THE PHYSICAL BASIS. 41 



meadows on the other. While the serai relations were very different, the 

 relation to water was much the same. On the broken or sandy ridges the 

 soil was porous and the competition relatively small, due largely to the bunch 

 habit, while in the moist meadows the grasses grew in a sod, the competition 

 for water was keen, and the amount for each plant correspondingly limited. 

 A similar inversion in hilly and mountainous regions has since been found for 

 the majority of grass dominants, as well as for an increasing mmiber of shrubs. 

 The breaking-down of the Miocene rim of the Bad Lands of Nebraska and 

 South Dakota yields a talus in which Rhus, Ribes, Symphoricarpus, Rosa, and 

 other shrubs occur, all of which form dense thickets in the valley several hun- 

 dred feet below. Chrysothamnus, Artemisia, and Atriplex grow far up the 

 walls and buttes of bad lands, and are found again as dominants in the 

 ravines and draws. In the Southwest the desert scrub consists of two major 

 dominants, Prosopis and Larrea. While they are often mixed in the 

 vast stretch over which they occur, Prosopis is typical of the valley and 

 washes. The valley plains and bajadas are characterized by a zone of Larrea, 

 above which lie Aristida-Bouteloua grasslands wherever broad sloping plains 

 occur. In these Prosopis again occurs as a consequence of increasing rainfall, 

 at an elevation of 1,000 to 2,000 feet above its position in the desert (plate 6). 

 Similar inversions occur in mountain regions, either as a consequence of 

 air-drainage or of exposure, or often indeed of both. In the case of exposure, 

 the general relations are obvious, though the relative importance of water 

 and temperature is usually uncertain. It seems probable that both are 

 directly concerned, and that water plays the primary r61e, except in mountain 

 regions characterized by a very short growing season .and minimum night 

 temperatures (cf. Shantz, 1906:25; Shreve, 1915:64; Weaver, 1917:44). The 

 effect of temperature inversions was pointed out by Kemer (1876 : 1) and 

 Beck (1886 : 3) in Europe and has been studied by MacDougal (1900) and 

 Shreve (1912 : 110; 1914 : 197; 1915 : 82). The latter's conclusions are as 

 follows (1914 : 115): 



"The influence of cold-air drainage might be expected to affect both the 

 upward limitation of lowland species and the downward occurrence of mon- 

 tane species. As a matter of fact the downward limitation of the forest and 

 chaparral vegetation of the desert mountain ranges is due to the operation of 

 the factors of soil and atmospheric aridity, and not to the chimenal factors. 

 The limitation of the upward distribution of desert species appears to be 

 attributable to chimenal factors, as the writer has shown for Carnegiea 

 gigantea. The writer has observed that a number of the most conspicuous 

 desert species range to much higher altitudes on ridges and the higher slopes of 

 canyons than they do in the bottoms and lower slopes of canyons. Samples 

 indicate that there is no essential difference between the soil moisture of ridges 

 and the bottoms of canyons during the driest portions of the year. Neither is 

 there any evidence that desert species would fail to survive in the canyon bot- 

 toms if they were somewhat higher in soil-moisture content. An explanation 

 of the absence of desert species from canyon bottoms and their occurrence at 

 higher elevations on ridges must be sought in some operation of the chimenal 

 factors rather than in the factors of soil and atmospheric moisture. An 

 analysis of the operation of the chimenal factors will be sure to discover that 

 cold-air drainage plays an important r61e in determining not only the lowness 

 of the mininaum, but also the still more important features of the duration of 

 low temperature conditions." 



