Figure 5-9. A climax stand 

 of Cladonia alpestris, with 

 intermixed dwarf shrubs 

 and herbs. Excellent winter 

 range for caribou or rein- 

 deer, widespread in Sub- 

 arctic in North America and 

 Scandinavia. July, 1957, 

 near Paxson, Alaska. 



first, is found in depressions in rough terrain and in more 

 sheltered sites, so that it has more snow cover and is less exposed 

 to the wind. The third, the Betula nana- Cladonia alpestris sociation 

 (Figure 5-9), is characterized particularly by the vigor and height 

 of C. alpestris. It occurs in still better-protected sites, the snow 

 cover is deeper and lasts longer, and it is less exposed to the wind. 

 The site is weakly mesic in contrast to the previous xeric ones, 

 and there are indications in the soil profile of weak solifluction. 

 The fourth community, the Betula nana- Cladonia rangiferina-C. 

 silvatica sociation — characterized by these two lichens, more 

 moss, and Salix glauca—iol^vdii&s still deeper and longer-enduring 

 snow cover, the substratum is more moist, and the soil profile 

 shows the effects of solifluction. This community often occurs on 

 solifluction terraces. The gradients in environmental factors from 

 the first to the fourth community are: increasing depth and 

 duration of snow cover associated with less exposure to wind, 

 probably a later inception of growth in the spring, increasing 

 soil moisture, and increasing likelihood of solifluction. 



The changes which are expected within the steady-state are re- 

 placement change and fluctuation change. The upgrade and 

 downgrade phases of a cyclic replacement change could be 

 prominent, and be misleading in determining that the steady- 

 state exists. It has already been noted that fluctuation changes 

 are usually not perceptible by simple observation, especially over 

 a short period of time. 



162 • Dynamics of Comnnmunities 



