(7) Prairie.—In the table of Fig. 90 a belt of prairie or steppe is shown in latitudes slightly higher than those of the subtropical dry forest. As a matter of fact, as appears on the map, subtropical dry forest, desert, prairie, and deciduous forest all occur in the same latitudes in both North America and Eurasia. The forests occur near the coasts and the deserts and prairies in the interior. The distri bution of prairies depends on the season of rainfall and the kind of soil. Grasses are such assertive, tenacious plants that they can drive out the trees in places where trees could grow if nothing else inter fered with them. Thus large parts of the American prairies and the steppes of Russia and Hungary are located in regions where certain kinds of trees can flourish if they are protected when young. The grasses, however, because of their more rapid growth and greater hardiness, have ,driven out the trees. Over most of the prairie region the rainfall is apt to be deficient in the spring when the trees especially need it. Hence when seedling trees begin to grow they are at a disadvantage and are strangled by the more rapidly growing grasses. If such a region is swept by fires or is grazed by herds of animals like the buffalo, the grass and seedlings both suffer, but the grass springs up again in a few weeks, while the young trees must start from new seeds and hence are ousted in the long run. Because of their stimulating climate and rich soil, the prairies hold high rank in both agriculture and civilization, as is shown by our own Middle West.
(8) Deciduous Forests.—In the eastern United States and western Europe the prairies give place to deciduous forests. These are com posed of trees like the maple, beech, oak, and poplar, that drop their leaves in the autumn. They grow in places where the winters are cold but not extremely long and where the summers are not only warm, or even hot for a while, but have plenty of moisture all the time. These are the regions of cyclonic storms and of abundant rains at all seasons. The regions of deciduous forests are so excellent for men that they have been largely cleared and to-day support some of the world's densest populations, and contain the great manufacturing centers and the countries that stand in the forefront of civi lization.
(9) Coniferous Forests.—In an average latitude of about the other types of vegetation merge irregularly into vast forests of spruce, fur, pine, hemlock, and similar coniferous trees which thrive where the winters are long and cold, and the short summers warm and rainy. This evergreen forest forms a great belt across Canada and another from Sweden through Russia and Siberia. On the whole the con iferous forest is too cold for agriculture. Hence it has been occupied by settlers only in the southern portions. The rest still stands as the world's greatest forest reserve outside the tropics. Where the con iferous forest region is inhabited, the people are generally in a high state of civilization.
(10) Tundra.—Nearer the poles the coniferous forest gradually breaks down into a belt of bleak, grassy tundra. The seasons are too short for any vegetation except grasses together with lichens and other small hardy forms. No agriculture is possible. The reindeer, caribou, and muskox, however, can get a living, though they must often paw away the snow to get at the plants beneath. Hence, civilization is very low as we see in the extreme northern part of Asia and North America.
(11) Polar Deserts.—Near the poles in latitudes above the temperature is almost everywhere so low that no vegetation can exist unless it be minute bacteria. Therefore this region consists of polar deserts like northern Greenland and Antarctica, which are wholly devoid of inhabitants.
It is worth noting that in polar deserts it is not the temperature alone which prohibits the growth of plants. The long period when the ground is frozen prevents the plants from getting enough water. There is no way in which loss of water by transpiration can be balanced by absorption of water through the roots.