Judging from how hot it has been, this year could end up being Austin’s hottest ever. Heat impacts health, happiness and the environment. So the city is trying a simple approach to reducing it: planting trees.

But those efforts have raised some interesting questions about what exactly to plant in a climate that is rapidly changing.

Trees can be an effective tool in the battle against heat, because our local climate is, in part, determined by our built environment. Cities are often around 5 degrees hotter in the day and more than 20 degrees hotter at night than surrounding country, according to the EPA.

“Cities are just natural heat pockets, where heat sinks” said the city’s Leah Haney. “It’s something that's special that happens when you have a lot of manmade materials.”

So, partnering with local nonprofits like TreeFolks, Austin distributes 4,200 trees every year for the specific purpose of combating urban heat.

“When trees have enough room to grow, and they’re healthy, they do this amazing thing where they actually cause the air around them to be cooler through a process called evapotranspiration,” said Emily King, with the city’s Urban Forestry Department.

The process can drop the temperature around a tree by up to 9 degrees.

“The whole impetus behind this program and this funding is to help cool Austin, to help shade it and help cool it,” said King. That's one reason tree planters even use satellite images to look for bald spots in the urban canopy where more trees can go.

Austin’s not the only place giving it a shot.

“The research shows that for every 10 to 15 percent of increase in canopy cover you can reduce the urban temperature by up to five degrees Celsius” said Cathy Oke, a city council member in Melbourne, Australia. “So we have a target to double our canopy cover.”

But there’s a problem in both Melbourne and Austin. Over the years, trees have been are dying faster than they can be replaced. Some of it’s because of development. Some of it’s because old age and, drought. And, in some cases, it’s because new invasive trees are taking over native species.

The power of invasive trees was on full display earlier this year in Walnut Creek Park, where volunteer Cliff Tyllick pointed to a spot where invasive ligustrum had sprouted up in a section of forest.

“The tree we see leaning there is a pecan. It died in the drought. Ligustrum is all around it," he said. He pointed to another tree, an escarpment black cherry, that also died in the drought. The ligustrum surrounding it were thriving.

KUT's Mose Buchele reports

Scenes like this illustrate how local trees that evolved to survive this harsh weather eventually fell victim to the drought, while some of the invasive species actually survived it. That begs the question: If the city is planting trees to prepare for Austin’s hotter future, should some of these exotic species – the very plants that seem so resilient in the dryness and heat – be included in that plan?

King said that's a loaded question in tree circles.

"Depending on who you ask, you’re going to get a different answer to what should we be doing ," she said.

Cathy Oke said Melbourne is planting a mix of native and exotic species.

But Meg Inglis of the Native Plant Society of Texas says, as far as climate change is concerned, she would hedge her bets with natives.

She says a lot of introduced species are just too destructive. They starve out local wildlife, suck up too much water and generally turn ecosystems on their heads.

“If the ecosystem collapses then we lose ecosystem services that are provided to us like sequestering of carbon, providing oxygen, putting more water into the aquifer,” she said.

But what if the ecosystem is already changing? Since we don’t know exactly how hot, dry or flood-prone climate change will make us, does it make sense to hedge our bets by bringing in some outside trees?

“I don’t think that we can be overly picky with where exactly something originated, if our end goal is to make sure Austin remains liveable,” said King. “And to be liveable, we do need to try to reduce the heat that we’re experiencing.”

A lot of this is just a question of how you define “native.” Is it a plant from a bit south of here that’s now spreading in? An introduced tree that’s been here for centuries?

Tyllick, the invasive species removal volunteer, said he prefers natives but adds that, within that range, you can find a lot of things to plant.

“A nice thing about this particular replanting program is that they’re planting lots of different species and the ones that do well in the conditions we end up having over the next five, 10, 15 years are the ones that will thrive."