One other pine of the widely extended Pacific forest demands attention even from the passing traveller, not on account of its size, for it is a dwarf in a land of giants, but for its wide distribution and the food its large, oily seeds furnish for birds, squirrels, and even for man. I refer to the piñon pine, of which there are several species. They are seldom over 35 or 40 feet high, and are not remarkable for beauty, although they furnish an agreeable feature in the sparsely forest-clothed and semiarid region where they thrive best, but they bear a profusion of small cones, each of which contains perhaps a dozen edible and nutritious seeds. These seeds were formerly used by the Indians for food on an extensive scale, and are still gathered in large quantities, and may be found in the markets of our cities. The Indian encampments in the piñon forests in the fall of the year are among the most picturesque features of these degenerate days of the aborigines.
In the southwest portions of the United States the forests are confined to the mountains and the higher table-lands, the hot, arid valleys being without trees other than the larger growths of cacti and yucca. Similar conditions are present in northern Mexico, but on the western side of that republic and throughout practically the whole of the peninsula of Lower California the mountains and valleys alike are treeless and desolate.
As stated by C. S. Sargent, the forests of North America, exclusive of Mexico, Central America, and the West Indies, contain arboreal representatives of 158 genera of plants, of which 94 genera occur in the Atlantic and 59 in the Pacific side of the continent, and 48 genera in the tropical portion of southern Florida. Of the number of genera of trees in the Caribbean forest we have no reliable census.
To the west of the Atlantic forest lie the broad natural meadows termed prairies, and still farther west the yet more extensive pasture-lands of the great plateaus which reach the eastern base of the Rocky Mountains (Fig. 28). The transition from the luxuriant and varied forest on the east to the treeless, thinly grass-covered plateaus on the west side of the interior Continental basin is gradual. The change occurs in the prairie region, where a struggle has been in progress for thousands of years between the conditions favouring and those adverse to tree growth. The balance of power, so to speak, is the amount of rain or of soil-stored moisture during the summer season. The gradual decrease in the mean annual precipitation from east to west on the inland border of the Atlantic forest continues westward, and on the plateaus adjacent to the Rocky Mountains the aridity is such that no trees can grow except along the immediate border of the stream, unless artificially irrigated.