The epic drought that began in the state of California in December 2011 and only officially ended in March of this year will have many lasting impacts—but one of the most severe is what it did to the trees in the central and southern Sierra Nevada mountains. A new study in the journal Nature Geosciences reports that by the drought’s peak, a combination of factors—including lack of rain, extreme heat and overcrowding stressed out the forest—lead to mass die-offs. In total, almost 150 million trees have died in California since the drought began.
Nathanael Johnson at Grist reports that to understand the tree-mageddon, researchers analyzed various forest communities in the mountains, including areas where 80 percent of the trees died and other where as few as two percent of the trees were lost.
They found that a number of factors came together to deal a devastating blow in the worst-hit areas of the forests. In many cases, the roots of Sierra Nevada forests stretch down between 15 and 50 feet, meaning that even during droughts trees can access moisture locked deep underground. But the recent drought short-circuited that safety net.
“Parts of the Sierra Nevada reached a ‘tipping point’ in 2015, where annual precipitation plus stored subsurface water were not enough to meet the water demand of the forest,” co-author Roger Bales of the University of California, Merced says in a press release. “This forest die-off can be viewed as a ‘perfect storm’ — the intersection of four years of low precipitation, hotter temperatures than in past droughts, and a heavily overstocked forest from centuries of fire suppression.”
Jacob Margolis at LAist reports that a similar drought between 1987 and 1992 was as dry as the recent drought period. The big difference, however, was the heat. This time around, temperatures averaged 2.16 degrees higher, meaning the trees lost water more quickly.
That, he reports, led to a cascade effect. The heat caused accelerated evapotranspiration, in which water evaporates from leaves, leading to increased water consumption by the trees. Once the trees used up all available moisture and the roots began to dry out, the green canopies above also began to shrink. The stress also made the trees more susceptible to pathogens and insects, like bark beetles. All of that added up to major tree mortality that slowly crept from lower elevations to the normally cooler higher elevations.
It came to head in 2016, when 55 percent more trees died off than the typical annual average—with nearly 80 percent of trees around 3,800 feet of elevation dying.
Even though the drought is officially over, the researchers say to expect more tree die-offs in the future as the planet warms. “We expect climate change to further amplify evapotranspiration and ground moisture overdraft (when more water is taken out of the soil than is replaced by precipitation) during drought,” co-author Michael Goulden of the University of California, Irvine, says in the press release. “This effect could result in a 15 percent to 20 percent increase in tree death for each additional degree of warming.”
Besides improving the climate crisis, Bales and Goulden suggest better forest management could help reduce future die-offs, including thinning forests or managing forests with fire. “There’s just too many stems in the ground sucking up water because of the fire suppression of the last century,” Bales tells Johnson.
“We need to manage the forest like we would agriculture or other systems that are water limited, and not have as many trees there,” he suggests to Margolis.
This year, the snowpack in the Sierra Nevada mountains is deep, and increased moisture is giving the forests a breather. Umfair Irfan at Vox reports there’s a small silver lining to the mass forest die-offs as well. It gives land managers the chance to start with a clean slate and manage forests so they are more diverse, less dense and more resilient. But that’s only possible if enough resources are appropriated—and as long as the mega-drought doesn’t return anytime soon.