The structure of natural young, mature, and old-growth Douglas-fir forests in Oregon and Washington

Structural characteristics of forest overstories, understories,and forest floor and coarse woody litter accumulations wererelated to stand age, geography, and relative site moisture in196 Douglas-fir stands in western Oregon and Washington.Stands ranged in age from 40 to 900 years and typically orig-inated after wildfire. A series of multivariate analyses wasused to identify structural attributes that distinguished old-growth (>195 years old) from mature (80-195 years old) andyoung (40-80 years old) age-classes. Separate analyses offour attribute sets-overstory, stand condition (degree ofdamage and disease), understory vegetation, and forest floorand coarse woody litter indicated that the age-classes differedin numerous attributes. Discriminant models that used over-story characteristics, such as tree density, density of largeDouglas-firs, and variation in tree diameter, and models com-bining variables from the four different attribute sets weremost successful in distinguishing the age-classes. Age-classesrepresent portions of a continuous gradient of structural variation associated with age, based on ordinations with can-onical discriminant analysis. Old-growth stands from differ-ent geographic provinces and relative site moisture-classesdiffer considerably in structure. Much of this variation in old-growth structure was associated with variation in densities ofshade-tolerant trees and characteristics of the understoryvegetation. Variation in old-growth-forest structure stronglysuggests that low to moderate disturbance during the life ofthe stands has had an important influence on stand structure.The study provides a basis for defining and taking inventoryof old-growth conditions as well as for managing the naturaldiversity of old-growth and earlier successional Douglas-firforests in western Oregon and Washington.