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Forests cannot penetrate the dense, intact, outer bark. Only if the bark has been broken by insect boring or some other injury can the roots penetrate into that protected, moist incubation zone beneath the bark, where the inner bark, cambium, and sapwood has already been softened and partially cleared away by insects, fungi, and other decomposers. Without an easy entrance, the roots must remain outside, where they dry up and die during the first summer drought. Even those that manage to penetrate, however, cannot yet pierce the heartwood, and this restriction, coupled with the initial lack of mycorrhizae, severely constrains early growth. [. . .] The lucky ones are clothed early with a soothing coat of mycorrhizal fungus, which provides moisture, nutrients, vitamins, growth regulators and antibiotics in return for photosynthetic sugars. The fungus not only cushions a seedling from the summer drought, but also gives it a competitive edge by accelerating nutrient uptake and growth. Plants that seldom form mycorrhizae (e.g., sedges and rushes) rarely grow among these vigorous mycorrhizal plants on rotting logs. [. . .] One [. . .] animal, whose life on the forest floor epitomizes the concept of symbiosis, is the California red-backed vole (Clethrionomys californicus). This poorly known rodent spends most of its time in subterranean burrows, where it mates, raises young, and, unlike any other North American mammal, feeds preferentially on the underground fruiting bodies of mycorrhizal fungi. Its diet includes the species Rhizopogon vinicolor, which fruits mostly in rotten wood and forms mycorrhizal associations with Douglas fir roots. The spores consumed by the vole pass through its digestive system intact, and are scattered in fecal pellets throughout the vole’s runways. The fungi thus depend on the vole’s feeding and burrowing to disperse spores from decrepit logs and stumps to new, fresh wood, where they once again infect any resident roots. The vole itself cannot survive without mycorrhizal fungi; after a clearcut in which all woody material is removed or burned, mycorrhizae stop fruiting and the voles die. In an old growth forest, the vole population is limited by the abundance of fallen trees. The optimal logs are those with well decayed sapwood or heartwood, in which the fungi have exhausted most nutrients and have begun to fruit. In this three-way symbiosis, both voles and trees die without the fungi, and the fungi might die without the photosynthetic products of the trees and the endless scurrying of the California red-backed voles that share their woody habitat. In addition to the voles, all other rodents west of the Rockies feed heavily on the underground fruiting bodies of mycorrhizal fungi, including mountain beavers, squirrels, chipmunks, pocket gophers, beavers, mice, rats, woodrats, muskrats, porcupines, and nutrias. Recent studies show that the fecal pellets of deer mice, chickarees, and northern flying squirrels contain not only

live, healthy spores of mycorrhizal fungi, but also a yeast and a nitrogen-fixing bacterium that lives within the fungal cells. The bacterium, azosporillin, is the most efficient nitrogen-fixing bacterium known. It apparently absorbs water and nutrients solely from the fungus, and perhaps indirectly from the plant roots, because it is incapable of surviving outside the fungus. It in turn supplies the fungus (and perhaps the plant roots) with nitrogen. Both fungus and bacterium grow more rapidly in the presence of the yeast, suggesting that the yeast secretes some regulatory compound. A single deer mouse produces an average of 66 fecal pellets every hour, and each pellet contains several hundred live mycorrhizal spores with their associated bacteria and yeast. Since it takes 1000 to 10,000 spores to inoculate a seedling, five deer mice produce enough pellets in 3 nights to inoculate 300-3000 seedlings. The pellets of other rodents contain similar amounts, and many of these species, especially voles, chipmunks, and squirrels, depend heavily on the feeding, cover, and nesting habitat offered by fallen trees, in whose crevices and cavities they liberally scatter these minute vitamin tablets. For these reasons, forest tree reproduction tends to be more rapid in stands with a greater abundance of fallen trees. At about the time the outer bark sloughs off and the seedling roots penetrate the heartwood, the community of plants and animals in and on a fallen tree reaches its maximum diversity. [. . .] By this time a lush plant community blankets the log, and includes conifer seedlings (mostly western hemlock, but also Sitka spruce and others), salal, two or three species of huckleberry, licorice fern, and a great variety of mosses, liverworts, and lichens. [. . .] Large snags. Many of the same decomposers also attack standing dead trees, or snags, but since they stand safely out of reach of most predators, large snags furnish critical habitat for a different community of birds and mammals (Figure 64). At least 46 birds and 6 mammals in the region depend wholly or heavily on snags for nesting and overwintering, sites for courtship rituals, food sources, and other activities (Proctor, et al., 1980; Franklin, et al., 1980). Among the birds are bald and golden eagles, osprey, peregrine falcons, mergansers, wood ducks, buffleheads, various hawks, owls, woodpeckers, chickadees, nuthatches, and bluebirds. The dependent mammals are martens, fishers, raccoons, chickarees, western gray squirrels, and northern flying squirrels. The largest snags are the most useful to these animals; hole-nesting birds prefer snags over 2 feet in diameter and 50 feet tall (Thomas, 1979). Birds that dig their own holes, such as the pileated woodpecker (Dryocopus pileatus), require a hard (more recently dead) surface as well. Generally, the larger the snags, the denser and more diverse is the community of birds that inhabit them (Franklin, et al., 1980).

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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...