Prevailing winds. — The effects of bodies of water and ocean currents, lati- 

 tude, altitude, and direction of slope are combined into a forest environment 

 by the prevailing winds. They may bring abundant rainfall warmed by 

 an ocean current, as on the Pacific slopes where the giant redwoods and 

 Douglas fir are produced, or they may be so hot and dry that only the 

 the mesquites and pinon pine may survive, or even a desert is produced. 



Forest fires and climate. — It is altogether probable that early forest fires 

 have had much to do with the kind of trees now growing in an area. The 

 same things that determine where trees will grow also contribute to or 

 alleviate the danger of forest fires. Frequent rains, of course, are natural 

 checks on fires. Forests of high altitudes, north slopes, and cold climates 

 generally are not as readily ignited as are those of low altitudes, south 

 slopes, and warm climates. Fire is a threat, however, whenever any forest 

 is dry. 



Within a region lightning fires are more numerous on higher ground than 

 on lower ground. 



The steeper a slope, the faster fire will burn because the fuel is in more 

 direct line with the upward movement of heat, gases, and flames. Wind 

 is the main factor in the rate of spread of a forest fire and the greater the 

 wind velocity, the greater is the difficulty of controlling the fire. 



The season of forest fires in different parts of the United States varies 

 largely with moisture conditions. In the South the most damage is done 

 in December, January, and February because those months have the least 

 rainfall and much of the undergrowth of shrubs and grass in the forest is 

 dormant and dry. In the Southwest spring and summer form an almost 

 unbroken fire season because of the lack of rainfall. In the Northwest 

 summer is the dry period and therefore the season of most forest fires. 

 Lightning storms at this time of year also contribute to the summer fire 

 hazard in these regions. In the Lake States forest fires are most common 

 in the late summer and early fall when rains are infrequent. In the Middle 

 Atlantic States forest fires are most common in spring and fall. The New 

 England States have comparatively few forest fires because of year-round 

 distribution of rainfall. The Central States, or hardwood region, have the 

 most fires in the spring when the fallen leaves have settled and dried. 



The kind of forest fires differs mainly with the species of trees. Crown 

 fires, which burn the tops of the trees, are rare in hardwood forests, except 

 in the dwarf hardwoods or chaparral of California. The firs, spruces, hem- 

 locks, and northern pines are very susceptible to crown fires. The southern 

 pines, except the sand pine of Florida, rarely support a crown fire, because 

 they are not so inflammable as are northern varieties and the stands are 

 much more open. Surface fires are closely associated with the southern 

 conifers, and with hardwood areas. 



The treeless Plains. — Many have tried to explain why trees do not grow 

 naturally on the Great Plains. In this region, except for fringes of trees 

 along the streams and on a few mountaintops cool enough to condense 

 some moisture from the passing winds, the land is treeless with seemingly 

 endless stretches of grasses, sagebrush, or bare soil. A combination of 

 factors rather than any one factor may be the cause of this condition. 

 However, trees along streams and the forests on the few mountains in the 

 area would seem to indicate that lack of rainfall is the major obstacle to 

 tree growth. It appears also that the extremes of heat and cold and the 

 periodic droughts are important reasons why the Great Plains are covered 

 with grasses instead of trees. 



