THE FOREST COMMUNITY. 



109 



That sufficient water ordinarily does not enter the soil to penetrate 

 deeply is shown by the following series of soil-moisture determinations 

 made during the summer of 1918: 



Table 21. — Water-content of the soil in the forest communities. 



* Samples taken the day following a rain of 1.84 inches. 



An examination of table 21 makes clear the fact that the greatest 

 amount of available water is in the surface 18 inches of soil. This 

 should account for the shallow root habit of forest herbs and shrubs. 

 The lesser extent of the root systems of these forest plants, when com- 

 pared as a group with the species of any of the preceding habitats, is 

 correlated with the more favorable aerial conditions for water con- 

 servation on the forest floor. Indeed, not only is the temperature 

 much lower, but the wind movement also is greatly checked. In the 

 fir forest the average daily wind velocity was only 29 miles as com- 

 pared with 67 miles on the half-gravel-slide. Likewise, in the forest 

 diffuse light and low evaporating power of the air are the rule. Com- 

 pared, for example with the half-gravel-sUde habitat, the aerial con- 

 ditions in the Douglas fir forest are only 54 per cent as severe. The 

 evaporation in the spruce forest is only 40 per cent as great as that on 

 the half-gravel-slide. 



