igi2] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 207 



and characteristic. Other members are Rhus trilobata, Nolina 

 texana (a long-leaved liliaceous evergreen), and the spiny-leaved 

 oak, Quercus undulatus. The Rhus is also imperfectly evergreen, 

 and indeed there is less difference between the winter and the 

 summer aspect of this formation than of any other, because there 

 is less difference in the relative humidity of the soil. 6 



PINON FORMATION 



This has been combined with the last by Merriam and other 

 writers, and they do shade into each other very gradually, even 

 imperceptibly, but no more so than do the Pinus ponderosa and 

 Douglas spruce formations, which are separated by these authors. 

 Furthermore, the pinon {Pinus edulis) never extends as far down 

 the mountain side as does the cedar, the differences being on the 

 average at least 500 ft. Other plants very characteristic here are 

 Yucca baccata or "amole," mountain mahogany {Cercocarpus 

 parajolius) , Philadelphus micro phyllus, Lesguerella Engelmanni, 

 and Tragia nepetaefolia. 



YELLOW PINE ASSOCIATION 



This is the " transition zone" of Merriam, which he states is 

 on the whole more closely related to the Sonoran than to the Boreal, 

 a conclusion which seems to the writer to be incorrect at least so 

 far as plants and insects are concerned. The latter are treated in 

 another publication. 7 The characteristic plants, after the Pinus 

 ponderosa scopulorum. are Geranium atropurpureum, white oaks, 

 red cedar {Juniperus scopulorum), the pasque flower {Anemone 

 patens jS iittalliana), wild gooseberry {Ribes divaricatum irriguum), 

 Ptelea mollis, wild grape {Vitis arizonica), cudweed {Antennaria 

 plantaginifolia) , and New Jersey tea {Ceanothus Fendleri). Here 

 occurs a sharp and complete change of flora. There is much more 

 difference between this formation (fig. 4) and the mesa or even the 

 pmon formation less than a mile away, than there is between it and 

 the woods of Ohio, as witness the preceding genera, if not species. 



W hether the oaks and Rhus drop their leaves early in the winter or carry them 

 until spring is determined by the soil moisture. In less dry winters and along arroyos 

 they retain them. Under more xerophytic circumstances the leaves are dropped. 



7 Report of the N.M. Resource and Conservation Commission. December 101 1. 



