There is an old Internet axiom, known as Poe’s law, which states that without a clear indicator of the author's intent, it is impossible to create a parody of extreme views that won't lead to some people believing that the author holds those views sincerely. This flat-Earther theory falls into this category, and we can’t tell for sure if people are serious about it or not. It appears some people who subscribe to the flat-Earth conspiracy believe that there is no such thing as trees.
According to this theory, everything we see today in forests, parks, and gardens, are not trees. They are bushes; saplings of the Earth's ancient trees that once extended up to 40 miles high into the sky with trunks 2 miles across but no longer exist.
This “theory” goes back to a video on YouTube posted in 2016 by a Crimean man calling himself Людин Рɣси, whose account is now deactivated. The video has been watched hundreds of thousands of time. Although the original video has since been removed, the theory lives on in other spin-off videos (here's the English-language dubbed version if you are curious).
The video claims that thousands of years ago a cataclysmic event destroyed 99 percent of Earth’s biosphere, and it took with it the “real forests”. If you are looking for evidence of these ancient giant trees, the video offers it up in abundance. It suggests geological features like buttes, mountains, plateaus, and mesas (flattop mountains) are actually the remains of these ancient trees. The video shows images of Uluru in Australia, the Giant's Causeway in Northern Ireland, and the Devil's Tower butte in Wyoming, USA next to pictures of real tree stumps, asking viewers to spot the difference. Because they look the same, they must be the same, it suggests, since the only difference is material and size. (Material and size, of course, are quite important, rather fundamental differences used to differentiate objects.)
The video claims all rock on Earth is not rock, it is the rubble left behind of these ancient leafy behemoths, and flattop mountains are just the severed stumps of trees that were destroyed in the earlier cataclysmic event.
There are some important fundamental differences between trees and rocks, as an arborist helpfully points out for Quartz. For example, one is a living organic thing made of mostly carbon and the other is an inorganic non-living thing made of minerals.
The video has been debunked many times by many outlets including Snopes, which has also debunked the claim famous flattop mountain national monument, Wyoming's Devil’s Tower, had a system of roots. That “claim” actually came from a satirical page, which is an excellent example of Poe’s law in action.
Among the many perplexing aspects of this conspiracy is why this theory is discussed in flat-Earther circles. It has no obvious link to the idea that Earth is not the globe we know it to be and is instead, against all probability and evidence, a flat disk. There are also consequences of this theory that have failed to be addressed, as it would be rather worrying to have a giant tree on a flat disk. There's an issue of balance and distribution, for one thing. What if a giant pine cone falls in America? Other than the destruction it would do, would it set the flat Earth disk spinning? And what would happen if a strong wind sets these trees swaying?
Of course, there's always the chance the whole idea was Poe's law in action. A Russian operative, perhaps, posing as a Crimean man, trying to stir up flat-Earthers to see what the most outrageous thing he could make them believe was. We'll leave it to the reader to consider if they needed any help with that.