Page 9

Forests layers of deep canopy. As the saying goes, the forest is a poor man’s overcoat.

Thus the few living things that can pull nitrogen out of the air are indispensible.

As a general rule, the animal species that live in a particular forest cannot be predicted from knowledge of the plant species growing there, but can usually be predicted [on the basis of the match between plant structure and the animal’s habitat requirements for nesting, roosting, and cover from predators]. [. . .] Roosevelt elk, for example, eat several species of palatable herbs, but need thick shrubby growth for cover and bedding. A northern flying squirrel eats underground fungi of several species, as well as nuts, fruits, green vegetation, insects, and meat, but to a large extent needs large trees with previously excavated trunk holes to nest in. Even the dependence on plant structure is unconnected to plant species; an elk accepts most shrub species and a squirrel any species of large tree.

Besides nitrogen, a tree needs phosphorus, potassium, calcium, magnesium, and sulfur in large amounts, and about seven other nutrients in minute amounts, as well as of course water and carbon dioxide. The minerals it extracts from the soil water, while the carbon dioxide diffuses as a gas through the leaf stomates. We might imagine that a forest eventually exhausts its nutrient supply and deteriorates gradually into a shrubfield or grassland. Although this occasionally occurs in dunes and other stressed habitats, as a rule it is prevented in Northwest forests by a constant recycling of nutrients, from rotting wood to soil to tree and back to rotting wood. Ultimately, however, the nutrients originate from rock. As the temperature rises, for example, the gases in pockets within rocks expand, and the rock repeatedly shatters, baring more and more surface to the constantly flowing groundwater and streamwater, which erode away the minerals and wash them through the soil and into plant roots (Proctor, et al., 1980).

So, using the above terms, forest animal composition does not depend on plant composition so much as on plant structure. And in any ecosystem, structure and function are as interdependent and inseparable as the limbs and roots of the same tree. In old growth western hemlock forests, the most important structural elements are massive trees: alive and dead, standing and fallen, on the ground and in streams. As described below, each of these performs its own special function and provides its own unique habitat.

Large Live Trees Function Western redcedar reaches a maximum diameter of 21 ft. at 1200+ years, Sitka spruce 17.5 ft. at 750+ years, coast redwood 16.7 ft. at 2200 years, and Douglas fir 14.5 ft. at 1200 years; typical diameters are a little less than half these values (Franklin and Dyrness, 1973). Although the average diameter of live trees in old growth is about the same as in young growth (1.5 ft.), the largest young growth trees reach only about 2-3 feet in diameter (Franklin, et al., 1981). Typical old growth heights are 250-330 ft. for coast redwood, 230-260 ft for Douglas fir, 230-250 ft. for Sitka spruce, and over 200 ft. for western redcedar (Franklin and Dyrness, 1973). Trees of these sizes are truly mountainous. [. . .] [. . .] The conifers, large and small, also perform most of the photosynthesis and primary production. [. . .] Production of course varies in daily and seasonal cycles. A coastal Douglas fir reaches peak photosynthesis in spring and drops to a minimum in winter; much more growth is outside the summer “growing season” than in eastern deciduous forests (Waring and Franklin, 1979). Photosynthesis is moisture-limited in summer and temperature-limited in winter; the optimum temperature is about 66°F (Proctor, et al., 1980). Nitrogen is usually the limiting nutrient and comes ultimately from the air.

The passage of water and nutrients from soil to roots hinges on an amazing and little-known but crucial interaction between the trees and certain kinds of fungi. Few people are aware that the underground strands of Boletus, truffles, and other fungi wrap tightly around the thin, absorbent ends of plant roots, and other kinds of fungi actually grow inside the root cells. These mycorrhizal (myco=fungus, rhizal=root) associations benefit both plant and fungus in several ways, and in many cases, neither could survive without the other. For example, water and dissolved nutrients absorbed by the fungi pass directly into the plant roots. The extra absorbing surface offered by the fungi augments the overall water and nutrient uptake of the plant, and facilitates absorption of rare nutrients. The antibiotics manufactured by some of these fungi help to protect the plant from infections, and the tolerance of fungi to high temperatures reinforces the plant’s high temperature tolerance. The fungi also produce growth regulators that can stimulate root growth, and prolong the life of the rootlets. And the benefits go both ways. The fungi, which cannot photosynthesize, absorb carbohydrates and other valuable photosynthetic products from the plant (Proctor, et al., 1980). The benefits bestowed by mycorrhizal fungi are indispensable to many plants that die without them, usually during the first summer drought. Countless other species gain considerable competitive advantage with the fungi even though they can survive in hospitable habitats alone. Perhaps because they rarely associate with mycorrhizal fungi, most ferns, rushes, and sedges grow optimally where the substrate remains moist or wet throughout the summer. A fascinating quality of this symbiosis lies in its lock-and-key quality: the fungus that works


Profile for University of Puget Sound

Bookends Reader  

Welcome new students! This reader is a collection of readings from and about Puget Sound and that will be at the heart of the Bookends orien...

Bookends Reader  

Welcome new students! This reader is a collection of readings from and about Puget Sound and that will be at the heart of the Bookends orien...