CLIMATIC FACTORS 



187 



The distribution of many climax communities is undoubtedly 

 governed by the saturation deficit of the air. This is especially shown 

 by the vegetation maps of Szymkiewicz (1923). Picea excelsa holds 

 strictly within moisture limits of 10 mm. (mean saturation deficit 



0.3 1.1 13 IZ 



xmm Juncetum trifidi e3S9 Vaccinietum viijrtilli >,^ I'olijtrichctum sexangiUaris. 



Fig. 73. — The profile of the vegetation and the saturation deficit in the Juncetum 

 trifidi and the Polytrichetum sexangularis, in the Tatra Mountains, 1,900 m. Length 

 of area 200 m.; height 25 m. {After Szafer, ct al.) 



of the air in the driest months at 2 p.m. 1895-1904) throughout all of 

 eastern and northern Europe, crossing these boundaries only rarely 

 and in insignificant amounts. The southeastern boundary of the 

 Russian forest region adjacent to the Sarmatian and Aralo-Caspian 

 steppe corresponds e.xactly with the 15-mm. line. 



D. Evaporation 



Without air movement, evaporation is closely proportioned to 

 vapor-tension deficit. The evaporation rate is the combined effect of 



Mar. April May June July Aug. Sept Oct Nov. 19 1? 

 Fiu. 74. — Grai)hs showing the reUition between mean monthly ev;i|><jrati<)n in 

 inches (£), wind flow in miles per hour (ID, and temperature in degrees ('(-n1igr;idc ( '/). 

 {After Shull.) 



humidity, wind, temperature, atmospheric pressure, and radiant 

 energy. Tempo rat ure far exceeds wind in its influence on evaporation 



