Mortality of both hardwoods and softwoods is trending upward over time as the average age of stands progressively increases (Figure 33). Weather patterns in some regions are also credited with increasing mortality. Hardwood mortality in the North has risen modestly over the past 50 years from about 0.61 percent of growing stock volume to about 0.73 percent. In the South, hardwood mortality has actually declined when the most recent figures are compared to 1952, but mortality rates increased markedly (37 percent) between 1976 and 2001. Recent insect outbreaks as well as hurricanes have been identified as significant factors in sharply rising mortality rates (Smith et al., 2004). Still, however, hardwood mortality rates in the South are currently almost identical to those in the North.

Exceptionally high hardwood mortality rates in the Rocky Mountain region are largely due to a lack of harvest activity or fire in aspen stands that would have the effect of stimulating regeneration of this pioneer species. In the absence of such events, relatively short-lived aspen and birch stands are experiencing high mortality rates, with stand conversion to softwood species.

Figure 33
Mortality as a Percent of Growing Stock Volume on Timberland by Major
Owner Group, 1952-200 1

Source: Smith et al., 2004.

When hardwood mortality is expressed in terms of volume, or as a percentage of hardwood growth, rather than as a percentage of growing stock volume, the magnitude seems a bit more impressive. Hardwood mortality in 2001 was estimated nationwide at 2.7 billion cubic feet, a volume roughly equivalent to 28 percent of hardwood growth nationally in that same year (Table 9). The most recent data suggests that rising mortality may have reached a plateau, although it is too early to tell whether a sustained leveling or downward trend is in the offing.

 

Table 9
Natural Mortality of Hardwood Growing Stock in
U.S. Forests, 1952-2001

 

Source: Derived from USDA-Forest Service, RPA Assessment, 2000; Smith et al., 2004.

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