The Atlantic forest reaches its western limit in the Mississippi basin (Fig. 28), and is succeeded westward by treeless prairies, which merge along their western margins with the drier and less completely grass-covered high plains adjacent to the east base of the Rocky Mountains. The forest does not terminate abruptly, as on the border of a cleared field, but by gradual transitions. As its western limit is approached, a change in the species is noted, trees which thrive on uplands and can sustain long-continued summer drought replacing the species best adapted for more humid conditions. The forest is most extended, however, along the streams where white-trunked cottonwoods, frequently of great size, with widely spreading branches, extend even into the region of the great plateaus. Much of the prairie region in Illinois, Iowa, etc., was originally nearly surrounded by forest growths. The natural condition of the prairies and higher plains adjacent to them on the west and the reason for the limits set to the western extension of the Atlantic forest will be considered later under the heading Prairies and Plateaus.
The Boreal Forest.—From Nova Scotia, Newfoundland, and Labrador a forest composed mainly of a few species of coniferous trees extends westward, and after passing the southern extremity of Hudson Bay, is prolonged northwestward
across the continent and in the region of the mouth of the Mackenzie nearly to the shore of the Arctic Ocean. It extends also through central Alaska to within about 100 miles of the border of Bering Sea. This vast transverse forest belt which unites the northern extremity of the Atlantic forest with the northern portion of the Pacific forest is over 3,000 miles long from southeast to northwest, and on an average fully 600 miles wide. On the north, more especially in arctic Canada and as it approaches the shore of Bering Sea, it thins out, owing to the severity of the winter climate, the trees become dwarfed and stunted in much the same way as the trees adjacent to the timber-line on high mountains, and is succeeded by the broad treeless plains of the Barren Grounds and tundra. Along its south-central border its extension is again limited by climatic conditions, principally the dryness of the hot summers. The trees are there scattered or form isolated groves to the south of the general forest boundary, and are finally succeeded by the treeless prairies and interior plains and plateaus. On the east the great northern forest merges with the pine of the northern portion of the Atlantic forest, and in a similar way at the northwest passes by insensible gradations into the north extension of the coniferous forest growing on the Pacific mountains. In each of the instances there is no well-defined boundary between the east and west belt of northern forests and the north and south forest belts adjacent to the Atlantic and Pacific.
The boreal forest presents a striking contrast to the forests of the torrid zone and to the greater portion of the forests of temperate regions in the fact that it is composed of but a few species of trees. Monotony which becomes oppressive to one who lingers long in its sombre shade is its most conspicuous characteristic. In the main it is composed of but eight species of trees, namely, white and black spruce, larch or tamarack, canoe-birch, balsam-poplar, aspen, balsam-fir, and the gray pine. Of these the spruces are the most abundant and most characteristic as well as the most northern trees of the continent. They
frequently reach sufficient size to make them available for building log houses and for lumber.
Four of the species mentioned above, namely, the white spruce, canoe-birch, balsam-poplar, and aspen, cross the entire breadth of the continent from Labrador to Alaska, but the pines and firs in the east and the west are of different species. The larch or tamarack, which forms such an important feature of the forest in eastern Canada and about the Laurentian lakes, extends westward to beyond Hudson Bay, but is represented by other species in the Mackenzie and Yukon basins, and in the northern portion of the Pacific mountains. The region occupied by the great northern forest is interspersed with lakes, some of them of large size, and by innumerable swamps. The spruces and the gray pine grow on the uplands between the lakes and swamps, while the cold, wet bottom-lands are occupied by poplars, dwarf birches, willows, and alders. In the north, near where the forest breaks into outstanding groves and finally gives place to grassy hills, as along the Porcupine River in Alaska, the foliage in the lowlands becomes golden in autumn and forms irregular, far-reaching avenues of brilliant colour separating the hills, which are black with spruce-trees or shimmer with the soft gray tints of ripened grasses. There is much that is beautiful and even lovely along the poleward border of the great forest, but within its deeper recesses the ground is covered with mosses and lichens, and the stiff, sombre trees have a monotonous similarity and unbending rigidity.
In spite of the great area covered by the boreal forest, it being one of the greatest, if not the most widely extended continuous growth of arboreal vegetation in the world, it is of comparatively small economic importance. Even if the trees were within the reach of a market, their wood is of inferior quality and not generally suitable for lumber. A modern industry has been developed, however, which may bring it into demand, namely, the manufacture of wood-pulp, so largely employed in the making of paper and for other purposes.
The Pacific Forest.—In the northern portion of the Rocky Mountain region in Canada and Alaska the boreal forest, as already stated, merges by insensible gradations with the forests occupying the Pacific mountains from Alaska southward to Mexico. The junction line between the two is irregular, and what are essentially outliers of the more northern forest occupy the higher portions of the mountains in the western portion of the United States.
The Pacific forest begins at the north near Mount St. Elias, and at first occurs on isolated areas separated by ice-fields and inland reaches of the ocean, but in southeastern Alaska and on the numerous islands adjacent becomes more continuous and extends eastward far into British Columbia. As the timber-line in that region has an elevation of but 2,500 feet at the extreme western extension of the forest, although gradually rising southward, large portions of the mountains are treeless and barren. In the United States, on account of increasing dryness of the valleys from north to south, the forest becomes broken into many detached portions, which occupy the mountains and higher plateaus and in general are restricted to higher and higher locations with decrease in latitude. This distribution illustrates in a striking manner the dependence of trees on humidity. The forest is densest and the trees in general of greatest size and occur at the lowest elevations on the northwest portion of the Pacific coastal region, where the rainfall is excessive and distributed practically throughout the entire year. The Coast Ranges from Alaska southward to central California, as well as the Cascade Mountains and Sierra Nevada, are tree-clothed. In the interior, and especially in the central and southern portion of the Pacific cordillera, where the valleys are hot and dry in summer, trees are absent, and even the borders of the rivers in many instances without shade. In Canada the trees frequently extend across the lowlands, but in Montana and Idaho the valleys resemble the treeless plains to the east of the Rocky Mountains, while the uplands and the lower mountain slopes are dark with firs and pines. Above the forest rise the barren and frequently perpetually snow-covered