iqi2] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 215 



but on the summit brown and frost-killed. Thus it is seen that the 

 growing season is at least a month shorter on the summit, but the 

 same oaks grow in both situations. 



Another illustration of the influences of moisture is seen along 

 the Jemez River. This fair-sized stream comes roaring down off 

 the Jemez Plateau through a rather shallow canon which faces the 

 south. This (altitude 6000-7000 ft.) is occupied by the Douglas 

 spruce formation, but the slopes on each side are occupied by 

 pinon, the yellow pine being largely omitted. The branches of 

 the spruce and pinon are in places at the same level and subject 

 to the same hot sun and consequently the same temperature, but 

 the roots of the Douglas spruces have access to the unfailing water 

 supply of the stream. 



This tendency of the " Canadian zone" to creep down the canons 

 and of the Sonoran to ascend the ridges is noted by Merriam. but 

 is explained as due to warm and cool winds, vertical exposure to 

 the sun, etc. It receives a much simpler explanation in that the 

 ridges are more xerophytic than the canons. At night, when a 

 plant is supposed to make a goodly share of its growth, these ridges 

 are little if any warmer than the adjacent canons. They are sub- 

 ject to greater and more sudden changes of temperature, to drying 

 winds, and are less able to hold their moisture — they are more 

 xerophytic. 9 



In most situations in the Sandia Mountains the oaks of the 

 transition zone entirely surround the colonies of the Douglas spruce 

 (Canadian zone). In watching the Sandia Mountains during four 

 winters I have been struck by the very close correlation between 

 the lower limit of the average winter heavy snow and that of the 

 lower limit of the yellow pine. I believe that it is chiefly these 

 snows that determine the distribution of this tree. Far be it from 



maintain 



this region. 



am sim 



not on one alone. 



important. Any scheme for mapping "life 

 d on all the factors determining the same and 



Valley 



and northern genera and even species (as Aster, Solidago, Cassia, etc.) which are 

 absent from the higher but drier mesa. 



