CLIMATIC FACTORS 115 



For the measurement of the amount of rainfall actually reaching 

 different plant communities and layers, rain gages must be set up in the 

 most varied places within definite plant communities. In stratified 

 communities, whose upper layers catch a part of the falling rain, 

 measurements must be made in the ground layer close to the earth, and 

 in the herb, shrub, and tree layers if present, in order to determine the 

 water-retaining effect of each layer. The water running down the 

 trunks of trees, so important for epiphytic vegetation, can be measured 

 by specially devised collecting vessels. 



Duration of Rain.- — Data concerning the kind and duration of rain- 

 fall are much more difficult to obtain than records of total precipitation, 

 although such data are important for an understanding of vegetation. 

 Equal amounts of rainfall produce very different ecological effects, 

 according to whether they fall in a few hours in violent downpours or as 

 a fine continuous drizzle. 



The mountain wall on the north of the Mediterranean basin receives 

 a large i-ainfall (Valleraugue 350 m., 165 cm.; Locarno 210 m., 194 cm.; 

 Tolmezzo 242 cm.; Crkvice in the bay of Cattaro 464 cm. per j'-ear). 

 It falls for the most part in heavy showers, which sometimes overflow 

 the rain gages so that the amount can only be estimated. For example, 

 Lasalle in the southern Cevennes on Sept. 26, 1907, received 40 cm., 

 the highest maximum for one day for Central and southwestern Europe. 

 But in spite of this temporary excess of precipitation, the vegetation of 

 the southern Cevennes has many xeric features: extensive Cistus scrub, 

 and the sclerophyllous Quercus ilex macchia covers the flanks of the 

 mountains up to 800 m. above sea level. The western slope of the 

 Cevennes, on the contrary, with less annual precipitation, is covered 

 with mesic deciduous forests and rich green meadows. The summer 

 drought is hardly noticed, for the number of rainy days is twice as 

 many as in the Mediterranean valleys, and fine and continuous rains 

 prevail. 



Effects of Heavy Showers. — Heavy gusts of rain are harmful because 

 they drown out the fine soils and puddle the ground. Plant coloniza- 

 tion of denuded and slightly sloping ground is made difficult or impos- 

 sible. Seeds that are blown in, if not immediately washed away by 

 violent showers, germinate poorly on the residual soil, devoid of loam 

 and humus. And since the seedlings are subject to drowning out as 

 well as to intense heat and drought, very few can survive such con- 

 ditions in a warm temperate climate. 



Thinly vegetated erosion slopes are especially common and striking 

 in semiarid regions. In the western I"'nited States large areas of suoii 

 bare «lopes give rise to an edaphic desert known locally as "Bad Land.s'' 



