ABSTRACT
Several studies suggest that recent changes in the condition of forest resources in western North America may be related to changes in global atmospheric conditions. For example, growth rates of Pinus longaeva, P. aristata, and P. flexilis in the southwestern United States have increased significantly since the mid-nineteenth century. Recent growth increases have also been reported for P. contorta and P. alhicaulis in the state of California, and for Tsuga mertensiana, Larix lyallii, and Abies amabilis in the state of Washington. Encroachment of subalpine tree species on meadows during the twentieth century in the Olympic Mountains, Cascade Mountains, and other areas of western North America suggests that recent climate patterns have affected treeline community composition and structure, as well as growth of established trees. Continued increases in tree establishment in these areas may ultimately lead to changes in the location and structure of the subalpine life zone. In addition, the combination of elevated CO2, climate change, and changes in seed source availability may create new community types.
