The shady expanse just east of Park Avenue, next to the Arizona State Museum, is some of the most inviting real estate on the University of Arizona campus. Lush grass is shaded by rows of towering pines and sheltering olive trees. Students are drawn there to study, to gather for quiet conversation or to doze.
"You couldn't get them to describe it, but they all say they like that part of campus," said Elizabeth Davison, director of the UA Campus Arboretum. "And you say, ‘Well, why?' ‘Well, the buildings are big and the trees are big and the shade's good and there's grass.'"
But another answer might be carbon sequestration.
Davison, as part of a street tree survey conducted with the city of Tucson, determined that the 2,000 campus trees that flank public streets sequester 246,620 pounds of carbon dioxide each year and absorb nearly 10,000 pounds of particulates. By lowering the temperature on campus and shading buildings, the trees of the UA save $18,230 a year in energy costs. They intercept more than a million gallons of storm water.
And, with an estimated 7,000 trees on campus in total, that's only a partial accounting of the benefits.
Davison said the data gathered using software developed by a former UA associate professor, Greg McPherson, who now heads the Center for Urban Forest Research at the University of California at Davis, translates "intrinsic value to dollars." Armed with this information, she said, "We could talk to municipalities and foresters and street taker-care-of-ers and governments and say, ‘This is what it costs to have a tree, yes, but here's the benefits that that tree produces.'"
The calculations are made by feeding the location (west side of a building, for instance) and size, to figure how much shade will be thrown, into the program. "And if you say we live in Tucson, Arizona, and it costs us 90 cents per a therm to cool our buildings," said Davison, "then it tells you how much you're saving, that tree multiplied by all the other trees that are doing basically the same thing at different rates because they're different species. It's magical."
The model can determine how much sequestered carbon is being held "in any kind of a tree trunk from now till the end of time," she said. "So when we want to say, ‘Look, the UA's carbon footprint is shrinking.' Well yeah, we're helping."
Davison's next goal is to gather data for the other 5,000 trees in the Campus Arboretum. The difficulty, she said, is that "oddball trees, which we have many of," do not have assigned values in the model. The Arboretum, as the oldest continually maintained public green space in the state, is Arizona's premier destination for rare specimens.
"They know about eucalyptus, they know about mesquites, they know about palms," she said. "That's used all over the Southwest, so they were all integrated into it. But when we talk about some of these small trees from Africa that we're the only ones in the state that's got it, then how do you measure that? It's a little trickier."
The campus landscape was officially designated an arboretum in 2002, taking its place as a member of the American Association of Botanical Gardens and Arboreta.
Davison is based in Herring Hall, the second-oldest existing building on campus (after Old Main). A Roman revival structure that looks as if it were plucked from Colonial Philadelphia, it was built in 1903 as the Men's Gymnasium.
Along with cataloguing and mapping every tree and adding signs to identify them, finding the history of the trees is an important part of Davison's job. "Finding out who planted that old one over there, that funny one over there," she said. The olive trees beside Park Avenue were planted in 1885, during the first days of the University, by Robert Forbes, the first head of the Agriculture Experiment Station.
"From the very first, when the state university was assigned to Tucson, we began to make a green landscape to try to make some kind of civilization out of the desert," Davison said. "To try to civilize and attract and cool off and create intellectual open space from the very first day all around Old Main. You see those old pictures with just creosote. Well, the next picture you see has trees and grass, because that's how you learn – it's not by freaking out in the desert. And they pushed the desert away, and the walls around all pushed the desert away and everything became like an oasis and all those palms and all those pines stood in standing water and it was just lush lush lush."
Some of the trees are nearly as old as the campus itself. Davison has been cataloguing the thousands of trees. "In a lot of cases, it's the only one in Tucson; it's the biggest one in the state; it's the biggest one in the Southwest; it's the only one in the state." Twenty such specimens are designated heritage trees. "Just about every one of these is one of a kind," Davison said.
There is the Bicentennial Moon Tree, an American Sycamore grown from seeds that had gone to the moon with Apollo 14. It can be found, appropriately, beside the Kuiper Building, which houses the Lunar and Planetary Laboratory. There is the towering Baobab next to the Administration Building that is "the only flowering example of its species in the Western Hemisphere."
Every tree on campus is numbered. "Every time a new tree gets planted, we have to take a picture of it, enter it in the database, put it on the map, give it a number, make a species page, get all that information there and then take care of it," Davison said.
"The whole message of this campus has been, Come shelter here. It's cool. It's green. You can escape. You can think. You can learn," she said. "This is the intellectual environment."
Part of the objective of the arboretum is preserving the heritage, Davison said. "Part of it is the conservation of the unique species and the conservation of the stories about those species. Part of it is keeping the campus as green as possible, and, of course, the new version is we're the negative on your carbon use, Mr. Campus. We're the way that you can keep it down."