Composition, Function, and Structure of Old-Growth Douglas-Fir Forests

Ecological characteristics of old-growth Douglas-fir forests are examined in terms of compositional, functional, and structural features. Old-growth forests typically include distinctive animal and plant species-that is, organisms that are most abundant in such forests. Functional behavior of old-growth forests differs from functional behavior under other forest conditions. Productivity is typically high, but most energy is used to maintain the large mass of living material; growth and mortality are in approximate balance over long periods. Nutrient and sediment yields from old-growth forests are typically very low. Old-growth forests also differ from younger forests in their effects on hydrologic cycles in areas where cloud or fog precipitation, rain-on-snow events, or both are important. The structural characteristics of old-growth forests are the basis for most of their unique compositional and functional attributes. Specifically, important structural components are large individual old trees-whether live, dead and standing (snags), or dead and fallen (logs)-in both terrestrial and associated aquatic environ-ments. Old-growth forests also have stand attributes that are very different from younger and, especially, from managed forests. The distinctive communities of microbes, invertebrates, higher plants, and animals that occur in old-growth forests are integrally related to the collective structural attributes. Abstract Sporocarps of hypogeous ectomycorrhizal fungi were harvested from 10 Douglas-fir stands representing a variety of natural habitats. Results are presented in a landscape context by assuming that the reported sporocarp production is representative of Western Hemlock Zone forests in the central western Cascade Range of Oregon. Over all stands, sporocarps were harvested from 5400 m 2 and yielded a total dry standing biomass equivalent to 1.3 kg per ha. The maximum single-stand sample biomass equaled 9.9 kg per ha. Forty-seven species of hypogeous fungi were recorded. Elaphomyces granulatus, Gautieria monticola, Hysterangium coriaceum, Leucogaster rubescens, and Rhizopogon parksii accounted for 73 percent of the biomass. In contrast to epigeous species in the study area, hypogeous sporocarp production was higher in spring than in fall, Individual fungal species showed strong seasonal trends; most had spring production peaks. Year-to-year variation in relative biomass for individual species was greater in spring than in fall. The major species in this study were also major species in studies from young Douglasfir stands in the Oregon Coast Range. Gautieria monticola, in particular, seems to be a widespread dominant species in Western Hemlock Zone forests of western Oregon. Abstract Structural characteristics of forest overstories, understories, and forest floor and coarse woody litter accumulations were related to stand age, geography, and relative site moisture in 196 Douglas-fir stands in western Oregon and Washington. Stands ranged in age from 40 to 900 years and typically originated after wildfire. A series of multivariate analyses was used to identify structural attributes that distinguished old-growth (>195 years old) from mature (80-195 years old) and young (40-80 years old) age-classes. Separate analyses of four attribute sets-overstory, stand condition (degree of damage and disease), understory vegetation, and forest floor and coarse woody litter indicated that the age-classes differed in numerous attributes. Discriminant models that used overstory characteristics, such as tree density, density of large Douglas-firs, and variation in tree diameter, and models com-bining variables from the four different attribute sets were most successful in distinguishing the age-classes. Age-classes represent portions of a continuous gradient of structural variation associated with age, based on ordinations with can-onical discriminant analysis. Old-growth stands from different geographic provinces and relative site moisture-classes differ considerably in structure. Much of this variation in old-growth structure was associated with variation in densities of shade-tolerant trees and characteristics of the understory vegetation. Variation in old-growth-forest structure strongly suggests that low to moderate disturbance during the life of the stands has had an important influence on stand structure. The study provides a basis for defining and taking inventory of old-growth conditions as well as for managing the natural diversity of old-growth and earlier successional Douglas-fir forests in western Oregon and Washington.