Overview of Agents and Patterns of Mortality and Resulting Coarse Woody Debris Recruitment in Western Forests

We present an overview of many of the mortality agents in western coniferous forests of the United States that are responsible for coarse woody debris recruitment under natural conditions. Patterns and scale of occurrence of mortality agents for the six western USDA Forest Service Regions are examined at both historic and projected-risk time periods. The data indicates that bark beetles have been the most important source of mortality in the western U.S., and with the exception of the Northern Region, this trend is expected to continue. Defoliators, although important in growth loss, have not had as large a mortality impact. Recent-past and projected-risk data indicate that root disease may be an even more important factor in tree mortality and recruitment of coarse woody debris than indicated in recent literature. Although regional measures of actual mortality are not available, all western Regions show high impacts by dwarf mistletoe (Arceuthobium spp.) during the past 20 years. Fire affects relatively few hectares compared to insect and disease, but the effects on coarse woody debris (recruitment and conditioning) are important. Additional concerns for managers wishing to augment coarse woody debris are also presented.