Pseudotsuga Menziesii - Forest Succession

Forest Succession

The shade-intolerance of Douglas-fir plays a large role in the forest succession of lowland old growth rainforest communities of the Pacific Northwest. While mature stands of lowland old-growth rainforest contain many Western Hemlock (Tsuga heterophylla) seedlings, and some Western Redcedar (Thuja plicata) seedlings, Douglas-fir dominated stands contain almost no Douglas-fir seedlings. This seeming contradiction occurs because Douglas-firs are intolerant of deep shade and rarely survive for long within the shaded understory. When a tree dies in a mature forest the canopy opens up and sunlight becomes available as a source of energy for new growth. The shade-tolerant Western Hemlock seedlings that sprout beneath the canopy have a head-start on other seedlings. This competitive advantage allows the Western Hemlock to grow rapidly into the sunlight, while other seedlings still struggle to emerge from the soil. The boughs of the growing Western Hemlock limit the sunlight for smaller trees and severely limit the chances of shade-intolerant trees, such as the Douglas-fir. Over the course of centuries, Western Hemlock typically come to dominate the canopy of an old-growth lowland rainforest.

Douglas-firs are seral trees in temperate rainforest, and possess thicker bark and a somewhat faster growth rate than most other climax trees of the area, such as the Western Hemlock and Western Redcedar. This quality often gives Douglas-firs a competitive advantage when the forest experiences a major disturbance such as fire. Periodically, portions of a Pacific Northwest lowland forest may be burned by wildfire, may be logged, or may be blown down by a wind-storm. These types of disturbances allow Douglas-fir to regenerate in openings, and low-intensity fires often leave Douglas-fir trees standing on drier sites, while less drought- and fire-tolerant species are unable to get established.

Conifers dominate the climax forests of the coast Douglas-fir. All of the climax conifers that grow alongside it can live for centuries, with a few species capable of living for over a millennium. Forests that exist on this time scale experience the type of sporadic disturbances that allow mature stands of Douglas-fir to establish themselves as a persistent element within a mature old-growth forest. When old growth forests survive in a natural state, they often look like a patchwork quilt of different forest communities. Western Hemlock typically dominate oldgrowth rainforests, but contain sections of Douglas-firs, Redcedar, Alder, and even Redwood forests on their southern extent, near the Oregon and California border, while Sitka spruce increases in frequency with latitude.

The logging practices of the last 200 years created artificial disturbances that caused Douglas-firs to thrive. The Douglas-fir's useful wood and its quick growth make it the crop of choice for many timber companies, which typically replant a clear-cut area with Douglas-fir seedlings. The high-light conditions that exist within a clear-cut also naturally favor the regeneration of Douglas-fir. Because of clear-cut logging, almost all the Pacific Northwest forests not strictly set aside for protection are today dominated by Douglas-fir, while the normally dominant climax species, such as Western Hemlock and Western Redcedar are less common. On drier sites in California, where Douglas-fir behaves as a climax species in the absence of fire, the Douglas-fir has become somewhat invasive following fire suppression practices of the twentieth and twenty-first centuries; it is becoming a dominant species in many oak woodlands, in which it was previously a minor component.