A walk through many San Francisco neighborhoods is an arboreal adventure, thanks to the nearly 125,000 trees lining the city’s streets. Within this urban forest are about 500 species — some native, others not.
But climate change is shaking up the city’s canopy faster than anyone imagined. To get a better sense of what the city can expect in the next couple decades, I checked in with Michael Sullivan, author of The Trees of San Francisco and once an avid planter as a volunteer for Friends of the Urban Forest.
Since Sullivan moved to the city in 1984, the street canopy has evolved from a few fast-growing species that thrived in SF’s cool climate — like ficus planted in the 1970s and ’80s — to a more diverse population.
But hotter, drier conditions, threatening many species, aren’t just bad for certain trees. They’re bad for neighborhoods that don’t have trees, something Sullivan, a member of SF’s Urban Forestry Council, and others want to address.
When the pandemic hit and we were not allowed to gather in large groups outside, Sullivan turned to guerilla sidewalk chalk tours, inspired by reports of similar activity in London. He started in his own Cole Valley neighborhood with 25 trees he picked at random, including the scientific name, place of origin, and an interesting fact, all off the top of his head.
I first saw one earlier this year while walking my dog along Clayton Street. An arrow pointed to a tree, as if to say “Pay attention to me!”
It was an American sweetgum, found in Mexico and Central America, among other places. The label included the hashtag #sftreetour, a website, and another arrow pointing to another tree up the street.
Here from Down Under
I caught up with Sullivan, a longtime Cole Valley resident, in front of the Boulangerie de San Francisco on the corner of Cole and Parnassus on a late-summer morning to take a portion of his neighborhood tour. Right off the bat, he pointed out specimens that we might see a lot more of in coming years.
In front of the bakery was an Australian sweetshade tree, and Sullivan said he was involved in its planting some 20 years ago. It’s still thriving. The sweetshade is from eastern Australia, where the climate gets enough rain some years and not enough in others — just like here.
The city’s 2016 tree census estimated the canopy helps sequester over 19 million pounds of carbon dioxide and filters more than 100 million gallons of storm water every year.
A few feet up the hill was a lemon bottlebrush, another Australian tree adapted for hot dry climates. Sullivan snapped off a twig to show numerous seeds that sit in wait for fire to release them.
Sullivan then showed me two Chinese elms on upper Belvedere and pointed out the bark. It was grey with random brown dashes and dots. He pulled off a piece to show how it looks like part of a puzzle.
He couldn’t say how the elms might fare as temperatures rise, but Sullivan had one more candidate for a warmer SF on his tour. On 17th Street, we paused at the city’s tallest cork oak, which comes from the Iberian Peninsula and thrives in hotter Bay Area climates, including Napa. Sullivan encouraged me to push my finger into the bark, which is soft and yields a little to pressure.
More chalk walks
The Cole Valley chalk tours caught on, and Sullivan has teamed up with friends — horticulturalist Jason Dewees and Pacific Horticulture editor emeritus Richard Turner — to push into other neighborhoods such as Forest Hill, Noe Valley, and the Bayview, with a focus on Quesada Gardens, a resident-driven initiative that began more than a decade ago.
Sullivan said residents approached them to share stories and information about their trees. Other times, the information flowed in the other direction. “Some people weren’t aware of what was in their yard, but they were proud that we’d selected and labeled it,” he added.
This type of outreach is important as the city tries to bring street trees into other neighborhoods that have so few of them, according to Mike Yarak, urban forestry program manager for Friends of the Urban Forest (FUF). “We can’t just plant a tree and walk away and say ‘Problem solved,’” he wrote in an email. “Forming genuine partnerships needs to be part of the process of increasing canopy.”
It is estimated that the current SF tree canopy helps sequester over 19 million pounds of carbon dioxide and filters more than 100 million gallons of stormwater every year, according to the city’s street tree census.
But the canopy isn’t distributed equally. Low-income communities of color have fewer trees, leaving them to endure hotter temperatures than wealthier, whiter parts of the city. For example, Quesada Gardens is a green oasis within a district that has very few trees. According to an ABC7 News interactive map, 4.3 percent of the census tract where the community garden is located is covered with trees. By contrast, the two census tracts that include Cole Valley have up to three times as much coverage.
To correct this shade imbalance, FUF directs funding for free plantings in underforested neighborhoods.
Killer heat wave
But can they plant fast enough? Rising temperatures are hurting SF’s trees.
Sullivan vividly remembered late September 2020, when the city experienced two and a half days of 90-plus-degree heat. The month before, some 75 very young trees were planted around Cole Valley, and the multiple days of extreme heat meant trouble because young trees don’t have a mature root system to tap into the water table. Sullivan rounded up gallon jugs and drove around to as many of the new trees he could.
“It doesn’t take much water — just a half gallon,” he explained, but he still couldn’t get to all of them, and he estimates 40 percent died.
Mature trees are at risk too. Rising temperatures and extended drought has stressed them, which makes them more vulnerable to pests and disease, according to Sullivan. The city is losing its myoporum to myoporum thrips, and the Mount Sutro eucalyptus is suffering from a beetle infestation.
The loss of more mature trees has a bigger impact, as they provide the most shade cover, noted FUF’s Yarak. Younger trees that replace them take decades to reach a similar size.
City tree experts are now looking for trees that can handle higher temperatures, with an eye toward what’s currently doing well in Southern California, like the island oak. “I think we’ll see the addition of more species that are already adapted to the climate we expect to see here in the future,” wrote Yarak.
That means climate change could do what many San Franciscans have resisted for so long: Make our city a little more like Los Angeles, at least when it comes to the street treescape. If that’s what it takes to keep our neighborhoods a little cooler, though, it’ll be a welcome evolution.
