Environment

Louisville has been thinking about trees a lot over the past few years. The city is losing about 54,000 trees a year to storms, disease and development, and isn’t planting enough new ones to keep pace. And that’s exacerbating growing problems like the urban heat island.

City leaders are taking steps to plant more trees — there was a recent million dollar anonymous donation to aid that effort — and an ordinance to protect public trees was recently introduced in Metro Council.

But as Louisville and cities around the country work on solutions to replant and revive the tree canopy, others are working to quantify the benefits of trees to help strengthen the case for planting more.

Non-profit The Nature Conservancy released a study of trees’ effect on air quality earlier this fall, and lead author Rob McDonald was in town this week to discuss it. The study looked at 245 cities around the globe, and quantified how much trees can help make the cities cleaner and cooler. Or, as McDonald puts it, “is tree planting a medical intervention?”

Perhaps.

“The average tree is cooling the area just around it by about two to four degrees Fahrenheit in the summer — that’s the air temperature difference,” McDonald said. “So, it doesn’t sound like much, but in a heat wave, that could be the difference between an elderly person having heat stroke, heat exhaustion, and being a little bit safer.”

McDonald said the study also looked at whether trees could provide these cooling and cleaning benefits less expensively than other solutions, like switching diesel buses for gas or painting roofs white.

“And what you find is trees are cost-competitive against those other strategies,” he said. “So, doesn’t mean trees solve the whole problem — they don’t — but there’s a part of the problem they can solve and they’re a cost effective way to do that part.”

As long as cities not only plant the trees, but maintain them, he added.

The Nature Conservancy is also involved in a Louisville-specific study, along with the Institute for Healthy Air, Water, and Soil, and the University of Louisville. The Green Heart Project is studying two comparable Louisville neighborhoods, and planting trees in one. Then McDonald and others will see if the trees have a measurable effect on the health of neighborhood residents.

“There’s been a lot of studies that show trees can keep the air cleaner,” he said. “So, we think that has health benefits, but this is going to be the first study that in an experimentally rigorous way has shown that, all the way from planting the tree to measuring health benefits of people.”

That study is underway, but McDonald said the results won’t be available for another five years.

Erica Peterson is WFPL's Assignment Editor.