134 



PLANT SOCIOLOGY 



cent and 1.2 mm. at 2 cm. above the soil, in the vegetation; 78 per cent 

 and 6.5 mm. at 13 cm., in the vegetation; and 57 per cent and 12.8 mm. 

 at 100 cm., in the open air. 



Relative humidity has been measured by foresters for long periods 

 and has been correlated with forest growth. But the relation of 

 humidity to the transpiration of forest trees seems to remain uninvesti- 

 gated (c/. Burger, 1925). Biihler (1918) found the relative humidity in 

 beech woods lower than in spruce wood. The spruce- wood air was 8 to 

 12 per cent moister than the air in the open. 



// rz 13 n IS 16 n m js zo 21 zz Z3 zf 

 Air femperafure "C 



Fig. 71. — Saturation deficit of the air at various degrees of relative humidity with 

 increasing temperature. {After Bolas.) 



Observations on relative humidity extending over 11 years, in a 

 Larix decidua forest at Interlaken, in a spruce forest at Berne, and in a 

 beech forest at Pruntrut in the Bernese Jura gave the following results: 

 larch forest relative humidity 69.5 per cent, excess over the open air 

 outside 4.1 per cent; beech forest relative humidity 78.9 per cent, excess 

 over open air 3.6 per cent; and spruce forest, relative humidity 85.5 per 

 cent, excess over open air outside the forest 9.9 per cent. 



C. Saturation Deficit 



Saturation deficit is obtained by subtracting the actual vapor 

 pressure from the maximum possible vapor pressure at the given 

 temperature. It is expressed in millimeters of mercury. A relative 

 humidity of 75 per cent at 15°C. corresponds to a vapor pressure of 

 12.73 mm. by 0.75, or 9.56 mm. The saturation deficit is the difference 

 between the highest possible vapor pressure (12.73 mm. at 15°C.) and 

 the iictual vapor pressure (75 per cent), that is, 12.73 — 9.56 = 3.17 

 mm. As Bolas (1926) pointed out, the saturation deficit may vary 



