andra, Trisetum spicatum, Silene acaulis, Pedicularis hirsuta, and Draba 

 oblongata which grow in one or a few kinds of habitats seem to have 

 narrow ranges. ^'^ Competition may, however, play some role in 

 the distribution of these species. In central Russia Menyanthes tri- 

 foliata occurs as a dominant in a number of marsh and bog com- 

 munities in part because of its wide range of ecological amplitude 

 with respect to water, concentration of mineral salts, and light 

 intensity, as shown in Table 1-1. Pinus silvestris can grow where 

 the water-table is 15 to 20 cm below the soil surface, but it is not 

 dominant unless the water-table is about twice that deep.'^^ 

 Zonation of plants is often caused by differences in ecological 

 amphtude of species. The causative factor may be the water con- 

 tent of the substratum, as on the borders of lakes; salt content of 

 the soil, as in saline depressions; or length of the growing season, 

 as on mountain peaks (Figure 1-4). The duration of snow cover 

 in arctic and alpine regions affects the length of the growing sea- 

 son and the amount of soil moisture. For example, in the bottom 

 of a small alpine valley in Rondane, Norway, where the snow 

 melts late, the ground is covered with meadow-like vegetation, 

 with Deschampsia Jiexuosa and Carex bigelowii as herbaceous domi- 



Figure 1-4. Variation in environmental conditions, with in- 

 creasing altitude and differences between species in require- 

 ments and ecological amplitude, are causes of zonation. The 

 zones shown here are (1) foreground, 8000 to about 9000 ft: 

 low shrubs (Gutierrezia, Chrysothamnus) and blue grama- 

 grass (Bouteloua gracilis); (2) oak brush to about 9500 ft; 

 (3) spruce-fir to about 11,500 ft, with much aspen in old 

 burns; (4) alpine to about 12,500 ft. West-facing slopes of 

 the Sangre de Cristo Range, Colorado. 



i 





^ 







^'^^Sii^'"^'*^'?^ 





-^'•^'W. 



.^ .>_ 



