Drought in California kills 12 million trees since last year

© U.S. Drought Monitor
This color-coded map shows drought conditions across the U.S., on April 30, 2015. Much of San Diego County, shown in red, is in an "extreme" drought. At this level, major crop and pasture losses are common, fire risk is extreme, and widespread water shortages can be expected, requiring restrictions.

    

An estimated 12 million trees across California's forestlands have died over the past year because of extreme drought conditions, according to an aerial survey conducted April 8-17 by the U.S. Forest Service.

In San Diego County, 82,528 trees, mostly Jeffrey pines across Mt. Laguna, have succumbed to a lack of rainfall, with many more struggling to survive, said Jeffrey Moore, interim aerial survey program manager for the U.S. Forest Service.

There is "very heavy mortality, a lot of discoloration in the pine trees that probably will expire sometime during this growing season, as well as oak trees that are suffering," Moore said.

Moore was part of a team that surveyed the trees visually, using a digital mapping system while flying in a fixed-wing aircraft 1,000 feet above ground.

A tree's survival often depends on its proximity to other trees, he said.

"A lot of trees are competing for whatever available moisture there is in a drought situation," Moore said. "When you have too many trees in an area, it makes it hard on all of the trees."

The extent and severity of tree mortality, which occurred after the 2014 aerial surveys in Southern California Forests.

    

Recently dead or injured trees were mapped visually by a surveyor using a digital aerial sketch-mapping system flying in a light fixed-wing aircraft approximately 1,000 feet above ground level.

    

In Southern California, the researchers tracked more than 4.2 million acres in Cleveland, San Bernardino, Angeles and Los Padres National Forests, where they found an estimated 2 million perished trees. They combed another 4.1 million acres in the Southern Sierra Nevada, where they documented approximately 10 million dead trees. Their findings were compared to similar surveys taken in July 2014, Moore said.

In San Diego County, Moore said they found substantial pine mortality near Descanso Road in the Cleveland National Forest, and throughout Mt. Laguna.

The team did not attempt to map gold-spotted oak borer beetle-related mortality in this survey, he said. Nor did they track black oak trees, since it's unclear whether those without leaves are dead or just "leafing out"—bare but in the process of growing their new leaves for the spring.

The county's forests are already reeling from the 2003 Cedar Fire that devoured 280,000 acres, including in the Cuyamaca Mountains. The region was formerly blanketed by a coniferous forest, but recovery has been poor, Moore said.

"Most of those areas aren't even coming back into trees at all," Moore said. "They're kind of being switched over now into Chaparral plants because they burned so hot the seed source is gone."

Large trees, such as the Jeffrey pine, are important for storing carbon from the air. They also provide food and habitat for various species, including squirrels, deer and birds, such as the Pygmy Nuthatch that probes into clusters of pine needles for small insects.

"When you start thinking about what it takes for a tree, which is usually a fairly hearty type of plant to die off, it's telling you a pretty clear signal of just how intense the drought has been," said Brian Fuchs, climatologist with the National Drought Mitigation Center.

"These dead forests are going to be more primed for any type of fire," Fuchs said. "Also, it's going to impact water quality as there's going to be more particulate that will go running off these hillsides into the rivers and streams."

Fuchs said 67 percent of California remains in an "extreme" or "exceptional" drought, and conditions are expected to worsen as the dry season sets in.

"The heat of the summer really amplifies some of that development," Fuchs said.