Someone want to engineer a tree with enormous, deep roots? Basically make the trees self-burying.
The tree already buries almost half of its biomass in the ground for you. And a lot of material goes through the food chain when the tree rots in situ. There's a big difference between regrown forest and converting prairie to forest and a lot of that has to do with quantity and quality of decaying matter on the forest floor. Dead trees are better than no trees.
Not all at once, though, and new trees spring up pretty quick. If you're replacing unforested land with forested land, you're still storing substantial carbon. Cutting down / burning an existing forest to plant trees doesn't work, sure.
This video makes me pretty proud to have graduated from UBC: https://www.youtube.com/watch?v=GHtdnY_gnmE
An extra reference: https://www.youtube.com/watch?v=2DPp2NcnTb0
Sure, the individual trees will eventually die and release a lot, though far from all, carbon back again. But by then, new trees will grow and act as the storage buffer.