The last 10 to 12 years have been drier than 

 average in eastern Oregon. In this drought index 

 for the Blue Mountains, below zero represents 

 drier than average years, and above zero is 



wetter than average. Source: SurfaceWater supply index. 

 Natural Resources Conservation Service. 



ity on dry sites, particularly where stands were 

 overstocked. Windstorms can cause stem 

 breakage and windthrow, which increases 

 stem decay in wounded trees and leads to bark 

 beetle buildup in windthrown trees. In the 

 Blue Mountains, severe winter storms often 

 cause extensive snow breakage or windthrow 

 in localized areas. Floods and ice jams fre- 

 quently damage trees growing in riparian zones 

 or on flood plains. 



Fire siq}pression charges forests — ^Natural 

 fire has played an important role in mixed co- 

 nifer and ponderosa pine forests. In the low- 

 elevation mixed pine and fir forests, frequent 

 low-intensity fires maintained stands at low 

 tree densities. At higher elevations, fires were 

 less frequent but of higher intensity, often lead- 

 ing to removal of the entire stand. With the 

 suppression of the low-intensity ground fires, 

 stands have become extremely dense and now 

 contain a large proportion of species such as 

 true firs that are particularly susceptible to 

 insects, diseases, and stand-replacing fires. 



Dwarf mistletoe is more severe where fire 

 is suppressed — Many conifer species east of 

 the Cascades are infected by dwarf mistletoes, 

 which are plant parasites. Before the 1900s, 



fires were frequent and reduced or eliminated 

 dwarf mistletoe in many stands in the eastern 

 Cascades and the Blue Mountains. For most 

 tree species, fire suppression in the past cen- 

 tury has resulted in more severe mistletoe in- 

 fections, particularly when stands are 

 unmanaged. 



In the eastern Cascades and on the Modoc 

 Plateau, Douglas-fir dwarf mistletoe is the most 

 pervasive and damaging. Western larch also 

 has high rates of mistletoe infection, despite 

 the distances between host trees on stands. 

 True firs, too, are affected by a dwarf mistle- 

 toe, but the effects on tree health are far less 

 intense than in Douglas-fir and western larch. 

 The combination of mistletoe infection and an 

 opportunistic canker fungus often kills branch 

 tips and produces a conspicuous flagging in 

 infected true firs. 



In the Blue Mountains, crown deterioration 

 from dwarf mistletoe is the most important 

 cause of western larch mortality in undis- 



More than 40% of the Douglas-fir in eastern 

 Oregon are infected with dwarf mistletoe, which 

 causes growth loss, topkill, and mortality. Note 

 the prominent brooms on the tree in the fore- 

 ground and the killed tops on the trees in the 

 background. 



Oregon — 34 



