Rebuilding Louisville's Urban Forest To Combat Climate Change
Before air conditioning, people had a different solution for keeping their homes cool in the summer and warm in the winter.
Take a look at the historic homes around town and you may notice towering oaks, lofty elms and sturdy sycamores blocking the southern and western exposures. In the summertime, their canopies offer sun-dappled shade. In the winter, the trees shed their leaves and let the sunlight beam through the windows.
People are often quick to point out the downsides of large trees: there are leaves to sweep, root systems that wring the plumbing and tear up the pavement, and then there's always the threat of a hefty limb crashing through the ceiling.
And that leads people to avoid planting large trees. But the irony is, any large tree planted today may take a century before it creates property hazards. Meanwhile, we are already dealing with the impacts of climate change.
The latest special report from the United Nation’s panel on climate change stresses that global warming is already altering our landscapes, exacerbating risks to human health and ecosystems, livelihoods and infrastructure.
The report also makes clear that sustainable land management can reduce and reverse these impacts. Consider that today human-use directly affects more than 70 percent of planet’s ice-free land surface.
Global warming increases the frequency of extreme weather including heatwaves, storms, droughts and floods. Cities can expect more flash floods and pot holes, hotter temperatures and worse air pollution.
But urban forests can help control many of these local and regional climate impacts while at the same time absorbing the very thing that has got us into this crisis in the first place — carbon dioxide.
Despite that, Louisville's tree canopy is shrinking at a rapid pace. Disease, pests, weather and the steady march of progress (i.e. development) are eroding the very thing that could help us manage the climate crisis.
WFPL News is taking a cue from the Lorax this week as part of Covering Climate Now — a week of concerted climate coverage leading up to the United Nations Climate Action Summit on September 23.
We Should Do This
Trees have amazing potential to help manage the dilemmas of city living. They help block out unwanted sound and light, they increase property values and they inoculate us to the effects of Louisville’s urban heat island, which is among the fastest growing and most intense in the country.
The city’s tall buildings, concrete and asphalt absorb and radiate heat throughout the day. That can make cities significantly warmer than their surroundings — a problem that’s only going to get worse with rising temperatures.
Trees, in turn, offer shade. Planting only three trees around a house — in the right places — can save as much as 30 percent of energy use, according to the U.S. Forest Service Center for Urban Forest Research. Shade also limits the formation of harmful ground-level ozone, which is a nice side effect considering Louisville is not currently meeting EPA standards for the pollutant.
But trees also have another mechanism for cooling. Through evapotranspiration, trees pull water up through their roots and emit the moisture through their leaves, cooling the surrounding environment.
The IPCC report says forest cover directly affects regional surface temperatures and increased evapotranspiration can result in cooler temperatures during growing seasons.
Urban forests also help buffer sudden fluctuations in temperatures. With heatwaves, it’s not so much the heat itself as the sharp rise in temperature that can cause more deaths, said Aruni Bhatnagar, director of the University of Louisville’s Envirome Institute.
“If you live in an area with lots of trees, there would not be these large fluctuations in temperature and that’s more gentle to our cardiovascular health than these rough spikes,” Bhatnagar said.
Trees also limit the impacts of other extreme weather events like large storms. As temperatures rise, the air can hold more moisture, making for larger storms. And when it rains, it comes down faster, causing flash flooding and river flooding.
Trees capture water in their canopies slowing the rate at which drops hit the pavement. The roots not only absorb water, but they help prevent erosion, which is particularly helpful for urban streams like Beargrass Creek when combating flooding.
But let’s go back to the leaves. Leaves are some of nature’s most elegant machines. They use sunlight to synthesize food from carbon dioxide and generate oxygen. They also absorb particulate matter pollution through pores on their leaves.
Which, again, is great because pollution gets worse in drier weather when more dust and particles get kicked up into the air. And those particles get into your lungs and enter your bloodstream, increasing the risk of heart disease, lung cancer, and asthma.
“In terms of urban environments, yes the amount of greenness does make a difference both in the distribution and dissipation of air pollution, as well as the amount of pollution that’s in any local area," Bhatnagar said.
Overall, people who live in green areas are healthier. Just last year, Bhatnagar released research indicating people who live in greener areas have lower levels of stress and better cardiovascular health.
Also, trees are pretty. Right?
But We’re Not Doing This
Back in 2012 Louisville had about 37 percent tree canopy, according to an eight-year assessment.
It found the city is losing about 54,000 trees every year.
At that rate — when factoring in losses from the Emerald Ash Borer beetle — the city could have as little as 21 percent canopy about 2050.
Trees Louisville is a local nonprofit that formed in the wake of the assessment to restore the city’s tree canopy. Executive Director Cindi Sullivan says the city is losing trees at an even faster rate now.
“We have incredibly old data,” Sullivan said. “I truly believe that our canopy loss is much more dramatic than the 2015 Urban Tree Canopy Assessment would lead us to believe.”
Sullivan says individual property owners need to take responsibility. Why? Because 70 percent of all the land in the county is held privately, she said.
The best trees to plant are big: maples, oaks and sycamores that provide vast canopies and grow several stories high.
Since starting in 2015, Sullivan estimates Trees Louisville has planted 10,000 trees, many of them in the South and West End where the city’s canopy is the sparsest.
But it’s not just about planting more trees, it’s about preserving the trees that we already have, she said.
That’s where city policies can really make a difference. Right now, the city is considering an update to its land development code that would increase the amount of canopy that must be preserved when developers build.
“We cannot plant our way out of this, period,” said Metro Councilman Brandon Coan. “To me, tree planting is a good, sort of, tactical urbanist strategy.”
If We Did Do This, Here’s What It Would Look Like
This October, the Green Heart Project will begin planting 10,000 mature trees and shrubs in a middle-income community in Louisville.
Bhatnagar, with U of L, is the lead researcher on the study. His hypothesis is that increasing greenness lowers the risk of cardiovascular disease. And one of the big questions he wants to answer is 'at what density does greenery begin to positively impact health and a community?'
“When we have a dose-response curve, we can say we need five trees per block, or two trees per block, at least we will know something about that, right?” he said.
But there’s no reason to wait for the results. Bhatnagar already sees the potential for Louisville and the rest of the state.
The cities of the future will be denser with more public transit and more green space. Vegetation will line streets and interstates, acting as buffers from noise, light and pollution. The greenery will help prevent soil erosion, improve public health and mitigate weather extremes.
Bhatnagar says Louisville could be the green city of the future where nature and human beings can co-exist in the same space. Our soil is fertile, the rainfall is plentiful and when we strip away the cement and the asphalt, the earth grows verdant.
“Our inability to actually grapple with this is a major social issue. It has nothing to do with our technological issues because we have the technology to at least make a big difference,” he said.
None of this is to say that trees can do it alone. In order to avoid the worst impacts of climate change, we need to overhaul our energy, transportation and agricultural systems to reach as close to zero carbon emissions as soon as possible (the IPCC’s first report gives us just about 11 years give or take).
Still, trees are a good start.
In the final part of this series, WFPL News will look at Kentucky's forests and how they too can help manage climate change while providing economic opportunities for the state.
This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.