Trees with their own conversion capacity are indeed a good interim solution, but in the long term this form of carbon storage isn't as permanent as rocks, especially given how prone trees are to dying and decaying in the biosphere of today. The Devonian didn't have that problem. For example, parts of the Amazon have become greenhouse gas emitters even when human-caused deforestation is discounted.

There's also the very real possibility of forest fires releasing huge amounts of carbon at once. Even dead trees preserved in peat bogs could burn and release carbon in this manner.

Rocks have their own weaknesses such as ocean acidification, but relatively speaking they're much more stable.