Figure 2-1. Cladonia sylvatica and C. alpestris form most 

 of the lichen layer in openings between the glandular birch 

 {Betula glandulosa), while C. rangiferina is most abundant 

 under the shrubs. Very good winter caribou range east of 

 Cantwell, Alaska, August, 1957. 



of mosses in the bottom of the depressions^ (ggg Chart 1). Impor- 

 tant associates in the first community are Vaccinium vitis-idaea, 

 Cornicularia divergens, and Cetraria cucullata. Species with wide eco- 

 logical amplitude such as Cetraria islandica and Dicranumfuscescens 

 were found in several communities but may be dominant in only 

 one, or at times in none. Competition also plays an important part 

 in the aggregation of the species; for example, where Cladonia 

 alpestris forms dense mats there is little opportunity for the growth 

 of less competitive lichens, mosses, and even low shrubs and herbs. 

 When the snow cover lasts late into the summer, Vaccinium myrtil- 

 lus cannot grow and the reduced competition improves conditions 

 for other species. In Alaska Cladonia alpestris suppresses even low 

 shrubs, including Empetrum nigrum and Vaccinium vitis-idaea, but it 



Groimpismg of Species • 4^3 



