FORESTS AND MEN

A juniper centuries old: "A man does not plant a tree for himself, he plants it for posterity."

This drawing is of a scene among the redwoods in California.

Trees Living Together

THE COMMUNITY OF TREES

JESSE H. BUELL.

AS ONE gets farther and farther beyond the centers of population he comes finally to the forests that human activities have not changed to virgin forests. In them, one can see better what man has done to forests and how he can more wisely mold them to his benefit. In these manless forests, also, a person comes to understand that a forest is a changing, living community, subject always to the forces of inanimate nature earth, air, sunshine, and rain; to the interaction within them of plants and the animals; to the changes that forests themselves can effect in their environment.

Green plants are the engines for the manufacture of the carbohydrates the basic stuff' that all vegetable matter comes from, all animals live on, and by which, ultimately, all of us are fed, clothed, sheltered, and kept warm. A forest is a vast battery of such engines. In a single growing season an acre well stocked with vigorous trees may produce 3 or 4 tons of useful wood, not counting the additional unmeasured pounds of the new growth on branches and roots and in leaves, buds, flowers, and seeds. The raw materials are carbon dioxide from the air, water and mineral nutrients from the soil, and warmth and energy from the sun. Of these, only water and warmth are likely anywhere on earth to be too scarce for forest growth. Carbon dioxide makes up only 3 parts in 10,000 parts of the atmosphere, but unlimited fresh supplies are continually brought by air currents. Mineral nutrients, although indispensable, are needed in such small quantities they make up only a small fraction of the dry weight of trees that they are abundant enough almost everywhere to keep forests growing.

But forests use up vast quantities of water. To make a summer's growth, the roots of the acre of healthy forest that grew 3 or 4 tons of wood may take up from the soil 4,000 tons of water. Much of this water, passing up from the roots through the trunk, branches, and leaves, escapes into the surrounding air. Its chief usefulness to the tree is to carry nutrients from the soil and organic materials from storage places in the trunk and roots to the leaves and the growing twigs. Although the water that is transpired into the air never goes into the building of woody tissue or leaves, trees cannot live without it. Such quantities are required that the climate over large areas of the earth is too dry to supply them.

And the circumpolar regions are too cold for tree growth. No plant can thrive where monthly mean temperatures are below freezing the year around. Just a few days in midsummer, warm enough to melt the snow and thaw out the soil to a depth of an inch or two, may bring into bloom tiny alpine plants on the bleak north shore of Greenland within 350 miles of the Pole. But so short a growing season would give a tree no chance to store up food for another season's burst of growth and it could not withstand the intense cold of the arctic winter.

So it was that before man began to use the forests their distribution over the continents was determined by the climatic pattern of the earth. Drought and cold are the barriers that limit tree growth, but the effect of each depends upon the other. It is the combination of temperature and rainfall that counts: A rainfall sufficient for vigorous tree growth in the Temperate Zone, to give an instance, might be inadequate in the Tropics, where water evaporates more rapidly from the soil and plants transpire faster, and would be useless in the polar regions, where temperatures are below freezing most of the time.

In general, forests occur only where the annual precipitation is more than 15 or 20 inches a year and where the frost-free period is at least 14 or 16 weeks long. In regions too dry for forests, grasses grow or they give way to desert; where it is too cold, tundras and icefields spread. The broad forest zones of the earth are the coniferous forests that stretch around the world above about 45 north latitude, follow the mountains farther south, and (in North America) extend down the Pacific coast and then reappear in the southeastern United States; the broadleaf, Temperate Zone forests of eastern North America, western Europe, and eastern Asia; the scrub or woodland forests that border the desert areas of all the continents; and the tropical forests of Africa and South America.

We in the United States are fortunate in our present and past climates, for they have given us the richest and most varied forests to be found anywhere in the Temperate Zones. In Maine or Michigan the forests are spruce and fir. In the South they are longleaf and loblolly pines; in between they are birch, maple, white pine, and hemlock toward the north, and oaks, hickory, and yellow-poplar toward the south. In parts of California are giant redwoods; in other parts are scrub chaparral and woodland that border dry lands where cacti are as big as trees; they, in turn, give way to deserts where almost nothing grows. West of the Cascade Mountains in Oregon and Washington are Douglas-fir forests; eastward to the far edge of the Rockies are ponderosa and lodgepole pines where the rainfall is sufficient, with spruce and fir showing up in the places where the mountains go high enough to reach the alpine cold. In the wide belt stretching from the base of the Rockies toward the Mississippi, the only trees you will find are cottonwoods and willows along the creeks or planted shelterbelts around the farms, for this is the great domain of the grasses. Within the broad pattern there are innumerable variations.

But neither the broad pattern nor the local variations are standing still.