Distinctive features and definitions of young, mature, and old-growth Douglas-fir/hardwood forests

We examined the structure of 70 Douglas-fir/hardwood stands ranging from 40 to 560 years old in northwestern California and southwestern Oregon for features that best distinguished among young (40-100 year), mature (100-200 year), and old-growth (>200 year) forests. Separate analyses of 11 groups of variables representing six components of forest structure identified features of trees, understory cover, snags, and logs as important discriminators. Independently, features of understory and dead-wood components were weak-to-moderate discriminators among forest age-classes. The strongest discriminators were features of tree height and diameter components. A discriminant model using features from several components proved to have the greatest overall discriminating power among forest age-classes. This multiple components model included seven variables: the basal area of conifers 240 m tall, the basal area of conifers 290 cm in d.b.h., the basal area of conifers <45 cm in d.b.h., the basal area of hardwoods 45 to 90 cm in d.b.h., the cover of hardwoods 18 m tall, the density of hardwood saplings 1 to 2 m tall, and the density of hardwood snags 220 cm in d.b.h. and 22 m tall. We present definitions for young, mature, and old-growth Douglas-fir/hardwood forests based on a wide range of structural features. In addition, we revise the interim old-growth definition for this forest type introducing new minimum standards along with average values + 95-percent confidence limits for certain key features.