In the last two weeks, at least 19 people have been killed by the powerful atmospheric-river storms walloping California in relentless waves. “We’ve had less people die in the last two years of major wildfires in California than have died since New Year’s Day related to this weather,” Governor Gavin Newsom said in Sacramento this week after being sworn in for a second term on the grounds of the capitol, where piles of shredded branches offered a survey of another looming disaster to come.
Century-old eucalyptuses and sequoias had been splintered and uprooted around the rotunda along with an estimated 1,000 downed trees in the city at large that split stately Victorian homes in half, snapped utility lines, and caused at least two deaths of unhoused residents when trees fell on their tents. Between the downpours — storms are forecasted to continue for at least eight more days — neighborhoods echoed with the sound of chainsaws, the smell of damp sawdust hanging in the air as Sacramento’s prized urban canopy toppled like so many bowling pins.
With about a million trees in its nearly 100 square miles, Sacramento ranks among the top cities worldwide by nearly every tree-related metric — from trees per capita to canopy coverage — proudly earning its “City of Trees” nickname. Similar scenes to Sacramento’s are playing out throughout the state — with fully grown trees captured on social media moving like animated giants through landscapes, wading into swollen rivers, and sliding down hillsides.
Yet the storms’ destruction isn’t just about losing a beautifully leafy landscape. Urban forestry is a crucial front against an increasingly hotter climate, according to a forthcoming paper published by the University of California’s Agriculture and Natural Resources Cooperative Extension. “While all plants cool the surrounding environment through transpiration, trees play a particularly important role in mitigating climate change by shading urban heat islands,” Janet Hartin and Rob Bennaton write. This is in addition to removing the carbon dioxide from the atmosphere that contributes to greenhouse-gas emissions in the first place. So when the hyper-saturated soil combines with 70 mph winds, it sends the city’s best tools of resilience smashing into sedans parked on side streets. This is the challenge to come in the wake of the current crisis: After the storms quiet and rebuilding begins, how do cities replace critical urban infrastructure that took up to 100 years to grow?
Trees succumb to old age and disease all the time, but what’s notable about these storms is how indiscriminately the damage has played out across the city, says Gabby Miller, a Sacramento Public Works spokesperson. “Urban foresters typically plan their forests and maintain them in such a way as to withstand normal and expected weather conditions — including strong storms and periodic flooding,” she says, “but storms at the intensity we’ve experienced are capable of damaging healthy, defect-free trees of all species and sizes.”
This may result in noticeable gaps in the canopy for the City of Trees but a similar storm could be downright devastating for a place like Los Angeles, which has less tree coverage compared to Sacramento’s more robust urban forest. Half of L.A. residents live in neighborhoods with tree canopies below 10 percent, and one-fifth of the city’s tree cover is concentrated in four of L.A.’s wealthiest census tracts. Yet despite the known risks of extreme heat and air pollution facing treeless low-income neighborhoods, L.A. continues to lose shade trees at an alarming pace while simultaneously failing to meet its tree-planting goals. This is the reality for many of the state’s urban communities. A survey of 50 California cities showed that the average tree cover hovers at about 15 percent — a number that’s lower per capita than anywhere else in the U.S. Mature street trees are too scarce of a resource for the state to squander.
The current destruction playing out across the state is a product of extremes, the kind of wild vacillations of weather known as “precipitation whiplash” that will become more common in years to come. While the supercharged storms of a warming atmosphere temporarily dumped an unprecedented amount of moisture on cities, there are lasting repercussions from California’s more than ten-year drought, which has caused instability for those street trees, says Joe McBride, forestry professor emeritus at the University of California, Berkeley. “In a field situation, the lateral roots are one-and-a-half times of the canopy of the tree, and as those roots get longer, they’re adding more support to the tree,” he says. But that’s not the case in a parched city like Sacramento or L.A., where trees already have limited room to grow. “Drought doesn’t support additional growth of the root system to keep up with the canopy.”
Street trees are tested for hurricane-speed winds in places like Florida, says McBride, but not so much in California, where towering, top-heavy American elms are among the gargantuan uprooted trees seen around Sacramento. He hopes that the disaster will urge cities to make changes in their replanting strategies.
In addition to considering high winds whipping through drought-weakened canopies, a future of extremes means planting species of street trees less susceptible to heat and even different types of insects and disease. “In the situation of Sacramento, where they have so many American elms, those at maturity can be very large trees with a moderate-to-high water requirement,” McBride says. “It may be appropriate to say, ‘Let’s not replace that with an elm.’”
Sacramento’s political prioritization of its urban canopy has been shown to help the city save on energy costs, and some residents may feel the loss of tree cover in their AC bills during hot summer days, says Hartin, an environmental horticulture adviser for the University of California’s Agriculture and Natural Resource research center in Riverside. “Surface temperatures of asphalt parking lots in inland California can exceed 170 degrees during hot summer days, and the shade and cooling resulting from a single well-placed landscape tree can reduce the surface temperature to a relatively cool 100 degrees,” she says. Hartin predicts that these localized impacts could last decades as replanted trees grow: “It will still take the next 20 to 30 years to offer the benefit of the trees that were there.” As part of her research, she is searching for hardy and fast-growing replacements in one of three “climate-ready tree” nurseries across California, where scientists monitor the growth of a dozen species for 20 years to make recommendations for their suitability as street trees in the state’s changing climate. (Promising candidates in Hartin’s Riverside nursery so far include the Bubba desert willow, Maverick thornless, honey mesquite, and Red Push pistache.)
But there’s another factor that needs to be addressed: Most urban trees live significantly shorter lives than the same species do in the natural world. “We have to choose species that are recommended for the right climate zone,” Hartin says, “but we need better tree care that’s more focused on the health of the tree.” She notes that planting a tree in a stressed-out urban environment without more attention (and dollars) paid to watering, pruning, and providing adequate space for roots to spread can mean missing out on up to 50 years of benefits.
Even in the best of weather, California city life as it currently exists is not optimal for keeping trees alive. And many factors determining the survival of the state’s urban canopy are dependent on policies beyond forestry practices, says Megan Lynch, a horticultural scientist at the University of California, Davis, who spent six years studying street trees in Southern California. The biggest factor? “Car culture,” she says, decisively. “A huge part of it is the paving and asphalting of everything around us, which is directly contributing to the intensity of the floodwaters we’re seeing.”
Squishing a street tree into a strip of grass on the parkway is extremely common, but it might be a death sentence: The tree is crammed between two slabs of concrete, and the turf keeps water at the surface. And during times of drought, many residents responsibly let their lawn go dormant yet neglect to sufficiently water their trees.
Instead of flushing rain to sewers, picking up pollutants on the way, stormwater should be diverted into tree wells, says Lynch, where it can assist trees in establishing deeper and stronger roots, filtering toxins, and fixing nutrients like carbon and nitrogen in the soil. Depaving and widening tree wells to make room for healthier vegetation can recharge groundwater locally, helping the state weather the unpredictability between very dry and very wet years. The question remains, though, if officials will heed these warnings, particularly as California’s just-released budget zeroes out urban-forestry funding — including tree-based climate-mitigation programs that are proven to work. More storms will come, after all.
“We have a new normal,” says Lynch, “but we still don’t know what it will shake out to be.”