This isn't the first time a catastrophic disease has decimated a U.S. tree species. Humans have a knack for deigning one species supreme—think of wheat, corn or pigs—and using it pretty much exclusively. We do this for our arboreal species, as well. Before ash trees were en vogue, elm trees were ubiquitous. Then American Dutch elm disease swept across the U.S. in the 1930s and 40s, devastating native populations. "In the earlier part of the 20th century, American elm lined the city streets and they created a cathedral canopy," says Smith. "Ash trees were used to repopulate American cities after the dutch elm disease epidemic."