CLIMATIC FACTORS 143 



Pinus silvestris, can endure long droughts in the seedHng stage. The 

 shghtest protection by shrub or tree layers alters the evaporation 

 relations and with it the chance for tree seedlings to develop. 



Larsen (1922) found the atmometer readings in the Pinus strohus 

 forest hardly half as much as in the open. The daily means (from 

 July 13 to Aug. 31), were 16.0 cc. in the forest against 35.3 cc. in the 

 open. Measurements of evaporation made at the German forest 

 reserve research stations, showed that evaporation in beech woods 

 during the summer months is only one-third of that in the open. The 

 annual evaporation in spruce woods and in pine woods averaged 48 

 per cent of that in the open; in beech woods, 42 per cent. 



Finally, we must consider the ability to reduce leaves and stems, to 

 shorten the duration of the life cycle, and to mature seeds in or close to 

 the ground (geocarpy, basicarpy). All of these capacities are adapta- 

 tions to a dry climate and hence are especially common in communities 

 of arid regions. 



E. Ratio of Precipitation to Evaporation 



In endeavoring to represent the water relations of the soil by a single 

 unit, Transeau (1905) employed the ratio between precipitation and 

 evaporation. He obtained his ratio (P:E) by dividing the normal 

 annual precipitation for each station in the United States by the 

 corresponding total evaporation from Russell's (1888) record of 

 evaporation data. The ratio values range from 0.4 for parts of 

 California and Nevada to 1.76 for portions of North Carolina and 3.84 

 for Tatoosh Island, Washington. From these data the United States 

 was divided by isoclimatic lines into four climatic zones which corre- 

 spond fairly well to the great plant-community types. 



Livingston and Shreve (1921) have somewhat extended this idea, 

 but there are yet too few evaporation data to permit this expression of 

 humidity to be very significant. 



N. S. Quotient 



In seeking a substitute for evaporation, Meyer (1926) argued that 

 air humidity as well as temperature regulates the amount of evapora- 

 tion. He therefore proposed the so-called "N. S. quotient" {Nieder- 

 schlag und Sdttigungsdefisit) — precipitation by saturation-deficit 

 Quotient. In spite of its imperfections, appreciated by Meyer, the 

 latio (quotient) appears to give significant results, and it has the 

 advantage of the abundant air humidity data which are of record. 

 The larger features of the distribution of both vegetation and climatic 

 soil types agree with the results of this factor, as seen in an N. S. 



