46 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



Table 22. — Relative humidity outside and inside the forest, in percent of saturation 



Region 



February-April 



May-July 



August-October 



November- 

 January 



Entire year 





Outside 



Inside 



Outside 



Inside 



Outside 



Inside 



Outside 



Inside 



Outside 



Inside 



Mountains of Al- 



Percent 

 80 

 80 



84 



Percent 

 85 

 84 

 85 



Percent 

 68 

 70 

 64 



Percent 

 75 

 80 

 68 



Percent 

 78 

 78 

 76 



Percent 

 84 

 85 

 81 



Percent 

 85 

 87 

 90 



Percent 

 89 

 90 

 92 



Percent 



77 

 79 

 78 



Percent 

 84 





85 



East Prussia 



82 



The forest litter also exercises an effect upon the relative humidity 

 of forests, according to the researches of Ebermayer, as recorded by 

 Morosov {159, p. 175), who found that the evaporation from the soil 

 in stands not covered with litter was reduced 62 percent in comparison 

 with soil outside the forest; but when the soil was protected by litter, 

 then the evaporation is reduced 23 percent more, making a total reduc- 

 tion of 85 percent. In other words, if soil in a given period of time 

 evaporates 100 units, the same soil under the forest canopy without 

 litter would evaporate 38 units, and with litter only 15. 



It is, therefore, easy to see that the especial conditions under the 

 forest canopy bring about a strong decrease in the transpiration of the 

 young growth and of the second story, and that the conditions under 

 which the lower story develops are much moister than those under 

 which the upper story is developing, as pointed out by Morosov 

 (159, p. 175). The young growth, possessing leaves not so heavily 

 cutinized and not so easily able to check transpiration, is thus develop- 

 ing in an atmosphere more nearly suited to it. Likewise, in clearings, 

 the relative humidity is less than in the stand, generally being between 

 that of the open country and the dense forest. Since the relative 

 humidity in clearings is frequently 10 to 25 percent less than that below 

 the forest canopy, when the mother trees are cut down the young 

 growth, which has been forming only shade leaves up to the present 

 time, is now suddenly plunged into a drier climate, so that the growth 

 of the tender leaves is often seriously checked. 



It is not surprising to learn, therefore, as reported by Adams (2), that 

 thinning a 20-year-old white pine plantation so that the basal area was 

 reduced 45 percent lowered the maximum relative humidity in the 

 crowns 3 to 4 percent, caused a slight increase in the evaporation rate 

 both in the crowns and near the ground, as measured by atmometers, 

 and increased the daily wind movement three to four times in the 

 crowns and two to three times at 8 inches above the ground. 



Altitude affects the humidity of the forest. According to observa- 

 tions in Bavaria by Eberma}^er, as stated by Rubner (178, p. 74), 

 the annual relative humidity of forests at high elevations is greater 

 than that in the lowlands. Also, in the high mountains the difference 

 between the relative humidity in the open and that in the forest is 

 greater, which is in agreement with the fact that the temperature 

 differences between the forest and the open increase with altitude. 

 This is of importance silviculturally, since in the higher altitudes the 

 humidity is frequently excessive, and attention needs to be directed 

 more in such cases to increasing the temperature which, of course, 

 then results in the lowering of the relative humidity. 



