This winter’s atmospheric rivers dumped rain, brought howling hurricane-force winds, and downed hundreds of San Francisco’s trees. At least three people died, traffic and transit was disrupted, and neighborhoods lost old favorites.
While the loss of sidewalk trees is bad — even dangerous — for urban areas facing climate change, some SF natives could benefit from the thinned canopy in parks and greenbelts, where the storms hastened a habitat reboot for quail, butterflies, and other birds and insects that once thrived in our natural areas.
“Ninety percent of the native birds are found in the first three feet of vegetation, so the understory now has room and access to sun and resources to grow,” Josiah Clark tells The Frisc. Clark runs a consulting firm that works with the city to restore and maintain green spaces.
To be clear, losing street trees is different. They shade sidewalks, cool homes and businesses, and trap rain runoff. But in Golden Gate Park, Mount Sutro Open Space Reserve, the Presidio, and other bigger green spaces, the low-lying vegetation — the understory — is crucial to support a diverse biome of native birds such as the spotted towhee and dark eyed junco, as well as insects that serve as food (spittlebugs) and pollinators (bees and butterflies), with many populations in deep distress.
A healthy understory is also key to reintroducing the state bird — the California quail — to places such as Golden Gate Park and the Presidio.
Many of SF’s grand stands of trees and urban forests came to be in the 19th century, replacing sand dunes, coastal scrub, and oak woodlands.
In 1886, Mayor Adolph Sutro planted thousands of eucalyptus, cypress, and pines on his land, and they grew into tall forests where the eucalyptus mainly won out.
On Mt. Sutro, which rises above the UCSF Medical Center, the winter storms accelerated removal of thousands of dead and dying trees that began in 2019 — controversial in some corners — and expected to continue for 20 years.
“The land is reclaiming itself, and you don’t have to worry about someone filing a complaint,” says Dominik Mosur, a local naturalist who leads birding tours for the Golden Gate Audubon Society.
The Sutro renaissance now includes an effort to restore a seasonal creek that runs down the moutain’s northeast slope. Currently the area’s understory is overrun with invasive Himalayan blackberry and cape ivy, so dense that young trees cannot grow.
But fewer eucalyptus, and more light shining through, means a more diverse groundscape and canopy could soon take root here and in other green spaces.
Filling in niches
As the sun peeks through late morning overcast in the Presidio, Lew Stringer and Stephen Duffy look over young Monterey cypresses growing alongside newly planted vegetation that will serve as the understory in a restored segment of forest along Park Boulevard, to the west of the national cemetery.
The Presidio Trust employees — Stringer is associate director of natural resources, Duffy is an arborist — say the project is going well so far, noting bumble bees buzzing around purple lupine and white crowned sparrows flitting about. The variable checkerspot butterfly, once the subject of a reintroduction effort, will also use this habitat, from the ground up to about ten feet, says Stringer: “It’s just so rich with the abundance of species that we’re bringing in that fill that niche.”
A reminder of past practices is visible to the north: a row of old spindly Monterey cypress, all roughly the same age, and mostly gray. Their only green is at the top, an unhealthy sign of the tree working hard to get nutrients to its sparse leaves and branches, notes Duffy.
The same scenario is playing out across San Francisco. While Adolph Sutro was foresting up his property in the late 19th century, the Army was doing the same in the Presidio, planting acres of eucalyptus, Monterey cypress, and pines. The Army wanted to cover up the scrubby dunes and grasslands and differentiate its property from the city, notes Duffy: “They were uninterested in biodiversity under the canopy.”
The Presidio’s old-growth trees, declining at the same rate, now create a logistical challenge, says Duffy. Clearing them all at once decreases carbon sequestration and worsens runoff, making it harder to re-establish a new stand of trees.
But financial and logistical complexities leave Duffy and colleagues little choice. These reforestation efforts already cost more than $1 million a year and depend upon — wait for it — an army of employees, contractors and volunteers to remove old brush and trees and plant anew. The storms did their part by downing roughly 100 trees throughout the park.
The toppled trees don’t just open more space for light to shine through. The fallen limbs and branches also supply habitat, and birds, which began nesting as early as February, have taken advantage.
But much of the material needs to be cleared, which means the new inhabitants, which raise broods through mid-summer, pose a problem. The Presidio uses birders to check for nests, and once an area gets the all-clear, workers have up to three days to remove trees and brush. After that, another nest search must take place.
In city-run parks and green spaces, however, crews are stretched thin, and more coordination like this is needed, says Audubon birding guide Mosur, who worries that a pair of nesting song sparrows might have been crushed by a wood chipper in Corona Heights earlier this month.
Mosur isn’t blaming the arborists. “I sometimes explain to tour groups that there’s a natural incongruity between humans and birds,” he says. “People need to get jobs done and birds are on a schedule to breed.”
The same disconnect between humans and nature was evident this winter. Our urban infrastructure and past attempts to shape nature to our liking revealed their limits in the face of historic downpours and other extraordinary weather conditions. The climate will only get more unpredictable, but if this winter was any indication, there are rays of hope shining for the birds and bees — and butterflies too.