1912] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 213 



a society dominated by box-elders, also rather scattered, and with 

 considerable grape (Vitis arizonica). These seem to be the canon 

 representatives of the cedar and pinon formations respectively. 



Higher up and in the narrower, more mesophytic portions of the 

 canon there occurs a society dominated by Populus angustifolia. 

 This corresponds with the pine formation on the whole, and if the 

 canon is open or U-shaped, the yellow pine will occupy the floor 

 with the poplar along the stream. Ascending still higher, where 

 the canon becomes decidedly V-shaped, the Douglas spruce forma- 

 tion holds full sway. And as one nears the head, above the per- 

 manent stream there usually occurs an association of quaking 

 aspens, somewhat less mesophytic than the Douglas spruce for- 

 mation. In the shade of the aspens grow Rubus deliciosus, Osmor- 

 rhiza nuda, Saxifraga bronchialis, Jamesia americana. Delphinium 

 scopulorum, Actaea spicata, Pedicular is procera, Frasera speciosa, 

 and nearly always young spruces. After a fire in the Douglas 

 spruce the quaking aspen always takes possession, but it has also 

 its natural place as a transition between the oak chaparral and the 

 Douglas spruce in the biotic succession. 



The biotic succession in the Sandia Mountains is as follows: 

 the bare rock first incrusted with crustose lichens, then foliose 

 lichens, mosses, herbs, oaks, followed in some cases directly by 

 Douglas spruce and in others by aspen and then the spruce; and 

 then as physiographic succession comes in, the poplars, pines, and 

 box-elders in the canon; and pine, pinon, and cedar on the slopes, 

 until the ultimate formation of the mesa is reached. 8 



Response to climatic factors 



This complex of associations is of course due to a complex of 

 causes, of which the most important are relative humidity of the 

 air and more especially that of the soil, and not the average tem- 

 perature of the growing or any other season, as some eminent 

 authorities have maintained. Temperature, of course, is a factor, 

 but principally as it affects the humidity. I have mentioned the 

 inability to grow certain cacti because of the winter's cold. There 



8 A study of Mt. Taylor indicates that the alligator juniper has a place between 

 the yellow pine and the pinon. 



