Most ecosystems follow a path of succession: first one tree grows quickly and dominates, but over time, if nothing is disturbed, another tree eventually surpasses the first tree and dominates.
For example, hardwoods like maples and oaks are the eventual dominant tree in New England, which is why "colors" there refers to red, orange, and yellow (i.e., more than just one color other than green). During the early centuries of settlement by white men, those trees were repeatedly cut down for building materials and fuel; as a result, the first dominant tree, evergreens, became very common, so in recent years people have been thinning the evergreens in some areas, trying to restore the dominance of the hardwoods.
Are you saying that in the Southwest, aspens are really the first dominant tree, and the stability over the past century has allowed the second dominant tree, in this case evergreens, to come back??