(CNN) A total of 18 million trees died in California last year, the US Department of Agriculture said Monday.
"Years of drought and a bark beetle epidemic have caused one of the largest tree die-offs in state history," said Wade Crowfoot, California Natural Resources secretary. California's drought began in 2010 and bark beetles are insects that reproduce under the bark of trees, the USDA said in a news release.
Despite the large number, the rate of tree mortality actually slowed in 2018, according to Thom Porter, director of the California Department of Forestry and Fire Protection.
"However, 18 million trees are an indication that the forests of California are still under significant stress," Porter said. "The stress of drought, insects, disease and prolific wildfire will continue to challenge the resilience of the state's forests."
The Camp Fire in Northern California burned for 2½ weeks in November. It is considered the deadliest and most destructive wildfire in state history. It left 88 people dead and destroyed almost 14,000 homes, 514 businesses and 4,265 other buildings. It covered more than 153,000 acres, roughly the size of Chicago.
Dead trees continue to threaten people and infrastructure. The USDA said most of the threats are centered on the west side of the southern Sierra Nevada range, near Sacramento.
"The Forest Service is focused on increasing the pace and scale of ecological restoration," said Randy Moore, regional forester of the USDA Forest Service, Pacific Southwest Region. Moore said this includes thinning dense areas so forests can become more resilient in fighting wildfires, droughts and bark beetle outbreaks.
"The Forest Service completed approximately 313,000 acres of restoration in 2018, which included over 63,000 acres of prescribed fire -- the largest number recorded since the National Fire Plan was implemented in 2001," Moore said.
More than 147 million trees have died across California's 9.7 million acres of federal, state, local and private lands since the state's drought began in 2010, the release said.
The state's drought officially ended in 2016-2017, the release said, but below-average precipitation in 2017-2018 slowed the recovery of surviving trees.
And since 2016, federal, state and local officials have had to take down 1.5 million trees that were a threat to life and property.
The USDA said the Forest Management Task Force was created in May 2018 "to help combat tree mortality, increase the ability of forests to capture carbon, and systematically improve forest management between state, local, federal and tribal agencies."
While the wildfires were burning last fall, President Donald Trump said in a series of tweets that the fires were a result of poor forest management and he threatened to cut federal aid.
Gov. Gavin Newsom called for a five-year, $1 billion forest management plan in his 2019-2020 state budget proposal as part of his commitment to forest health, the USDA said.