GENERAL CONCLUSIONS. 109 



the highest elevations to which they attain. The streams of cold air 

 are not more than 75 to 100 feet deep, however, and can not, therefore 

 be functional in preventing the occurrence of plants on the middle 

 and upper slopes of canons. An apparently valid explanation of the 

 high occurrences on ridges is in accordance mth the theory already 

 mentioned, that the upper limits of the Desert species, and possibly 

 of the Encinal species also, are set by winter temperature conditions. 

 The ridges are obviously the localities which receive the fullest and 

 longest insolation on the short winter days with low sun. This cir- 

 cumstance would not only warm the plants themselves but would 

 warm the soil and rocks in a manner such as to lessen the severity of 

 the coldest nights. With the pronounced low temperatures in the 

 canons, due to cold-air drainage, and with the favorable conditions 

 of the ridges for a pre-warming of both plant and habitat, it may be 

 expected that there will be great differences between the vertical 

 limits of species in canon bottoms and on ridges. 



GENERAL CONCLUSIONS. 



The desert mountain ranges of the southwestern United States stand 

 in the midst of a region which presents severe conditions for plants. 

 The relative richness of the vegetation in this region is due chiefly to 

 the occurrence of two yearly seasons of rainfall. The entire annual 

 vegetational behavior is related primarily to the moisture seasons and 

 much less pronouncedly to the thermal seasons. The perennial plants 

 lead an existence which permits of rapid growth during the warm 

 humid season, together with an extremely low ebb of activity during 

 the arid seasons, and with the possible loss through drought-death of 

 much of the growth that has just taken place. 



The severe conditions of the desert environment cause the vegetation 

 to exhibit a high degree of sensitiveness to slight topographic and 

 edaphic differences. Wherever the character of the soil or the topo- 

 graphic location is such as to present a degree of soil moisture slightly 

 above that of the general surroundings, or as to maintain it for a longer 

 time in the periods of extreme aridity; or in whatever locations plants 

 are protected from the most extreme conditions of transpiration — in 

 such places are to be found heavier stands of vegetation or else particular 

 species of plants. 



The higher mountains of the desert region exhibit strong gradients 

 of change in climate and in vegetation. Both of these gradients are 

 much more pronounced than those of mountains of equal elevation 

 in more humid regions. They lead from arid to humid, or at least 

 semi-humid, conditions of moisture, and from sub-tropical to tem- 

 perate conditions of temperature; from low, open microphyllous and 

 succulent desert, through a sclerophyllous semi-forest to heavy conif- 

 erous forest. 



