figure 1.2. White spruce– ponderosa pine forest of the northern Black Hills. Some trees
have arboreal lichen hanging from their branches, as do balsam fir and white spruce
forests of northern Maine. Photo by Gail Jenks.
figure 1.3. Castle Creek Valley, shown in 1874 during the Custer reconnaissance of the
Black Hills (left) and more recently (right). The photos show the transition from an aspen- to
a pine- dominated landscape. Progulske 1974; photos by permission from South Dakota State
University.