CHANGES IN FOREST 

 VEGETATION 



The amount, variety, age, 

 and size mix of trees on a site 

 determine the extent and se- 

 verity of damage by distur- 

 bance agents. Changes in 

 forest vegetation qffectforest 

 health. 



Thirteen thousand years 

 ago, glaciers still covered 

 much of North America. As 

 the continent warmed, about 

 10,000 years ago, glaciers re- 

 ceded and coniferous forests 

 expanded their range. Fos- 

 sils from Mount Rainier sug- 

 gest that the period from 

 6,000 to 3,400 years ago was 

 actually warmer and drier 

 than the current climate. 

 Subalpine fir, Douglas-fir, ponderosa pine, 

 noble fir, and lodgepole pine were common. 

 California chaparral vegetation extended as 

 far north as Vancouver Island. Fires were 

 probably very frequent. The current cooler, 

 wetter period began about 3,500 years ago, 

 and fire frequency declined. 



Forest Succession 



Forest succession is the change in species 

 composition as plants grow, die, and are re- 

 placed over time. A tree that thrives in a 

 sunny opening created by fire may not be 

 able to reproduce in the shady environment 

 of a mature forest. It will be replaced by a 

 more shade-tolerant species. In the absence 

 of disturbances that create openings, shade- 

 tolerant climax species eventually dominate. 

 Type, diversity, and frequency of distur- 

 bances interact with site factors such as soil 

 type, topography, weather, climate, and sur- 

 rounding vegetation to influence which 

 plants invade a site after disturbance and 

 how communities develop. People can affect 

 plant succession by altering the type, sever- 

 ity, and frequency of disturbances. 



Millions of logs were moving out of the Northwest by the turn of the 



century. Photo courtesy of Oregon Historical Society (neg. 45791). 



American Indian and Pioneer Influences 



Native people modified the vegetation of 

 the Pacific Northwest — both accidentally and 

 deliberately. Fires set on sites such as Puget 

 Sound's Whidbey Island, the Willamette Val- 

 ley, and the eastern slopes of the Cascade 

 Range enhanced the growing of bracken, ca- 

 mas, huckleberries, blueberries, and 

 grouseberry and attracted browsing animals 

 like deer and elk. 



Early non-native visitors and settlers also 

 modified the forest environment in Oregon 

 and Washington. In many places, the vir- 

 tual elimination of beaver by trapping for 

 their pelts drastically altered riparian sys- 

 tems. Settlers copied the American Indians' 

 technique of attracting grazing animals by 

 setting many, sometimes devastating fires. 

 Settlers also brought new species to the area: 

 sheep, cattle, cheat grass, wheat, potatoes. 

 Use of forests was initially limited to local 

 demands for construction materials, fire- 

 wood, and fencing. Some forest lands were 

 converted to agriculture, town sites, and 

 residential areas so, in some places, forest 

 depletion became an issue. 



Overview — 6 



