728 Forestry Quarterly. 



does the juniper, the differences being on the average at least 500 

 feet. 



The yellow pine association descends in some places to 7,000 

 feet, and extends upwards to 10,000 feet, and its distribution coin- 

 cides very closely with that of the deep winter snow. In the 

 drier situations the pine arises scatteringly from a chaparral of 

 dwarf oaks and dwarf locust. The associated herbaceous vege- 

 tation on the east slope with its greater precipitation is strikingly 

 similar in genus and often in species to that of the hard pine 

 forests of the eastern States. The association represents a ten- 

 sion line between the flora of the arid southwest, as for example, 

 on the adjacent mesas, and that of the more humid north and 

 east. The higher points of the mountains are covered with Abies 

 concolor and in some places with Englemann's spruce. 



In ascending the canyons, the first trees met are the hackberry 

 (Celtis reticulata). Next comes a society dominated by box- 

 elder. Higher up in the narrow and more mesophytic portions 

 of the canyon one finds a society dominated by Populus angusti- 

 folia. If the valley is wide and open, yellow pine may occupy 

 the floor with poplar along the stream. If narrow or higher up 

 where the valley is V-shaped, the Douglas fir association controls. 

 At the head of the canyon, above the permanent stream, there 

 usually occurs an association of quaking aspen. 



Among the conclusions of the author the following may be 

 noted: The chief factor determining the abrupt changes of vege- 

 tation is moisture, the supply of which is largely determined by 

 precipitation, the ability of the soil to hold it and the protection 

 from drying winds and sun. This conclusion is based upon the 

 fact that the same plants occur throughout a great range of alti- 

 tude and temperature, but in a soil of about the same relative 

 degree of humidity. Spruces and piiions (separated into distinct 

 temperature zones by Merriam) will grow with their branches 

 almost touching if the roots of the former have access to an un- 

 failing water supply. 



The tendency of the higher zones to creep down canyons and 

 the lower zones to creep up the ridges receives a much more 

 plausible explanation in connection with the supply of moisture 



