in weather will also create changes in the in- 

 tensity and frequency of natural disturbances, 

 such as wildfire and wind storms, which in- 

 fluence forest composition and structure. 



If the climate becomes significantly warmer 

 and drier, forest communities may shift up 

 slopes to higher, cooler, moister elevations; 

 move to more northern latitudes; or move from 

 southern to northern aspects. Because each 

 plant species responds individually to the 

 unique interactions of temperature, moisture, 

 and site characteristics, some of the new com- 

 munities are likely to be completely different 

 assemblages from any community dominant 

 today. Insects and diseases are likely to ex- 

 pand or contract their ranges along with their 

 hosts, but they may also change their behav- 

 ior or habitat in unexpected ways. 



Because tree species are long lived relative 

 to people, some of the forest changes caused 

 by climatic fluctuations might not be obvious 

 for decades or even centuries. Changes would 

 be greatest in places where many species are 

 already at their physiological limits (southwest 

 Oregon, for example). An increase in insect 

 and disease activity on relict populations, 

 stressed by harsh site conditions, would be ex- 

 pected. 



The Future 



A challenge facing climatologists, ecologists, 

 and land managers is to improve understand- 

 ing of how dominant weather patterns are 

 likely to change — in both the short- and long- 

 terms — and how these changes will affect for- 

 ests. Although the climate has shifted in the 

 past, determining if current trends in climate 

 indicate a true climate shift or just a change 

 within the normal range of variation is diffi- 

 cult. If a climate change is predicted, forest 

 managers will need answers to many ques- 

 tions. How large will the change be? How soon 

 is it expected to affect vegetation? How will 

 plants, insects, and diseases respond? How 

 will the industries and services that depend on 

 forest communities be affected? Can forests 

 be made more resilient to such changes? 



EXOTIC PESTS 



Trees have coevolved with their native pests 

 for thousands of years. Forest health can be 

 greatly affected when exotic pests are intro- 

 duced and upset the balance. 



Exotic plants and animals — those intro- 

 duced from places outside of their native 

 range — can be harmful to native species. Many 

 introduced organisms are beneficial, such as 

 crop plants, ornamentals, game animals, and 

 livestock; these organisms were deliberately in- 

 troduced and are essential to United States 

 commerce and society. In the Pacific North- 

 west, the exotics that cause the most damage 

 to forest trees are accidentally introduced in- 

 sects and fungi. Introduced weeds are also de- 

 structive, competing with native forest vegeta- 

 tion for space, nutrients, and water. 



Problems With Exotics 



Without natural checks, the population of 

 an introduced pest can grow rapidly and wreak 

 havoc on the host organism. After a fungus 

 disease — white pine blister rust — ^was intro- 

 duced 86 years ago, western white pine has 

 been significantly reduced in many Oregon and 

 Washington forests where it once was common. 

 The balsam woolly adelgid, an insect that was 

 introduced to the Pacific Northwest in the 

 1930s, has damaged grand fir at low elevations 

 in the Willamette Valley to such an extent that 

 most are unable to reproduce. 



Exotic pests seriously affect Northwest for- 

 ests. Damaged trees diminish the value of 

 property and recreation experiences. Revenue 

 is lost from recreation, forest products, and 

 real estate. Quarantines to prevent pest spread 

 disrupt and affect the costs of transporting 

 local forest products. Control efforts (such as 

 pesticide treatments or resistance breeding 

 programs) are expensive, and additional money 

 must be spent to replace killed or damaged 

 trees. In Oregon and Washington, the cost of 

 trapping, analysis, and eradication of gypsy 

 moth from 1985 to 1995 exceeded $50 million. 



Most important, undesirable exotics change 

 forest ecosystems. Potential effects range from 

 slight decreases in native populations to per- 



Overview — 16 



