by mild temperatures; prolonged cloudy peri- 

 ods; wet winters; cool, dry summers; and a 

 long, frost-free season. Coastal mountains 

 cause rain shadows that alter this maritime 

 climate in the Puget Trough and Willamette 

 Valley, making the summers warmer and the 

 winters colder and reducing annual precipita- 

 tion. With the rain shadows compounded by 

 southerly latitudes, southwestern Oregon's 

 valleys are still hotter and drier. 



East of the Cascade Range, temperatures 

 fluctuate more widely than in coastal areas, 

 winters are colder, summers are hotter, and 

 frost-free seasons are shorter. Although pre- 

 cipitation is considerably less than on the west 

 side, it is much more uniform throughout the 

 year. May and June are the months with the 

 highest precipitation in some areas east of the 

 Cascades. 



Short-Term Weather Events 



Short-term weather events damage trees but 

 are part of normal climate variation. Various 

 kinds of weather kill trees outright or affect 

 their susceptibility to injury, disease, or insect 

 attack. Spring freezes shrivel tender foliage. 

 Lightning damages portions of trees and tem- 

 porarily reduces their defensive capacity. Al- 

 though most trees can survive short-term 

 flooding, complete submersion and silt buildup 

 around trunks restrict oxygen intake and 

 cause damage. Fungal spores enter tree 

 wounds caused by wind or snow breakage. 

 Many bark beetles take advantage of daily 

 water stress, attacking host trees on warm 

 summer afternoons when defenses are low. 



The late 1980s and early 1990s in the Pa- 

 cific Northwest were significantly drier than the 

 long-term average. Many places — including 



Precipitation Patterns 

 (Washington and Oregon) 



tonroa: Ormgon C fciiila t tf W e 



Precipitation patterns in Oregon and Washington. 



Extreme weather, such as wind storms (top), can 

 kill trees outright. Some types of weather, such 

 as hail (bottom), damage only parts of the tree 

 but make it more susceptible to diseases 

 and insects. 



Overview — 14 



