If you stand on the deck outside of Beth Pratt-Bergstrom's home and look over the railing, you can see prime wildlife habitat. There are lots of trees and even a small pond that, in the spring, is home to croaking frogs and their tadpoles. But to Beth, there's something missing. Until recently, her deck was shaded by a massive, 100-year-old Ponderosa pine tree. She decided to remove the centenarian along with three others from her six-acre property just outside of Yosemite National Park.
"It was a very difficult decision," she says, "[but] these are trees that, if they fell, would fall on the house." The Ponderosa was most difficult for her. It was the first thing she could see every morning after waking up. "When it started turning brown on top, I started crying, because at that point, it's a death sentence."
In a sense, Pratt-Bergstrom is one of the lucky ones. Her neighbor had to remove 50 pines.
The California drought officially began in 2012, but it took four years before its effects on the state's trees became obvious. By 2015, some 29 million trees were declared dead, the worst spate of tree mortality in recorded history. Today that figure has topped 100 million. The very fabric of the iconic Sierra Nevada forests are changing, but trying to identify which parts of the state are at greatest risk of losing trees has been a challenge. A new study led by researchers from UC Davis offers up several clues that can aid researchers in identifying the most susceptible forests.
After noticing that so many trees were dying – an obvious, visceral phenomenon for anyone familiar with the Sierra Nevada – ecologists Jens T. Stevens and Derek J. N. Young and their colleagues turned to a dataset compiled by the US Forest Service over many years. Flying helicopters up and down the state, USFS had mapped out where all the dead trees were.
"If you drive around the state or look at the maps, you see that mortality is not evenly spread out. It's concentrated in different pockets," says Stevens. "We wanted to figure out what would predict where we would see mortality, and where we would see lots versus a little mortality."
It's not as simple a question as it might at first seem. For example, you might expect that trees in historically wetter areas would be less prepared to deal with drought because they never had to evolve efficient water use practices. On the other hand, you might imagine that trees in historically drier areas were already living closer to their physiological limits, making them more vulnerable to drought, despite their evolved mechanisms for dealing with limited water supplies.
To tease apart these possibilities, the researchers combined the tree mortality maps produced by the Forest Service with information on tree coverage and drought severity. The results were published last month in the journal Ecology Letters.
Their analysis allowed them to draw two broad conclusions. First, the parts of the state that are already drier on average had higher tree mortality. Trees in these places simply couldn't cope with even lower access to water.
Second, forests with more tree cover were more likely to experience high levels of tree mortality than those that were more sparsely covered. That's because these are places with more trees drawing from the same, finite water source. All other things being equal, a water glass with fewer straws will take longer to empty than a glass that's forced to contend with higher demand. So too with tree roots.
"So there's a climate piece and there's a forest structure piece to this story," says Stevens. And in both humans are ultimately complicit. Our influence on global climate requires no explanation, but we are also responsible for the high tree density that results from a century of active fire suppression. When wildfires and the cultural burns Native people used to manage the landscape can't do their job of thinning out forests, what we're left with is more trees than would otherwise exist in a given landscape.
"Fire suppression has caused a change in forest structure, and that change is interacting with changes in climate, to drive mortality," says Stevens. But the story gets a bit more complicated still, because it's rarely thirst itself that kills the trees.
Stevens explains that in the 1980s, ecologists developed the notion of a "spiral of death." Just like a human, when a tree is faced with one stress, that makes it more vulnerable to the next stress. Eventually, those stresses pile up and the tree becomes unable to survive. If stressed by competition for water or sunlight, then a tree becomes more vulnerable to drought. If stressed by drought, then a tree becomes more vulnerable to disease or infestation, like the bark beetles spreading through California's forests.
"A lot of the dead trees were killed by beetles directly, but the drought predisposed [them] to mortality from beetles," says Stevens. In other words, if a tree is already immune-compromised, even a simple flu could herald death.
So the millions of dying trees in California, especially in the Sierra Nevada, are but a symptom of a larger problem: our forests lack resilience. And while researchers, conservationists, and policymakers debate the consequences of altered fire regimes and fossil fuel emissions, we are faced with the more urgent question of whether we can save the rest of California's trees from a death sentence. By identifying forests that are at a greater risk, Young and Stevens and their team have provided a means of doing conservation triage, helping to identify higher- and lower- priority targets.
Back at home, Pratt-Bergstrom says that she couldn't quite let go of her one hundred year old pine. She paid the tree removal service to carve a couple benches for her out of the Ponderosa's corpse. She sits on them to watch the sunset most nights. It's little comfort, though; her small town is located smack dab in the center of one of the worst hit parts of the state. "I can't even go for a walk without feeling terrible. Everything's dead, it looks like a tree graveyard," she says. Ironically, the town she lives in is called Midpines. "We are probably going to have to rename our town."
Banner: Dead trees in the Sierra Nevada. Photo: U.S. Forest Service