April 6, 2005

San Francisco's Urban Forest May be Stranger than Necessary

By Erica Gies
Special to the Neighborhood Newswire

Cities are often defined by their skylines, but what makes them "livable" – that nebulous quality that sooths urban dwellers' jangled nerves – may have more to do with patches of nature than architecture. In San Francisco, these patches are stitched together through backyards, boulevard medians, and almost 100,000 street trees to create a diverse planted area containing 700,000 trees, almost one for every person who lives in the city.

"I think we need trees around us. Studies have shown that trees relieve tension in people," said Alexis Harte, coordinator for the Urban Forest Council, a multi-agency advisory body nested within the San Francisco Department of the Environment. "We need shade as we're walking down the sidewalk."

According to a recent University of California (U.C.), Davis study about 20 percent of the city’s street trees are tended by the Department of Public Works (DPW). The rest are cared for by private property owners, sometimes with help from San Francisco’s urban forestry nonprofit, Friends of the Urban Forest (FUF).

The U.C. Davis study identified a number of benefits from San Francisco’s urban forest, including reduced energy bills, principally by helping with heating and cooling; the sequestration of more than 2,200 tons of carbon dioxide, a “greenhouse gas” that contributes to global warming; and the trees' ability to intercept more than 13 million cubic feet annually of rain, thereby reducing stormwater runoff. All told trees save the city more than a half-million dollars a year, according to the study.
The study found 115 different tree species on San Francisco's streets, a diversity that should protect the urban forest from blights like those that devastated the American chestnut and elm in eastern cities. However, like many of its human residents, almost all are visitors from other places.

"If we were limited to planting native vegetation in San Francisco, we'd have very few trees," said Harte. "The native vegetation here was akin to the Marin headlands. It was mainly grasses and forbs, scraggly oaks, toyon. Here the city was laid down upon sand dunes."

According to Doug Wildman, a FUF program director, most native trees have low, spreading canopies that don't mesh with the public safety requirements for street trees, part of which includes an 8-foot initial clearance.

"The way a lot of natives grow, it would be difficult to have the tree be healthy and express its natural form and adhere to the guidelines that are in place for safety," said Harte. "So you think about a street tree, and the ideal is one that grows up fairly straight and doesn't have low branches that will knock you off your bike. [The natives] are not appropriate from the public safety point of view."

However, Wildman says coastal live oak is one native that can be safely planted on city streets. In fact, on Arbor Day FUF planted this species in front of Charles R. Drew Elementary School.

Trees from all over the world grow well in the city. Australian natives flourish in microclimates near the oceans, while more tropical species, like the jacaranda, enjoy the Mission’s warmer climes. "These trees were chosen because of their performance and toughness and their known value of that day," said Wildman. "So if a tree was planted 60 years ago, it was believed to be a really great tree, resistant to pests or strong winds."

However, some argue there are better alternatives to planting trees from Australia and Chile. Peter Berg, director of Planet Drum, an organization that advocates a bioregional basis for urban sustainability, admits that, with the exception of coastal live oak, few species from vegetation zone Franciscan I (San Francisco), are suitable as street trees. However, he believes the city could do a better job attracting regional species into San Francisco.

"Yes it’s true that only a few trees allegedly did well in Franciscan l geography," he said. "Yet Northern California has a wide range of trees that do well here. What I would contrast is using exotic species from all over the world with using native Northern Californian plants that are much more likely to contribute to ecosystems and habitat in San Francisco. A city is a fairly exotic environment anyway. But I'd much rather see a mazanita or a ceanthothus than an English privet.

"I'm tired of exotics,” he added. “You know what's exotic now? Natives! Natives are exotic!"

City officials base their planting decisions principally on studies of what’s already in the soil. Many existing trees were planted during a time when decisions were dominated by aesthetics, sentimentality for other places, or easy adaptability to the local climate.

The emphasis on trees, rather than on other types of shrubs and plants, is in part the result of city planning criteria that emphasize safety and permit requirements that impose higher costs to create more planting space. For example, if a property owner wants to expand the green area on the sidewalk in front of their house, they must file for a minor sidewalk encroachment permit, which costs $716, with DPW. The fee covers the cost to inspect the site for safety but has perhaps the unintended effect of limiting the number of property owners willing to pony up the fee to increase their square dirt area.

Nick Elsner, senior plan checker for Bureau of Street Use and Mapping, DPW, said, "We look at a couple of things. We look at the neighborhood pattern first. Is it a 15-foot-wide sidewalk? Does everyone's front lawn extend out? Is pedestrian traffic heavy? Light? Nonexistent? Could a blind person meander their way from the parking space to the walking area of the sidewalk without walking through the plantings? We're addressing these on a case-by-case basis."

Drum has managed to plant the area in front of his offices at 451 30th Street, although decreasing stormwater runoff and increasing aesthetic beauty weren't his only motivations. He sowed his beds with native plants, including snapdragon, low juncos, coffeeberry, coastal sage, and yerba buena, which he says makes a "marvelous tea." His garden has just been certified as an urban wild habitat by the National Wildlife Federation.

"[Native plants] are not damaging to the soil," he said. "They do not require constant irrigation. They can make it through the winter wet/ summer dry. We have ruby throated hummingbirds in our garden. We have California sister butterflies."

However, concerns over traffic safety dominate the planting choices on boulevard medians, says Bureau of Engineering landscape architect John Thomas. "It depends on the median width," he said. "If the median is narrow, the canopy may interfere with vehicles that are driving by on either side. However, trees with an upright form or that are tall enough that their foliage doesn't interfere can work."

Thomas also cautioned that groundcover and shrubs can require more maintenance than trees, which can end up being a safety issue for both the gardener and drivers, as a lane of traffic is shut down while maintenance is being done. "The trees are highly visible and are relatively low maintenance," he concluded.