Ecological characteristics of old-growth Douglas-fir forests.

Abstract

SummaryThe major ecological features of oldgrowth coniferous forests in the Douglas-fir region are reviewed. Special attention is given to characteristics that distinguish oldgrowth forests from managed and unmanaged (natural) young stands. The primary exemplary type is 350-to 750-year-old Douglas-fir-western hemlock forest typical of the western slopes of the Cascade Range, but other types and locales are discussed. Management techniques for maintenance of oldgrowth forests are also considered. Major conclusions are:1. Approximately 175 to 250 years are required to develop old-growth forests under natural conditions in both Coast and Cascade Ranges. Development of old growth is faster on good sites than on poor sites.2. Few plant or animal species are solely confined to old-growth forests, although many speciesincluding several vertebrates, saprophytic plants, and epiphytic lichens-find optimum habitats in such forests. Some organisms, however, may require old growth to maintain viable populations. Moreover, there are substantial differences in composition and relative abundance of species between young-and old-growth forests.3. Gross productivity is maintained at high levels in most old-growth stands, but mortality generally balances growth. Thus, the merchantable board-foot volume tends to remain constant for several centuries or gradually decreases because the amount of defect increases. Total organic matter keeps increasing because of accumulated masses of dead tree boles, mostly as down logs.4. Old-growth forests are highly retentive of nutrients; large amounts are incorporated into living and dead organic matter. Losses of limiting nutrients, such as nitrogen, are low.5. Nitrogen-fixing epiphytes are abundant in old-growth trees, and bacterial nitrogen fixation appears to be common in the large woody debris characteristic of old-growth forests.6. Small-to medium-size streams in old-growth forests depend mainly on forest litter for an energy base. These materials are invariably partially utilized before they are exported downstream.7. The structure of old-growth forest is more heterogenous than that of young forests; coefficients of variation in tree sizes are greater, and understory patchiness is much higher than in young-growth stands.6. Most of the distinctive features of old-growth forests can be related to four structural features: (1) large, live old-growth trees, (2) large snags, (3) large logs on land, and (4) large logs in streams. The structural features are related over time.9. A large, old-growth Douglas-fir is individualistic and commonly has an irregularly arranged, large, coarse branch system, and often, a long crown. It is ideal habitat for specialized vertebrates, such as the red tree vole, northern spotted owl, and northern flying squirrel, as well as nitrogen-fixing lichens.10. Large snags are valuable as habitat for a variety of vertebrates and invertebrates and as a future source of logs.11. Logs on the forest floor are important habitats for small mammals, including species that dispe...

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