REDWOODS, RAINFALL AND FOG 



189 



sary for redwood forest is an ample supply of soil water ; but that 

 this alone is not sufficient. The second, a summer fog blanket, 

 cutting dowTi the rate of evaporation, is just as essential. The 

 results may be expressed in tabular form (table 2). 



TABLE 3 



SUMMARY 



The redwood requires a high ratio of water supply to water 

 loss, and is unusually sensitive to the danger of rapid transpira- 

 tion, even when the supply is ample. 



Soil moisture studies in the Palo Alto region indicate that 

 during the rainless season the soil becomes dangerously dry, 

 even in the more mesophytic habitats. 



The redwood may exist in regions where the rainfall is deficient, 

 but only close to permanent streams. 



For full development of redwood forest, covering mountain 

 slopes which may become relatively dry as well as the inmiediate 

 environs of permanent streams, heavy wdnter precipitation is 

 necessary, but alone is not sufficient. Abundant summer fog 

 is also essential, its effects being to decrease the water loss, and 

 in some degree to add to the soil water supply. \ATiere summer 

 fogs do not occur, or where they occur infrequently, no true 

 forest of redwood is possible, even though the rainfall be as high 

 or higher than in fog-frequented areas. In areas of the former 

 kind the infrequent redwoods are confined to the banks of streams, 

 as in regions of deficient precipitation. 



For the rainfall studies involved in the above work a simple 

 type of rain gauge, making possible sunmiation of precipitation for 

 long periods, was used, and is described in the body of the paper. 



