Intense Drought Blamed for California Forest Die-Off, Climate Change Could Make it Worse

There was a spike in tree deaths in 2015 and 2016 following California’s worst drought on record. (Photo courtesy United States Forest Service)

LOS ANGELES – Thanks to our ultra long-rainy season, we finally said goodbye to drought conditions in California. But the reality is we’re still living with the aftermath of one of the worst droughts on record, possibly the worst in 1,200 years.

Just look to the Sierra Nevada where more than 110 million trees have died since 2010, with the biggest spike in deaths came right after the hottest and driest years of drought.

New research published in Nature Geoscience says that the spike isn’t a coincidence. That years of extreme conditions can be directly tied to a 55% increase in the number of tree deaths.

Driest Four Years in a Century

2012 to 2015 were the four driest in a century, with each year hotter than the last.

“During these extreme dry periods, trees basically run out of water,” said Roger Bales, professor of Engineering at UC Merced and co-author on the paper.

  • First, the earth around the deep roots of trees dried out. These roots can 30 to 50 feet deep, which typically helps trees to survive drought conditions.
  • Then, because they were stressed, the size of their canopies – the green foliage above – shrank and their ability to fight off pathogens suffered.
  • Then, the trees started to die.

Pines, which have also been hit hard by bark beetles, suffered the most. Though mature conifers like white fir and incense-cedar, perished as well.

This June 6, 2016 photo shows a tree stained blue by a fungus carried by the bark beetle, which diminishes the trees value as lumber, near Cressman, Calif. (AP Photo/Scott Smith)

The pattern of death was particularly present in the Southern and Central Sierra, where it started at lower altitudes and crept upwards to the usual cooler and wetter locations on the mountains.

It’s true that tree mortality and drought aren’t new in California, but according to the authors, compared to the 1987-1992 drought, while this most recent one saw about the same amount of precipitation it was 2.16 degrees Fahrenheit hotter, which meant that the trees had to use more water.

Scary Implications

As the climate crisis progresses, rainfall patterns are expected to become more extreme and temperatures are going to increase.

A greater number of dead trees means more wildfires, decreased air quality, and a loss of habitat for insects and animals.

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The authors also found that extreme conditions limited how much CO2 the remaining trees were able process, or uptake. While uptake usually occurs nearly year round, when the trees were stressed out uptake was largely limited to the spring and winter wet seasons.

The future of California’s forests in an ever-changing climate is uncertain, though the authors postulate that they could use the new data to predict how much water trees across the Sierras will need to survive droughts in the future.

Bales explained that forest managers could decide to reduce the number of trees in the forest depending on the area and the weather conditions.

“When you have fewer trees drawing water out of the ground then there’s enough to go around,” he said. “We need to manage the forest like we would agriculture or other systems that are water limited, and not have as many trees there.”

This story is part of Elemental: Covering Sustainability, a multimedia collaboration between Cronkite News, Arizona PBS, KJZZ, KPCC, Rocky Mountain PBS and PBS SoCal.