Conditions for winter wind 

 damage often are restricted to 

 a particular elevation, resulting 

 in a band of damage called 

 "red belt." 



Winter Wind Injury: The process of transporting water from roots to foliage is passive. 

 Evaporation from the foliage, called transpiration, creates a deficiency of water at the top 

 of the tree. Additional water molecules are drawn up along the trees' conductive tissues 

 to replace molecules that leave the foliage. The resulting water column is like a drinking 

 straw. Additional liquid is drawn upward to replace any liquid removed from the top. 



Injury can result to trees on warm, bright winter days. Pores on the foliage open to 

 receive carbon dioxide from the air. Water begins to evaporate from the leaves and addi- 

 tional water is drawn into the roots to replace it. Water moves slowly through cold soils, 

 however. The foliage loses water faster than roots can acquire it, causing the foliage and 

 upper branches to desiccate and die. 



Precipitation 

 Willamette Valley 



S-year running average 

 - 100-year average 



u^ O irt O 









Year 



The precipitation record for the Willamette Valley 

 illustrates the cyclic pattern of moist and dry 

 periods in the Pacific Northwest source. Oregon cumate 



Service, Oregon Stote University. 



southwest Oregon, the Blue Mountains, the 

 Willamette Valley, and the Puget Sound cirea — 

 experienced this drought. Outbreaks of some 

 insects, related to increased moisture stress 

 on trees particularly in overstocked stands, of- 

 ten follow dry years. Over the last century in 

 the Pacific Northwest, the most recent drought 

 appears to be part of a cycle of wet and dry 

 periods each lasting 10 to 30 years. This cli- 

 mate pattern is within a currently accepted 

 normal range of variability. 



Long-Term Weather Changes 



Long-term climate changes have and will 

 continue to produce changes in forests in Or- 

 egon and Washington. The environmental vari- 

 able that limits forests most and defines for- 

 est community gradients is available moisture 

 during the dry summers. Long-term changes 



Overview — 15 



