The driving force here is that the rising global temperature is wiping out seedlings. In many spots around the U.S. West, summer temperatures are already high enough to cook young trees before they can develop thick protective bark. Others have become so dry that seedlings shrivel before their roots can grow deep enough to reach #groundwater. Both circumstances can thwart forest regeneration. Mature trees can survive in these areas long after they stop reproducing. But when fires wipe out these forests and seedings can’t get a foothold, they are replaced with grasses and dense #brush.