National Geographic: The U.S. West’s old-growth forests may look quite a bit thinner in the future. A new study suggests these forests could feature fewer and smaller trees, and global warming may be driving the change. (Learn about global warming.) “[Tree] death rates have doubled over the last two decades in old-growth stands across the Western U.S.,” said Phillip van Mantgem, a U.S. Geological Survey scientist and co-leader of the research team. Forests are losing trees faster than new …