Forests store much carbon in their wood and soil. Annual additions to forest carbon in the US take up 10-15% of the CO2 from our fossil fuel use, mostly because US forests are regrowing after clearing for agriculture and past harvests. When regrowth stops, these annual additions and CO2 offset will also stop. In the West, the largest challenge for forest carbon is to retain current stocks when fire, bark beetles, and drought are killing many trees, and high intensity fires change vegetation to grass and shrublands in montane forests. Fuel reduction in frequent fire forests may help retain montane forests, but greatly reduces forest carbon stores. Because montane forests are denser now than is sustainable, they have more forest carbon than is sustainable. Over the longer term, fuel reduction may retain more forest carbon by retaining forests at a cost of large current forest carbon loss.