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- Embed this noticeThe CO2 in a tree is gathered over years and years, whereas rotting can occur in a relatively short period of time. It ends up back in the air, and plants around them which take years and years to gather carbon through photosynthesis won't collect it immediately.
Moreover, you can't say for certain whether a spot will even stay a forest on geological timeframes, and the odds are it will not. There have been 5 mass extinction events on earth, and 3 of them happened since the end of the carboniferous period. The End Permian event was caused by volcanic activity releasing large amounts of CO2 and H2S which caused acid rain and ocean acidification (killing 96% of species), the End Triassic event was caused by underwater volcanic activity which caused global warming and a change in composition in the oceans (killing 80% of species), and the End Cretaceous event was caused by a meteor impact which caused global cataclysm including global cooling (killing 76% of species). Besides that, there have been 13 other mass extinction events if you include the current Holocene mass extinction event. Antarctica was once part of a massive forest and today is an icy waste, and Australia was once almost entirely forest and today is mostly desert.
That's why the coal exists for the 60 million years after plants evolved to grow cellulose and before something else evolved the ability to digest cellulose, and essentially disappears. During the carboniferous period, anywhere there was forest (particularly swampy forests), that carbon essentially became part of the landscape and over 60 million years accumulated and was exposed to anerobic conditions thanks to the swampy conditions, and if the forest died, the carbon remained because there was nowhere for it to go and often got driven underground by geological processes over millions of years. I'd expect that millions of years of sedimental deposition by itself (even through processes like wind) would be enough to cover up the tree beds over time. The reason it stops after that is the tree beds don't stick around and become deposited carbon, they become CO2 through the metabolic processes of fungus.
By contrast, the process of life producing rock such as carbonates is a long term place for carbon to go. The white cliffs of dover for example are formed from the bodies of millions of years of aquatic life forms dying and falling to the ocean bed, and the parts that don't rot, oxidize, and aren't eaten by other creatures end up sticking around and packing down, creating entire mountains of carbon impregnated rock.
Honestly, one of the biggest shocks in my life was reading geological history and realizing that stuff we think would be important ended up being meaningless, while stuff we think of as insignificant ends up becoming incredibly important when you're talking about geological timeframes.