The weekend before Derby I went for a hike in the Parklands. Back in the city, it seemed like spring was well underway: Most trees had full canopies, and the dogwoods and Magnolias were in bloom.
But on the hike, my partner Caitlin McGlade noticed that wasn’t the case in the Parklands.
“The trees were a lot more bare than they were on our block and a lot of them hadn’t flowered yet,” she said. “There was only one or two that had flowers and it was a lot chillier”
That got me wondering, does spring arrive sooner in the city than in the country?
We know there’s a pretty serious urban heat island here; that’s where the city is a lot warmer than the surrounding countryside. A 2016 study found Louisville was warming faster than any other city in the country. But does that affect how and when trees bloom?
Turns out, there’s no easy answer. I began my inquiry reaching out to Louisville Metro Government’s urban forester Erin Thompson.
We met in Willow Park, in the Highlands, where there are plenty of trees to look at: red maples, yellowwoods, dogwoods and a towering Norway spruce.
Louisville lost about 54,000 trees a year between 2004 and 2014. Thompson said the resulting loss of shade has made the urban heat island worse.
But when I pose my hypothesis that trees bud faster in the city because of the heat, she says:
“I think that’s a really good question and I don’t know the answer to it.”
But she did offer one explanation for why trees in the city and the Parklands seem to be on different blooming cycles: Louisville is in a different hardiness zone than the Parklands.
The U.S. Department of Agriculture designs these hardiness zones, which describe which plants grow best in which regions. But Thompson also said Louisville’s hardiness zone has changed over the last two decades, because of warming temperatures.
And that could be due to climate change and the urban heat island effect.
That’s something local arborists have seen firsthand. I caught up with Robert Rollins, with Green Haven Tree Care, while working in the woods near Prospect.
“So we’re right in the thick of spring here, if you look around us you’ll see the maples and the oaks and the sycamores and the walnuts and they haven’t fully pushed out their leaves, they’re about three-quarters of the way out,” he said.
Rollins has been an arborist in the city for more than 30 years, and has seen Louisville’s hardiness zones change. But he said he’s also witnessed the effects of the city’s urban heat island.
“These trees in the city are already leafed out. They are further along, because again the heat, the temperature is what pushes it, and of course we’ve had plenty of rain this spring,” Rollins said.
So — is it hardiness zones or urban heat? It’s both, in this case.
Researchers know that the more days you have above a certain temperature, the more likely trees are to begin pushing out their leaves.
A study from a couple years ago from the University of Wisconsin, Madison used thermal sensors and satellite imagery to compare the growing seasons of plants inside and outside the city.
“What we found was that, on average, the growing season was about five to seven days longer than it was in the surrounding rural areas,” said Sam Zipper, a post-doctoral researcher, who worked on the study.
Zipper says there’s a strong correlation between the amount of impervious cover — things likes roads, buildings and sidewalks — and how the plant growing season is changing.
Basically, the more concrete and asphalt, the more heat. The more heat, the longer the growing season.
And that’s made worse by climate change.
“This is definitely a situation where climate change and the urban heat island are kind of overlapping on each other. So as the air warms due to climate change, similarly the urban heat island is going to be exacerbating that effect in urban areas,” Zipper said.
Spring does seem to arrive sooner in Louisville’s urban core, and that’s likely because of both the urban heat island effect and a changing climate. But all of it is the result of the impact humans have on the environment.