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Forests

Forests From The Northwest Coast: A Natural History by Stewart T. Schultz The Pine of fur species, or spruce Pine grow here to an emence size & hight maney of them 6 & 7 feet through and upwards of 200 feet high. — William Clark, Tuesday, Nov. 12, 1805. The chapter excerpted below has been edited for length and clarity.

DOMINANCE OF CONIFERS The forests of the Pacific Northwest have no equal on earth. The largest and heaviest trees in the world, and most of the runners-up, grow in the lush, fog- and rain-drenched forests of the Coast Ranges. The total weight of trunk, branches, and foliage in a mature forest ranges from 330 to 790 tons per acre in Oregon Cascades and Coast Range non-redwood forests, and up to 1800 tons/acre in the coastal redwoods. Forests in the eastern U.S. and elsewhere, including the tropics, rarely accumulate more than 300 tons/acre (Waring and Franklin, 1979). Not coincidentally, the dominant trees in these forests are magnificent evergreen conifers rather than deciduous hardwoods. Twenty-five conifer species grow in the Northwest, several times the number in any other region in North America. By comparison, only 12 hardwood tree species are native to the Pacific Northwest. Moreover, both abundance and biomass of conifers in the Oregon Cascades and Coast Range are 1000 times that of the hardwoods (Harris, 1984); hardwoods dominate in nearly all other North Temperate forests. What might explain the dominance of conifers? As early as 30 million years ago, a diverse hardwood forest occupied the lowlands of the Pacific Rim from northern California to Alaska, Siberia, and Japan, consisting of beech, elm, oak, hickory, and many other genera now common in the eastern U.S (Waring and Franklin, 1979). At this time the conifers were small, stunted, and restricted to the more stressful conditions at high elevations. By about 10 million years ago the Cascade and Coast ranges had arisen, and promoted a wetter, cooler climate on their western slopes. During the late Pliocene, some 2.5 million years ago, many of the hardwoods became extinct, and the rest gradually shrank into specialized habitats, as the conifers spread down into the lowlands. By early Pleistocene, about 1.5 million years ago, the Northwest forests had developed an overwhelming preponderance of coniferous species, and appeared very much the same as today.

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What happened in the late Pliocene to usher in the conifers? Apparently this was the period when the two distinctive features of the present Northwest climate coincided for the first time: the low rainfall of summer, and the mild temperatures of winter. Why should these favor conifers?

Mild Winter Temperatures Botanists believe that deciduous trees evolved about 100 million years ago in areas with wet soils, perhaps river bottoms or flooded meadows (Stebbins, 1974). By this theory, in these places the leaves of plants were saturated with water most of the year, most importantly in the winter when temperatures often fell below freezing. To prevent ice from forming in the soft, vulnerable tissues of the leaves and young stems, some trees effectively “hardened” themselves by dropping their leaves well before the first frost. These were the first deciduous trees. The deciduous strategy still abounds in wet areas of the Northwest: alder, willow, maple, and ash are common in bogs, fens, and along riverbanks. Before the leaves fall, most of their metabolites, notably chlorophyll, and energy reserves are transferred back into the tree to be recycled next spring. As these substances exit and unveil the minor pigments, the leaves flush crimson and yellow. Since our forests lack the abundant deciduous trees of the eastern U.S., we have nothing to compare with the brilliant “New England autumn.” On the Northwest Coast, however, the proximity of the ocean warms the winter air enough to prevent frequent freezes. Even in the mountains subfreezing day temperatures are rare. In the subalpine zone, although snow often accumulates and persists until July, the soil usually remains unfrozen all winter. As a result, photosynthesis is possible throughout the Northwest winter, but only for those plants that retain their leaves, namely the conifers and evergreen hardwoods. While the deciduous trees lay dormant, the conifers gain a tremendous advantage by continuing photosynthesis, from October to April accumulating 30% to over 50% of their total budget of carbohydrates (Waring, 1982). Another aid to winter photosynthesis is the crown shape of conifers. While the bowl-shaped crowns of deciduous trees capitalize on abundant direct summer sunlight, the long, conical crowns of the conifers maximize the absorption of the oblique, reflected light common in the winter. [. . .]

Summer Drought Plants are faced with a dilemma in hot and dry climates. Stomates need to remain open in order to take in the carbon dioxide necessary for photosynthesis, and to allow nutrient absorption by roots during transpiration. But on the other hand, open stomates allow water loss. Moreover, the fungi

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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...