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 cafions. An apparently valid explanation of the 
high occurrences on ridges is in accordance with 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 
cafions, 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 cafion 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. 
