Urban Ecology

A Shady Situation

by Diane Boudreau

It’s an Arizona thing. People in other parts of the country just don’t understand. Here in the Sonoran Desert, it’s pretty common to pull into a parking lot and find the spots closest to the store open and available.

All the shoppers have parked at the far end of the lot just to snag the few spaces half-shaded from the sun. A long walk to the store is far better than driving home in a car that has baked for hours in the desert heat.

Business owners plant trees around their parking lots to provide shade. They often plant trees known to have a broad, full canopy—the leafy part of the tree. But when the trees grow on the shores of an asphalt sea, they don’t always live up to their potential.

“People plant trees for what they’re supposed to look like. But in a parking lot they don’t always end up looking as they should,” says Sarah Celestian, an ASU graduate student in plant biology.

Celestian wants to know how parking lots affect tree growth, and which kinds of trees do the best. She thinks that the heat absorbed and reflected by asphalt pavement is an important factor.

During a typical Arizona summer, parking lot surfaces can reach temperatures of more than 160 degrees Fahrenheit. The lots also hold onto their heat well into the night. Natural desert surfaces cool down much more quickly. Celestian studies the relationship between parking lot temperatures and growth among six popular types of trees.

“I drove around town and looked for parking lots with a minimum of five trees of one species located in the median, and five trees of the same species on the perimeter landscape. I’m comparing the median trees to the perimeter trees,” she explains. “The perimeter serves as my control group.”

The perimeter is the area surrounding the parking lot. This area is usually covered with dirt, gravel, or grass instead of pavement.

During the summer of 2001, Celestian studied 15 parking lots in the greater Phoenix area. The lots contained 149 trees.

Celestian took surface temperatures around each tree at several distances from the trunk. She took readings in all four compass directions. She also recorded surface types (e.g. asphalt, gravel, etc.) and whether or not the surface had plants growing on it. Next she measured tree size using canopy volume, height, and trunk diameter.

Celestian took her measurements from June to August between 1 and 4 p.m.

“It was so hot! I did about three parking lots a day,” she says.

She found that parking lot surfaces were up to 30 degrees hotter than the perimeter. Parking lots had higher temperatures than any other type of surface.

Celestian’s parking lots contain six different kinds of trees. She studied bottle tree, Arizona ash, Chilean mesquite, Aleppo pine, Canary Island pine, and Chinese evergreen elm. Only three of these trees—bottle, ash, and mesquite—are native to hot dry climates like that in Arizona. Not surprisingly, these trees did the best overall, especially the bottle and mesquite trees.

All of the parking lot trees had smaller canopies, heights, and trunk diameters than the trees on the perimeter. But the mesquite and bottle trees suffered the least.

The Aleppo pine and elm trees were the most reduced in parking lots in all the size categories. However, elms were the most common parking lot trees that Celestian found. They appeared in six of the 15 parking lots she studied.

“Elm seems to be a really good shade tree. It has a nice big canopy. I worked in a nursery and if someone came in looking for a shade tree for their yard, I’d say, ‘Buy an elm!’”

But Celestian wouldn’t recommend the elm for a parking lot. She also wouldn’t recommend pines. In her study, the pines had an 80 percent smaller canopy in parking lots than they do in other environments.

Celestian hopes that her work will help business owners make better choices when planting trees around parking lots. Ultimately, the Arizona native wants to help create more of those shady parking spots for desert drivers.