Donald Trump gestures while talking to the media.

If a tree falls in the forest, will only Donald Trump hear it explode?

The US president, once mocked for asserting that California's devastating wildfires may be caused by a lack of "raking" the forest floors, on Monday waded again into the debate over the wildfires ravaging swathes of the western US.

"With regard to the forests, when trees fall down after a short period of time, about 18 months, they become very dry," he said during a visit to California.

"They become really like a matchstick... They just explode."

His comments elicited widespread bewilderment and scorn on social media, with people speculating that the president may have misunderstood briefings about the effects of fire on trees, many which have died due to drought that weakens their defenses against pests.

Some live trees, like Australia's eucalyptus, have been reported to have exploded at times by firefighters, as the sap superheats and expands, while freezing trees have been reported to let off sounds like gunshots as the saps freezes and snaps off limbs.

Trump blamed the fires on poor forest management in states led by rival Democrats, and sparked skepticism when he said that global warming -- which he has repeatedly denied -- would naturally abate.

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"It will start getting cooler. You just watch," he insisted to Wade Crowfoot, the head of the California Natural Resources Agency.

The official responded: "I wish science agreed with you."

Some Twitter users noted the similarity between his misplaced optimism on climate change and over the coronavirus outbreak, which has killed almost 200,000 Americans.

"It's going to disappear. One day, it's like a miracle, it will disappear," Trump said of the pandemic in February.