroundings – is not the only culprit, the researchers say. Factors such as pollution, changes in humidity and fertiliser runoff may also influence plants’ seasonal patterns.

The research was led by Daijiang Li, from the Florida Museum of the Natural History and the University of Florida in the US, with the findings reported in a paper in the journal Nature Ecology & Evolution.

Li and colleagues analysed millions of observations of 136 plant species across the US and Europe to study how regional temperature and the local density of people – a proxy for urbanisation – affects when plants sprout leaves and blossoms.

They found warmer temperatures and higher population density both spurred earlier springs. 

An increase of two degrees Celsius in temperature bumped up plants’ production of leaves and flowers by about five and six days respectively, they say. A fourfold increase in human population density advanced flowering and leaf production dates by about three days. {%recommended 8908%}

However, when these two factors worked together, local temperature was found to have an outsized influence.

In cold regions – areas with an average November-to-May temperature of about minus eight degrees Celsius – 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 20 degrees Celsius, 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.

The researchers found that plants’ response to changes in temperature and urbanisation differed by species. 

However, while closely related plants generally produced leaves and flowers on similar timelines, these similarities were erased by urbanisation and changes in regional temperature. “That makes prediction for the future harder,” Li says.

Florida Museum’s Robert Guralnick says many studies that pointed to the urban heat island effect as the main culprit for shifts in the seasonal patterns of plants had relied on remote sensing data, which only capture what’s happening in the plant canopy, or local observations of a few species, which don’t show broad scale patterns of seasonal change.

“They didn’t have the species-level data to pick apart what’s happening to different species on the ground,” he says. “The question is whether we can look at what’s happening in everyone’s backyard at once. The real power of this study is that we can actually do that.”

The new study accessed three large monitoring programs: the National Phenology Network and the National Ecological Observatory Network based in the US, and the European Plant Phenology Network. Between them they contain more than 22 million data points.

A key area for future research, says Li, is how suburbs and exurbs (areas just outside city suburbs) affect plants’ seasonal growth. 

“These are where the effects are probably going to be the most important because they cover the most land area,” he said. “There’s not much truly urbanised land in the US.”