114 PLANT SOCIOLOGY 



from north to south, dividing them into a western slope with a rainfall 

 varying from 200 to 300 cm. yearly and an eastern basin with an annual 

 precipitation of only 25 to 40 cm. 



Rainfall in Small Areas in Relation to Plant Communities. — Places 

 with equal rainfall are connected on a map by lines called isohyets. 

 The study of the distribution of specially significant species and plant 

 communities has shown that their boundaries within a small, climati- 

 cally uniform region agree to a remarkable degree with certain isohye- 

 ts. Thus, the most peculiar xeric colonies and associations of the 

 Alps are strictly confined to the centers of extreme drought with 55 to 

 70 cm. annual precipitation. These include such Sarmatic species as 

 Carex stenophylla, C. supina, Kochia prostrata, Astragalus austriacus, A. 

 vesicariiis, A. exscapus, Seseli varium, Dracocephaliim austriacum, and 

 Achillea tomentosa, and the dry-turf communities of Festuca vallesiaca, 

 Poa concinna, Stipa capillata, C. supina found mostly as sharply 

 defined associations. The regions of minimum rainfall of the interior 

 of the Alps can be located by the presence of these communities. 



A few Sarmatic species of the oak woods and the Xerobrometum of 

 northern Switzerland and southern Germany have similar indicator 

 value. The Xerobrometum erecti with its characteristic companion 

 species is strictly confined to the region with 70 to 90 cm. annual 

 precipitation. In habitats that are edaphically especially favorable, on 

 dry southern slopes of the porous Jurassic limestone, this association 

 occasionally oversteps the isohyet of 90 cm. 



From 90 to 130 cm. rainfall, in the northern alpine foothills, the 

 place of the Xerobrometum is everywhere taken by the more mesic 

 Mesobrometum, as shown by Scherrer (1921) and by Koch (1926). 



Measuring Raijifall. — The characterization of climate according to 

 rainfall is the business of the weather service. Daily precipitation is 

 measured and from this are obtained the monthly and annual means 

 and the number of rainy days and their distribution throughout the 

 year. The last two points are of especial importance for biology. In 

 uninhabited places, especially in high mountains, the total annual 

 precipitation alone is taken by means of rain collectors. The ample 

 collecting vessel, with windshield, is supplied with 6 kg. of calcium 

 chloride to prevent freezing. A thin layer of oil poured over the 

 contents prevents evaporation. Thus one annual measuring and 

 emptying suffices. This "totalizer," by acting as dust collector, also 

 gives important data on soil formation (c/. p. 176). The establish- 

 ment of rain collectors at high altitudes in the Pyrenees and the 

 Alps has brought out the important fact that the total precipitation in 

 mountains increases up to very great heights. 



