232 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



THE PETRAN ALPINE MEADOW. 

 CAREX-POA ASSOCIATION. 



Extent.— The alpine meadow of the Rocky Mountains reaches its typical 

 development between 12,000 and 14,500 feet, though it descends to lower 

 altitudes in Montana and Alberta. In Colorado it is found in more or less 

 characteristic form in lake basins at 11,000 feet, but this is apparently due to 

 cold-air drainage and the influence of water. A number of dominants and sub- 

 dominants may be found still lower, but these are chiefly subalpine in nature, 

 or occur merely as fragmentary outposts. The northern limit of the associ- 

 ation is thought to be in southern Alberta, since the alpine plants of Mount 

 Robson (Standley, 1913: 77) are largely those of the Pacific association. The 

 southeastern outposts are in the Sangre de Cristo Range of northern New 

 Mexico, and the southwestern on the San Francisco peaks of northern Arizona. 

 The general western limit is thence northward along the Wasatch Mountains 

 of Utah into eastern Idaho and southwestern Montana. The association 

 occurs in reduced form in some of the ranges of Nevada. The central portion 

 is most typical and extensive. It occupies Colorado and Wyoming and in- 

 cludes the Uinta Mountains of Utah and the San Juan and Sangre de Cristo 

 ranges of New Mexico (plate 56). 



DOMINANTS. 



The Petran association exhibits 62 dominants which play a r6le of more or 

 less importance in the climax. Of these, 30 are sedges, 23 are grasses, and 

 8 are rushes. The majority of them occur also in the Pacific association, but 

 18 or nearly a third are lacking there. For the sake of brevity, only the most 

 typically alpine or the most abundant dominants are included in the following 

 list. In Carex and Poa the order is that of relative importance on the 

 alpine peaks of Colorado. 



Carex rupestris. Carex engelmanni. Poa rupicola. 



Carex filifolia. Carex nardina. Poa pattersoni. 



Carex pyrenaica. Carex illota. Poa grayana. _ 



Carex nigricans. Carex concolor. Poa lettermani. 



Carex festiva. Elyna bellardi. Trisetum subspicatum. 



Carex atrata. Poa alpina. Festuca brachyphylla. 



Carex nova. Poa arctica. Deschampsia caespitosa. 



Carex capillaris. Poa alpicola. Juncodes spicatum. 



Carex tolmiei. Poa epilis. Juncus triglumis. 



Carex alpina. Poa crocata. Juncus castaneus. 

 Carex petasata. 



Groupings. — As a result of the large number of dominants and the con- 

 sequent equivalence, the number of groupings is exceptional. Pure con- 

 sociations are extremely rare, except in areas of a few square meters, and 

 mixed communities are universal. The number of dominants in each mix- 

 ture is large, and the groupings consequently merge into a more or less indefi- 

 nite pattern. Carex rupestris and C. filifolia are the most important domi- 

 nants on a score of Colorado peaks, as well as on the San Francisco Mountains 

 of Arizona, though apparently absent on those of New Mexico. Poa, Elyna, 

 Trisetum, and Juncodes are commonly associated with them. Carex pyren- 

 aica and C. nigricans are found on a number of alpine summits in Colorado, 

 but they are much less important. They nowhere seem to have the dominance 

 characteristic of them in the Pacific association. 



