Il Vegetation

IL VEGETATION If the great physiographic divisions are exempted, the earth's surface is marked more distinctly by vegetation cover than by any other feature. This fact early attracted man to the study of vegetation as distinguished from the study of the individual plant. Primitive man recognized the plant cover not only as a source of sustenance and protection, but also as a guide in his hunting and planting his food crops. Studies in vegetation and its relation to the factors of the environment began very early.

In the recognition of a plant community (a term used in a gen eral sense to include all groups, large and small) the following criteria may be employed : Physiognomy, or the general appear ance, historically stands in first place, followed closely by flo ristic composition which has formed the basis of the science of floristic plant geography. Succession seeks to determine whether the vegetation has reached its final stage, complete adjustment to environment. The study of Habitat (see PLANTS: Distribution and Ecology) seeks to evaluate climate, soil and biotic factors.

The terrestrial vegetation of the world may be classified into the following great plant types : Forest, Grassland and Desert. The forests of the world may be divided into seven great formation groups covering a world area of about 22,000,000 square miles. Most of this area is valuable timber land, and about 14,000,000 sq.m. are capable of being converted into crop-producing land, of which about 6,000,000 are suitable for warm-weather crops, and 8,000,000 suitable for cool-weather crops. About 3,000,00o sq.m. of forest land are suitable for grazing only. The grasslands of the world are divided into seven great formation groups covering 13,000,00o sq.m. of valuable grazing land. Of this area 3,500,000 sq.m. are suitable for the production of cool-weather crops such as wheat, rye and oats; while 6,800,000 sq.m. are suitable for warm weather crops such as sorghum and cotton; and 2,700,00o sq.m. are suitable for grazing only. The deserts may be divided into five great formation groups covering a world area of 17,000,000 sq.m., valuable chiefly as grazing land of low carrying capacity

and, where water is available, for irrigation farming. The world contains about 26,000,000 sq.m. of land suitable for grazing; about 12,800,000 sq.m. of land capable of producing warm-weather crops, and about i i,5oo,000 sq.m. capable of producing cool weather crops. Allowing no waste land it would limit the produc tion area of the world to less than 25,000,000 square miles.

Forests.

Tropical Rain Forest is a luxuriant, evergreen forest with trees of unequal size and height and of varying age form ing a dense canopy of thick, leathery leaves, and composed chiefly of Leguminosae, Lauraceae, Myrtaceae, Moraceae and Diptero carpaceae—a true climax forest with trees of all ages and a mix ture of many species. Temperature is uniformly high, air damp, and rainfall 6o to 200 in. per year. Soil is greyish or reddish-brown, poor in humus and containing a large amount of hydrolyzed iron and alumina. Animal life is abundant but chiefly arboreal. Primi tive man has lived for the most part at the edge of this forest, and civilized man has utilized natural products such as rubber, gum, wood and bark. Cultivated crops are chiefly tropical such as rub ber, sugar cane, cacao and tropical fruits, vegetables and fibre plants, though temperate ones can be grown. This forest covers 3,800,00o sq.m., in Africa, South America and the East Indies.

Temperate Rain Forest is even more luxuriant and varied than the tropical rain forest. It is a many-storied forest, rich in mosses, tree ferns and vines. It consists of the southern conifers such as Podocarpus and Araucaria, as well as Ficus, Olea, Juniperus and Nothofagus. The temperature is moderate and the rainfall 4o to 8o inches per year. Under cultivation it will produce temperate cereals and fruits. This forest occupies about 55o,000 sq.m., chiefly in the highlands of Central Africa, South America, south eastern Asia, East Indies and Oceania.