Insect outbreaks can kill millions of trees, particularly when susceptible 

 trees are grouped together, giving the insect an unbroken source of food 

 and breeding habitat 



Normal Mortality 



Many scientists and forest managers use 

 historical conditions (from 1600 to 1850, be- 

 fore European settlement of the West) as the 

 yardstick for determining "normal" conditions. 

 Old journals, photographs, maps, and tree-ring 

 analysis can be used to trace and compare his- 

 torical forest composition, distribution, and 

 disturbances with current conditions and dis- 

 turbances. Such analyses have shown that 

 disturbance patterns and mortality are outside 

 the historical range of variability in some ar- 

 eas of Oregon and Washington. 



For example, western spruce budworm out- 

 breaks in northeastern Oregon are more fre- 

 quent and severe now (some stands have more 

 than 80% overstory mortality) than they were 

 in the 1800s, largely as a result of the selec- 

 tive harvest of nonhost species (ponderosa pine 

 and western larch) and fire suppression. These 

 two practices have led to dense, multistoried 

 stands of predominantly budworm-susceptible 

 species (true fir and Douglas-fir). 



Many places in eastern 

 Oregon and Washington 

 have dense second- 

 growth ponderosa pine 

 stands that replaced 

 open, parklike stands of 

 old-growth ponderosa 

 because of fire suppres- 

 sion. These overstocked 

 stands are extremely 

 susceptible to attack by 

 bark beetles. The num- 

 ber of stands in this con- 

 dition and the resulting 

 mortality is higher than 

 in the past. 



Fire frequency and se- 

 verity have changed sig- 

 nificantly over the past 

 century as a result of fire 

 suppression practices 

 and increases in tree 

 densities and flammable 

 materials. When fires oc- 

 cur, they are much more severe and kill more 

 trees on each burned acre. 



Mortality Trends 



Recent trends in mortality can be tracked 

 through forest inventories or surveys. Most 

 survey and inventory data are relatively recent. 

 The first aerial insect surveys began in the 

 1940s, and the first forest inventories began 

 in the 1930s. Comparing data from year to 

 year is useful and shows areas with potential 

 problems. 



Tree mortality in western Washington and 

 northwestern Oregon has been, and probably 

 will continue to be, relatively low with occa- 

 sional local areas of high mortality from dis- 

 turbances such as flooding, wind storms, fire, 

 and insect outbreaks. The most significant 

 mortality agent on the west side is root dis- 

 ease. Although not highly visible, root disease 

 causes mortality on 10% of all lands in the 

 Pacific Northwest and may kill more than 50% 

 of a stand over a period of years. 



In eastern Oregon and Washington, forest 

 inventories show mortality has been above 



Overview — 12 



