Guest Commentary

Keep Tucson cool by paying for a tree

Steven Haas might be on to something. After the Catalina State Park manager announced in late December that people could honor their loved ones by paying $350 or so to plant a tree, some 20 people called to lay their money down. As he mentioned during a phone call, the enthusiastic response is almost more than the park can handle.

Tucson residents, take note: Donations might be a way to get the cloak of greenery so desperately needed to keep our pavement, sidewalks and red-rocked xeriscapes from heating the city beyond bearable. It turns out that the Tucson Parks Foundation can accept donations for planting city trees, with memorial trees running $100 each.

The city has room for another couple of hundred trees just in the square mile centered on downtown, according to a 2011 analysis by Alison Meadow while she was a landscape architect intern for the city. Some 237 spaces are ready and waiting. It's a matter of finding the funding to fill them with trees.

Planting 20 donated trees every few months could help the core of the Old Pueblo reach a more-sustainable state. Everybody with skin knows that shade cools things off. Meteorologists regularly put numbers on the extra heat we feel from the humidity factor, or the extra kick of cool from wind chill. But you won't find a systematic reporting of the shade-chill factor.

We measure the temperature of air, while the shading effect occurs on a surface. Air doesn't have a surface—but people do. So do streets, buildings, sidewalks and other heat-radiating structures that cover the cityscape. For another thing, it's tough to pinpoint the temperature difference of shaded versus sun-soaked surfaces. It changes with the color and density of the surface, time of day and season.

To get a feel for how it works, I borrowed an infrared thermometer from UA climatologist Michael Crimmins and wandered around town pointing it at the ground. At about 4 p.m. on a May day, the surface temperature of unshaded soil was running about 90 degrees, some 30 degrees hotter than shaded soil. The difference was even more extreme for rock, which was reaching 130 degrees. My informal findings echo the results of more-refined studies by others.

Of course, the benefit of a cooler city comes with a cost. The $100 for a city tree or $350-plus for planting a bigger one in the state park goes beyond the price of the tree, because it considers factors such as the amount of water it will take to keep the tree alive.

The cost in water might raise more eyebrows among desert dwellers than the price tag; we obviously have limited local supplies. Spending water on cooling, though, is one of the better uses for it in a town where temperatures can run dangerously high.

Much of the water that trees use goes to take the edge off the heat in the same way that our swamp coolers do. When water evaporates, whether from leaves or cooler pads, it takes heat with it. This evaporative cooling can chill the air in an enclosed house by some 30 degrees. It can cool the air around trees and grass by several degrees—perhaps dropping the air temperature in the immediate environment from 104 degrees to 98 degrees on a good day.

For heat-sensitive people, every degree closer to body temperature makes a difference. It's especially crucial in the city, where pavement, concrete and buildings crank up the heat. So it was nice to hear that Peg Weber, who calls herself a "happy public servant" for Tucson Parks and Recreation, is willing to act as the point person between the parks foundation and the various city departments. People can contact her to donate trees, whether they'd like to see them in city parks or along city streets. A $100 donation covers a memorial tree and its maintenance, including irrigation.

Trading greenbacks for greenery yields benefits that far outweigh the costs. A lot of residents evidently already get this, based on the number of people ready to pay for trees in Catalina State Park. Let's hope city-dwellers will put their money where their house is.

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