A Shady Situation

Sarah Celestian may be the only person who voluntarily loiters in Phoenix-area parking lots on blazing summer afternoons. She has her reasons.

It’s an Arizona thing. People in other parts of the country just don’t understand. Here in the Sonoran Desert, it’s not unusual 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 handful of spaces half-shaded from the sun. A long walk to the store is far better than driving home in a car that’s baked for hours in the relentless desert sun.

Business owners liberally plant trees around their parking lots. They often plant species known to have broad, full canopies. But when the trees grow on the shores of an asphalt sea, they often fail to 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 trees fare the best. She thinks that the excessive 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 retain their heat well into the night. Natural desert surfaces give back the heat much more readily. Celestian studies the relationship between parking lot temperatures and growth among six popular species 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.”

During the summer of 2001, Celestian monitored 15 parking lots in the greater Phoenix area. The lots contained a total of 149 trees. All of the trees had been established for at least five years.

Using an infrared thermometer, she took surface temperatures around each tree at several distances from the trunk. Celestian took readings in all four compass directions. She also recorded surface types (e.g. asphalt, gravel, etc.) and whether or not the surface was vegetated. The ASU researcher then recorded tree size using canopy volume, height, and trunk diameter.

“Trunk diameter is often the best measure of size for urban trees,” says Celestian. “Pruning can reduce canopy volume artificially.”

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.

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

All of the parking lot trees had smaller canopy volumes, heights, and trunk diameters than their perimeter counterparts. However, mesquite and bottle trees were the least stunted in size. In fact, the reduction in trunk diameter among mesquite trees was so small that it was not statistically significant.

Conversely, the Aleppo pine and elm trees were the most reduced in parking lots in all the size categories. Ironically, elms were the most common parking lot trees that Celestian found, appearing in six out of 15 parking lots.

“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!’”

Unfortunately, elm trees just don’t grow as well in a parking lot environment. Neither do pines, which had an 80 percent smaller canopy in parking lots than they do in other environments.

Celestian found that parking lot surface temperatures were up to 30 degrees higher than the perimeter surfaces. Parking lots had higher temperatures than all other types of surfaces studied.

Celestian is currently taking measurements underground, below the asphalt. She wants to find out what is occurring where the tree roots grow. She buried temperature loggers under one of ASU’s large parking lots. The devices will log temperatures every hour for 85 days.

Celestian says the belowground heats up, but doesn’t get as hot as the surface. “It stays pretty warm throughout the day. What’s interesting is that the hottest temperature under the asphalt occurs at midnight. How does that affect the growth of the plant?”

She says that heat may not be the only factor influencing tree growth in parking lots. For example, soil under pavement is very compacted and may restrict root growth. Pavement might also limit water availability to the roots.

In addition to studying underground temperatures, Celestian measures the chlorophyll content of leaves on ash, elm, and bottle trees. She is also takes gas exchange readings on the leaves to see what the stomata—openings that take in and release gases—are doing during the heat of the day.

Ultimately, the Arizona native would like to help create more of those coveted shady parking spots for desert drivers.—Diane Boudreau