California Storms’ Tree Destruction Will Have Climate Impact

Photo: Kathleen Ronayne/AP/Shutterstock

In the past two weeks, at least 19 people were killed by the powerful atmospheric river storms that roll over California in relentless waves. “We’ve had fewer deaths in the last two years of major California wildfires than there have been since New Year’s Day because of this weather,” Gov. Gavin Newsom said. in Sacramento this week after being sworn in for a second term on the court. from the Capitol, where heaps of jagged branches offered a glimpse of another impending disaster to come.

Century-old eucalyptus and redwood trees had been splintered and uprooted around the rotunda, along with about 1,000 downed trees in the city as a whole, which split stately Victorian homes in half, snapped power lines and caused at least two deaths. homeless residents when trees fell on their tents. Between the downpours — the storms are expected to continue for at least ten more days — neighborhoods echoed with the sound of chainsaws, the smell of wet sawdust hanging in the air as Sacramento’s prized urban canopy crumbled like so many bowling pins.

With about a million trees Within its nearly 100 square miles, Sacramento ranks among the best cities in the world by nearly every tree-related metric — from trees per capita to canopy cover — which proudly deserves its nickname of “City of Trees”. But scenes similar to those in Sacramento are playing out across the state – with mature trees captured on social media moving like animated giants across the landscapes, wading through swollen rivers and sliding down hillsides.

Yet the destruction of storms is not just about losing a beautifully green landscape. Urban forestry is a crucial front against an increasingly hot 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,” write Janet Hartin and Rob Bennaton. This is in addition to the removal of carbon dioxide from the atmosphere which contributes in the first place to greenhouse gas emissions. So when the hyper-saturated ground combines with 70 mph winds, it sends the city’s best tools of resilience crashing into parked sedans on side streets. This is the challenge ahead in the wake of the current crisis: after the storms calm and reconstruction begins, how do cities replace critical urban infrastructure that has taken up to 100 years to develop?

Trees succumb to old age and disease all the time, but what’s remarkable about these storms is how indiscriminate damage has occurred across the city, says spokeswoman Gabby Miller of Sacramento Public Works. “Urban foresters typically plan their forests and maintain them to withstand normal and expected weather conditions, including heavy storms and periodic flooding,” she says, “but storms of the intensity we have experienced are capable of damaging healthy, defect-free trees of all species and sizes.

This can lead to noticeable gaps in the city’s canopy of trees, but could be downright devastating for a place like Los Angeles, which has less tree cover compared to Sacramento’s more robust urban forest. Half of Los Angeles residents live in neighborhoods with tree crowns below 10%, and one-fifth of the city’s forest cover is concentrated in four of LA’s wealthiest census tracts. Yet despite the known risks of extreme heat and air pollution focused towards poor neighborhoods without treesLA continues to losing shade trees at an alarming rate while simultaneously failing to meet tree planting goals. This is the reality of many urban communities in the state. A survey of 50 California cities showed that the average tree cover fluctuates around 15% — a number that is lower per capita than anywhere else in the United States. Mature street trees are too scarce a resource for the state to waste.

The current destruction unfolding across the state is the product of extremes, the kind of savage time wobbles known as “precipitation kickwhich will become more common in the years to come. While supercharged storms from a warming atmosphere have temporarily dumped an unprecedented amount of moisture on cities, there are lasting repercussions from California’s decade-plus drought, which has caused instability for these street trees, says Joe McBride, professor emeritus of forestry at the university. of California, Berkeley. “In a field situation, the lateral roots represent one and a half times the canopy of the tree, and as these roots lengthen, they add more support to the tree,” he says. But that’s not the case in an arid city like Sacramento or Los Angeles, where trees already have little room to grow. “Drought does not promote additional root system growth to keep up with the canopy.”

Street trees are being tested for hurricane-speed winds in places like Florida, McBride says, but not so much in California, where towering, top-heavy American elms are among the gargantuan uprooted trees being sees around Sacramento. He hopes the disaster will prompt cities to change their replanting strategies.

In addition to considering high winds whipping awnings weakened by drought, a future of extremes means planting street tree species that are less susceptible to heat and even different types of insects and diseases. “In the Sacramento situation, where there are so many American elms, the mature ones 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 tree. “”

Sacramento’s policy 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 air conditioning bills during hot summer days, Hartin says, consultant in environmental horticulture for the University of California. Agriculture and Natural Resources Research Center at Riverside. “Surface temperatures in interior California asphalt parking lots can exceed 170 degrees on hot summer days, and the shade and cooling from a single, well-placed landscape tree can lower the surface temperature. at 100 degrees relatively cool,” she says. Hartin predicts that these localized impacts could last for decades as the replanted trees grow: “It will take another 20 to 30 years to deliver the benefit of the trees that were there. As part of her research, she is looking for hardy, fast-growing substitutes in one of three “climate ready tree» nurseries across California, where scientists are monitoring the growth of a dozen species for 20 years to make recommendations about their suitability as street trees in the state’s changing climate. (Promising candidates from Hartin’s Riverside Nursery so far include Bubba Desert Willow, Thornless Maverick, Honey Mesquite and Pistachio Red Push.)

But there is another factor that must be taken into account: most urban trees live much shorter lives than the same species in the natural world. “We need to choose species that are recommended for the right climate zone,” Hartin says, “but we need better tree care that is more focused on the health of the tree.” She notes that planting a tree in a stressed urban environment without paying more attention (and money) to watering, pruning, and providing enough space for the roots to spread can mean losing up to 50 years of benefits.

Even in good weather, California city life as it currently exists is not optimal for keeping trees alive. And many of the factors determining the survival of the state’s urban canopy depend on policies that go beyond forestry practices, says Megan Lynch, a horticultural scientist at the University of California, Davis, who has spent six years studying trees. street in Southern California. The most important factor? “Car culture,” she says firmly. “A big part of that is the paving and asphalting of everything around us, which directly contributes to the intensity of the flooding that we see.”

Crushing a street tree in a strip of grass on the boardwalk is extremely common, but it could be a death sentence: the tree is piled up between two concrete slabs and the grass is holding water on the surface. And during times of drought, many residents responsibly let their lawns sleep, but neglect to water their trees enough.

Instead of draining rain into sewers, picking up pollutants along the way, stormwater should be diverted to tree wells, Lynch says, where it can help trees establish deeper, stronger roots. filter out toxins and fix nutrients like carbon and nitrogen in the soil. . Unpaving and widening tree pits to make way for healthier vegetation can recharge groundwater locally, helping the state overcome the unpredictability between very dry and very wet years. The question remains, however, whether officials will heed these warnings, especially as the California budget just released cancels funding for urban forestry – including proven tree-based climate mitigation programs. More storms will come, after all.

“We have a new normal,” Lynch says, “but we still don’t know what it will become.”

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