(CNN) — An estimated 36.3 million trees died last year in California, a massive jump from the 9.5 million trees that died in 2021, according to an aerial survey report from the U.S. Department of Agriculture Forest Service.

The trees, which are on federal, state ,and private land, were killed by the effects of the state's prolonged drought in its overgrown forests, insect outbreaks and disease, the Forest Service said in a news release earlier this week, adding that it is working with its partners throughout California to remove the dead trees to improve forests' health.

"Since 2020, California has experienced the driest and warmest years on record causing serious drought conditions. Without enough water, trees are susceptible to bark beetle attacks and disease. Their susceptibility rises when trees are crowded and temperatures are abnormally high," the Forest Service said in the release. "Even with the recent storms from atmospheric rivers, increased tree mortality should be expected in forests until precipitation returns to normal or above normal for a few years."

The dead trees span nearly 2.6 million acres compared to 1.2 million acres in 2021, with the average severity of mortality becoming "significantly higher." The highest mortality rates were in the central Sierra Nevada Range and areas farther north, the Forest Service said.

"Mortality was particularly severe and widespread in the north interior in several conifer species where drought conditions were most exceptional," the report said.

The survey's findings come weeks after torrential rain from a series of deadly storms caused extensive flooding, mudslides and landslides — which killed at least 20 people, turned streets into rivers and destroyed many homes.

The back-to-back downpours helped alleviate California's severe drought conditions, but most of the state is still experiencing some level of drought, according to the US Drought Monitor's latest report. That's mainly due to the dire severity of the drought before the storms lashed the state.

After more than a century of suppressing fires, the USDA said California's forests are overgrown. And human-induced climate change has been leading to hotter, more intense fires that threaten the increasing number of homes being built in what was wildland. All these factors are behind "what is now a full-blown wildfire and forest health crisis," the Forest Service said.

In December, CAL Fire announced it is offering its local partners up to $120 million in grants to improve forest health by reducing fuels, conducting prescribed burns and managing pests. The grant money may also be put toward reforestation, conservation easements and or land purchases.

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