You need to hydrate more of the world. For example, in our Earth, you might hydrate the Sahara. That way, the trees replace desert rather than grasslands. Even if the trees will release the carbon dioxide back to the atmosphere, increasing the amount currently in living plants helps.
Where do you get the water? Saltwater greenhouses. Instead of clearing out rain forest to grow crops, grow crops in the greenhouses and pipe the excess fresh water into the desert. Rain forests are teeming with organic life, all of which traps some carbon.
Don't just consider trees either. We actually have more trees in the United States now than a hundred years ago. That hasn't been enough to offset increasing fossil fuel use.
Look at what produces fossil fuels. Coal was produced from peat moss in bogs. So really overhydrate your Sahara. Dying vegetation will drop to the bottom of the swamp mud and the carbon will be trapped rather than releasing into the air.
Oil comes from algae and plankton. Encouraging greater growth in the ocean can be more effective, as the dead plants sink to the bottom. Again, trapping the carbon rather than releasing it back into the atmosphere.