pearance in spring or eariy summer of dead 

 lateral branches, dead tops, or entire dead 

 trees (particularly Douglas-fir) can be alarm- 

 ing. The primary cause of damage is water 

 stress in the tree resulting from drought in the 

 previous year or accumulated over several 

 years. The water stress reduces the tree's abil- 

 ity to defend itself against insects and patho- 

 gens. Damaged trees are most common in 

 urban areas on the fringe of forested areas, in 

 overstocked stands, on compacted or disturbed 

 soils, and on droughty or shallow soil types. 

 Such damage was particularly noticeable dur- 

 ing the late 1980s and early 1990s, a period 

 of drought in the Willamette Valley. Similar 

 observations were documented in the 1950s 

 and the 1970s. 



Winter weather kills needles or tree 

 tops — Winter drying of needles is common on 

 Douglas-fir in the northern end of the 

 Willamette Valley and in the Columbia Gorge, 

 where dry, easterly winds and sunny weather 

 cause water loss from needles to exceed water 

 uptake by roots. Needles often dry out and fall 

 from the tree in late winter and early spring. 

 Although the damage is highly visible, the buds 

 usually are not damaged and new growth re- 

 sumes in spring. 



Low temperatures alone can kill needles, 

 buds, twigs, or inner bark. Large trees, par- 

 ticularly Douglas-fir and ponderosa pine, are 

 often topkilled when low temperatures follow 

 an unseasonably warm period. In the Novem- 

 ber 1955 "deep freeze," and again to a lesser 

 extent in the winter of 1985, 

 widespread topkill was re- 

 ported throughout the 

 Willamette Valley. Mortality 

 and other symptoms of low 

 temperature injury often do 

 not show up until one or two 

 growing seasons after injury. 



Douglas-fir on droughty soils show typical symptoms of 

 canker diseases after a very dry year. 



Symptoms of Moisture Stress. Trees respond to mois- 

 ture stress in several ways. Under moderate stress, stem 

 and root growth are reduced. As stress increases, trees 

 become increasingly susceptible to certain insects and 

 diseases, particularly canker diseases and twig beetles. 

 Under severe drought, water content in the tree may drop 

 so low that the entire tree or portions of it may die. Roots 

 and the lower main trunk are the last to die, and often 

 remain living even though aboveground parts are dead. 



Sudden low temperatures after 

 a warm period in winter can kill 

 the tops of large, exposed trees, 

 especially Douglas-fir. 



Oregon — 3 1 



