Trees ultimately burn or rot, releasing a large portion of that carbon back into the atmosphere. Cutting them down and burying them to grow more would be ideal, but takes additional work.
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.
We're essentially doing just that when we use the wood for housing and other products and then eventually landfill, then you can plant more trees on that land.
It's a buffer as long as there is a forest. Once you establish a forest where there previously was none, you have tied up a certain amount of carbon in all those tree trunk/root/branches.
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.
Make more stuff from wood. Replace lots of throwaway plastic goods with wood that lasts. Slow the rate at which it rots. Engineer landfills to retain the carbon.
What percentage of houses burn down out of all houses ever built? I think not that many. Most probably stand for a hundred years and then are torn down and their component parts recycled.
Someone want to engineer a tree with enormous, deep roots? Basically make the trees self-burying.