Spruce trees, members of the genus Picea, occur throughout the boreal forest and on mountains farther south. For example, spruce grow in the Rocky Mountains south from the heart of boreal forest all the way to the deserts of the southern United States and Mexico. How do you think they would be distributed in the mountains that rise from the southern deserts? In particular, how do altitude and aspect (see chapter 5) affect their distributions in the southern part of their range? Would spruce populations be broken up into small local populations in the southern or the northern part of the range? Why?