Although the area of timberland, both in the East and nationally, is quite stable, major changes are occurring in timberland area by forest cover type. In the East, large increases in the area of the maple-birch-beech and oak-hickory cover types have occurred over the past 50 years, while the extent of several other major cover types have declined. South and Buckner (2003) documented a substantial shift from southern pine to the oak-hickory forest type in the South, resulting from fire suppression, natural hardwood regeneration, insect infestations, and other factors (Table 5) that accounts for most of the gain in oak-hickory depicted in Figure 11. Changes in other species are due to a number of factors as well, including ravages of disease (i.e. the Dutch elm disease), and the fact that annual forest growth, and particularly hardwood growth, has for decades substantially exceeded removals, the latter meaning that the average age of hardwood forests has been progressively increasing. The rapid increase in maple-beech-birch, a phenomenon that is also reflected in timber volume statistics, is almost totally due to the aging of the Eastern forest. In the Western U.S., the aspen-cottonwood cover type predominates, with relatively minor volumes of other species such as birch, alder, maple, and oak. Overall, the area comprising the western hardwoods category is increasing.