Rip out mature shade trees and put in palms?

That's the debate going on now in sunny San Diego—and it is shedding light on the complexities of a resource that has long been taken for granted but is increasingly recognized for its value: the urban forest.

It all started when some groups proposed removing 107 magnolia and ficus trees on Broadway, a busy downtown street. The idea was to replace them with medjool date palms, which already line the city's nearby waterfront.

To some people, the idea makes about as much sense as ripping out your front teeth and replacing them with tooth picks. They view the trade as an environmental misstep, because a palm provides comparatively few environmental benefits and absorbs only about half as much carbon as a similarly sized magnolia. And they say the idea is driven by an outdated notion of Southern California's identity.

Others love the concept and say it will beautify and unify an important thoroughfare and visually tie it with the waterfront, a major tourist attraction.

If anyone can pin a value on a particular tree, it would probably be Kelaine Vargas, founder of a San Francisco-based company called Urban Ecos.

"Trees provide us with ecosystem services," she said. "They do work that goes beyond sitting out there in the landscape and looking pretty."

Vargas is trying to help cities recognize what trees can do for them. She developed a software program with the U.S. Forest Service to help municipalities map their trees, both public and private, and quantify their myriad benefits.

The maps, which have been created for Sacramento, Philadelphia, San Francisco and San Diego, keep a running tally of the trees' monetary contributions. San Francisco's "yearly eco-benefit" from its 86,815 mapped trees, for example, is $3,446,292.

The calculation, based on each tree's species, GPS coordinates and trunk diameter, shows San Francisco's trees are taking in 26,947,681 pounds of carbon from the atmosphere, a contribution worth $538,953; saving 139,655,264 gallons of water, worth $558,621; conserving 13,554,893 kilowatts of energy, worth $1,179,312; and reducing 94,027 pounds of pollutants from the atmosphere, valued at $555,405. (Last week's Weather Insider column focused on a recent recalculation of trees' ability to sequester carbon.)

Trees' "eco-benefits" are a big part of the debate in San Diego, a much larger and more geographically diverse city than San Francisco.

According to Mike Palat, an arborist and chairman of the San Diego Regional Urban Forests Council, shade trees not only sequester far more carbon than palms, they also produce much more oxygen, capture more storm-water runoff and air pollutants, and provide more shade to cool an urban area loaded with asphalt and concrete.

"It is truly night and day, the environmental benefits from a mature shade tree compared to a palm tree," Palat said. "Plus, the palms are higher maintenance" because date trees can be very messy.

"The existing trees are in good health and provide benefits. If you have to tear out trees, at least have a good reason to do so."

The dispute in San Diego was triggered by an effort that has other environmental implications: a push to make rapid transit downtown more accessible and appealing. A regional planning agency, known as SANDAG, plans to put eight new bus stations along Broadway. Doing that will require reconstructing sidewalks and medians and removing about 25 magnolia trees.

SANDAG, which has $1.5 million in taxpayer funds for the trees, held meetings with downtown business and property owners to discuss the proposal. That's where someone came up with the idea to replace all the trees on Broadway with palms. Janelle Riella is director of policy for the Downtown San Diego Partnership, one of the groups that wants the palms installed. Riella didn't return calls for this story, but she told KPBS, a local public radio station, that the palms would link the street with the North Embarcadero waterfront.

"If you bring that all the way through Broadway, it could be really beautiful," she told the station.

Vicki Estrada, a land planner who wrote the current landscape manual for downtown San Diego, said she recognizes that the business group was trying to make the environment as pleasant as possible.

"But they didn't realize what it does to the character and usability of the streets," she said. "If you ask 100 people, would you rather sit under a broad shade tree or a palm tree, they'd all say a shade tree.

"This is done mostly for tourists, at the expense of those of us who live and work downtown."