CORVALLIS, Ore. — While large fires have had dramatic impacts in some Pacific Northwest forests, only about 10 percent of the forested lands in the eastern Cascades have burned in the last 30 years, and young trees and dense forests are continuing to grow at a rate that outstrips losses from disturbance. As a result, many forests across this region are becoming denser. Efforts to reduce their vulnerability to future high-severity fires — through tree thinning, prescribed burning and harvesting — have had little overall effect on forest structural conditions across the region as whole. Those are among the results of a comprehensive analysis of forest structure and biodiversity based on satellite imagery and on-the-ground field work in the eastern Cascades of Washington, Oregon and Northern California from 1985 to 2010.