104 



ing 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 with- 

 out 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 tempera- 

 tures 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 de- 

 pends upon the other. It is the com- 

 bination of temperature and rainfall 

 that counts: A rainfall sufficient for 

 vigorous tree growth in the Temperate 

 Zone, to give an instance, might be in- 

 adequate 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, fol- 



Yearboo^ of Agriculture 1949 



low the mountains farther south, and 

 (in North America) extend down the 

 Pacific coast and then reappear in the 

 southeastern United States ; the broad- 

 leaf, Temperate Zone forests of east- 

 ern North America, western Europe, 

 and eastern Asia; the scrub or wood- 

 land 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 fortu- 

 nate in our present and past climates, 

 for they have given us the richest and 

 most varied forests to be found any- 

 where in the Temperate Zones. In 

 Maine or Michigan the forests are 

 spruce and fir. In the South they are 

 longleaf and loblolly pines; in be- 

 tween they are birch, maple, white 

 pine, and hemlock toward the north, 

 and oaks, hickory, and yellow-poplar 

 toward the south. In parts of Cali- 

 fornia are giant redwoods; in other 

 parts are scrub chapparal and wood- 

 land 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 Doug- 

 las-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 pat- 

 tern there are innumerable variations. 



But neither the broad pattern nor 

 the local variations are standing still. 



IN THE FIRST PLACE, climates are 

 continually shifting. Geological revolu- 

 tions, such as inundations, mountain 

 uprisings, and ice ages, can profoundly 

 alter weather and vegetation. We do 

 not need to go into details, but one 

 example is especially interesting. When 



