26 SEEDING AND PLANTING 



either the moisture of the air or the moisture of the soil but to 

 both. Thus, a dry atmosphere and a relatively abundant absorp- 

 tion may cause the same effect as a moist atmosphere and a 

 relatively small absorption. 



Woody species thrive on the western slopes of the coast moun- 

 tains of southern California because of the frequent fogs and the 

 relatively humid atmosphere. Farther inland, because of the 

 less humid atmosphere, a similar vegetation is possible only when 

 a much larger amount of soil water is available for absorption. 



All sites undergo great fluctuations in the relative humidity of 

 the air. Where soil moisture conditions are suitable, however, 

 there is no known region where the air is too dry to sustain forest 

 growth. Thus, in the most arid regions forest trees of one species 

 or another thrive when the soil is supplied with water. But 

 even if the soil is amply supplied with moisture, not all species 

 have the same power for resisting the dry air. Thus, most 

 species with a large transpiration surface, thin cutis, and other 

 hygrophilous structures fail when planted in desert regions even 

 under irrigation. The roots are unable to supply a transpiration 

 current of sufficient volume to overcome the loss through the foliage. 



We know from observation that different species exhibit a 

 wide range in the humidity conditions under which they thrive. 

 Thus, Douglas fir appears to thrive under a wide range of con- 

 ditions while the redwood of California is limited in its distribu- 

 tion to the area subject to frequent fogs and consequently with a 

 high relative humidity. Most species appear to do best in a 

 humid or a moderately humid atmosphere. 



In seeding and planting operations in arid and semi-arid regions 

 or in regions having a relatively low degree of atmospheric hu- 

 midity, species which have structures well developed to resist 

 the loss of water through transpiration should be selected. In 

 regions where the amount of soil moisture is very limited, many 

 species can be successfully grown if fogs are frequent and the rela- 

 tive humidity high. On the other hand, an annual precipitation 

 of 20 inches or more will not maintain a forest, except of xerophilous 

 type, in the extremely dry air of Arizona. 



13. Precipitation in Its Relation to the Choice of Species. - 

 The aqueous vapor of the atmosphere is deposited upon the 

 surface of the earth as rain, snow, dew, rime or sleet, hail, and 

 frost. All of these forms of precipitation add to the water con- 



