FACTOR INDICATORS. 89 



water-content. Perhaps its most significant effect lies in emphasizing the 

 effects of exposure toward or away from the sun. Together the two increase 

 temperature and evaporation, and decrease humidity and water-content on 

 all southerly exposures, while they have just the opposite effect on northerly 

 ones. In arid regions, the effects upon plants are often most pronounced. 

 Succession moves much more rapidly and the climax is reached much sooner 

 on the north side, with the result that the communities often differ greatly 

 on the north and south slopes of the same hill. Growth usually begins earlier 

 on south slopes, but the plants are taller and denser on north ones. The 

 indicator differences deal with the presence or absence of various species and 

 the corresponding communities, and with the growth and abundance of the 

 individuals. Such indications are related primarily to water-content and 

 evaporation, though temperature plays a direct rdle of some consequence 

 (plate 16, b). 



Alternation in vegetation is largely a, matter of slope-exposure (Clements, 

 1904 : 165; 1905 : 285; 1907 : 289). Much attention has been given to the 

 alternation of dominants and subdominants on different slopes in the rolling 

 prairies of Nebraska and the mountains of Colorado. Shantz (1906 : 25) 

 has shown the variation in temperature and light intensity during the day for 

 different slopes in the short-grass association at Colorado Springs. Weaver 

 (1917 : 43; 1919) has made a detailed study of the evaporation, water-content, 

 and temperatures of northeast and southwest slopes in the Palouse region of 

 Washington and adjacent Idaho. All the factors agree in showing that the 

 southerly slopes are much more xerophytic, and readily explain the absence 

 of a large number of species, or their greater abundance on the northerly slopes. 

 Spalding (1909 : 43) studied the occurrence of species on two opposite slopes 

 in the desert scrub at Tucson. He found that they had 15 perennial species 

 in common, while the northeast slope had 24 not found on the southwest, 

 and the latter 9 not present on the other. Shreve (1915 : 97, 61) has given a 

 detailed account of the differences in the vegetation of the Santa Catalina 

 Mountains due to slope-exposure, and in the factors concerned. 



Altitude indicators. — Altitude is not so much an edaphic factor-complex as 

 the expression of a specialized climate, of which elevation above the sea-level 

 is the remote cause. This expression occurs in some degree at all altitudes, 

 but its accumulation becomes most striking at the higher ones and especially 

 above timber-line. Because of the close relation between altitude and latitude, 

 the actual level of a particular effect, such as timber-line, varies from sea-level 

 at the northern tree-limit to 12,000 feet or more in the southern Rocky 

 Mountains. As is well known, the direct effect of increased elevation is seen 

 in reduced pressure and a correspondingly rarefied atmosphere, which is the 

 primary cause of most of the changes. The factor most affected is tempera- 

 ture, the rays passing readily through the rarer air during the day, while for 

 the same reason radiation is very rapid at night. As a consequence, the soil 

 and the air immediately above it may become very warm on a sunny day and 

 then drop to freezing at night. On Pike's Peak the surface of the soil may 

 show a temperature of 140° F., while in the air 5 feet above, the temperature 

 is but 70° F. Probably still more important is the shortness of the growing 

 season. The frostless season is nearly 5 months long at Colorado Springs 

 (6,000 feet), while on the top of Pike's Peak (14,100 feet) frost occurs fre- 



