WILLAMETTE VALLEY (242A) 



Ecology 



The forests of the Willamette Valley are 

 stands, groves, or savannas of Douglas-fir and 

 Oregon white oak, with scattered ponderosa 

 pine. Western hemlock, grand fir, and west- 

 ern redcedar grow on the wetter sites, as do 

 bigleaf maple, black cottonwood, red alder, and 

 Oregon ash. Most of the land lies between 50 

 and 1,000 feet in elevation, with a few hilltops 

 on the eastern edge reaching 1,500 feet. The 

 summers are hot and dry; annual precipita- 

 tion ranges from 25 to 60 inches, most of it 

 between October and June. 



Although the Willamette Valley is character- 

 ized by a summer drought, weather records 

 show that the severity of drought fluctuates in 

 10- to 20-year cycles. The mid 1980s to the 

 early 1990s was generally drier than normal, 

 and the next 10 to 20 years are predicted to 

 be wetter than normal. 



All Species 

 Lack of fire has caused species shifts in 

 the Valley — Historically, fire was periodic in 

 the Willamette Valley, either started naturally 

 or set by people to maintain open areas. Fre- 

 quent, light, ground fires maintained oak and 

 pine stands because the large thick-barked 

 trees tolerated fire better than did young oaks, 

 grand fir, or Douglas-fir. In the last century, 

 fire has been excluded, and many oak stands 



are now losing the competitive race with Doug- 

 las-fir and other species. Without the return 

 of fire, and with increased urbanization and 

 agriculture, oak and pine stands will likely di- 

 minish slowly in the Willamette Valley. 



Air pollution increases with increasing 

 people— The population of the Willamette Val- 

 ley has greatly increased over the past few de- 

 cades, mostly in the Portland area. Future 

 projections are for continued growth. Popula- 

 tion growth will be accompanied by increased 

 pollution from automobiles and light and heavy 

 industry. Air pollution not only affects visibil- 

 ity and human health, but also damages for- 

 ests. 



Ozone is a pollutant that affects forests. 

 Monitoring of both vegetation and air has 

 shown that ozone concentrations in some ar- 

 eas are high enough to cause plant damage. 

 In the Willamette Valley, the number of lichen 

 species, which are more sensitive to ozone than 

 other plants, has decreased in some areas. The 

 number and diversity of lichens, as well as the 

 concentration of pollutants in lichen tissue, are 

 being monitored to identify forested areas with 

 ozone damage. 



Drought interacts with insects and dis- 

 eases to damage conifers— The sudden ap- 



In the Willamette Valley, wet and dry periods have 

 Puctuated in 1 0- to 20-year cycles during the last 

 century; 1 985 to 1994 was a period of drought 



Source: Surface Water Supply Index, Natural Resources Conservation Service. 



Oregon — 30 



