Vol. IX] HALL & GRIN NELL— LIFE-ZONE INDICATORS 39 



case that the distribution of a taxonomic group can be ac- 

 curately stated in terms of latitude and altitude only by 

 making the range so inclusive as to render the statement of 

 but general interest. 



It is just this failure to recognize the importance of local 

 disturbing conditions that has led some investigators to under- 

 value both the scientific basis and the utility of the life-zone 

 concept. The more important local influences which interfere 

 with the normal, orderly succession of life-zones may be 

 specified and briefly discussed as follows : 



1. Slope exposure. Next to altitude and latitude this is 

 apparently the most important determinant of temperature 

 and consequently of zone positions in our western states. As 

 may be determined mathematically, the amount of heat 

 received by a slope with the most favorable gradient is 1.4 

 times as great as that received by an equal area of level land, 

 other conditions being constant (Hall, 1902, p. 34). It is 

 evident, from such considerations, that any life-zone will 

 occur at higher altitudes on southerly facing slopes (in the 

 Northern Hemisphere) than it will on level areas, and that 

 it will be correspondingly depressed on northerly facing- 

 slopes. 



2. Air currents. It is well known that cold air flows 

 down the caiions in mountainous districts, especially at night. 

 This results in a depression of life-zones, particularly in 

 narrow valleys and canons. On the other hand, proximity 

 of a desert area or other source of wann air often greatly 

 elevates the position of life-zones on mountain slopes, be- 

 cause of the warm ascending currents. (See Merriam, 1899, 

 p. 51; Shreve, 1914, pp. 197-202.) 



3. Streams carrying cold zvater. The effect upon the 

 growth conditions of plants whose roots are in moist soil 

 near these streams is considerable, as indicated by the per- 

 sistent manner in which species of the higher zones fringe 

 the water courses at lower levels. As an alpine stream 

 descends to the plains, its rise in temperature is much less 

 than is generally supposed. This is indicated by thermo- 

 metric observations (not yet published) made in the moun- 

 tains of Colorado by Dr. Gideon S. Dodds. 



