[fluoridation]

It's not locked up, it's continuously flowing in and out of that ecosystem. It's only locked up when it gets buried and can't return back to the atmosphere (except through technology). There's any number of reasons why a forest could get disrupted and die out besides intentional deforestation.

[developer93]

Admittedly I haven't checked for sources, but I'm willing to bet that it's mostly man made these days, either direct deforestation or climate change.