city when it is growing within its optimum range of ecological 

 amplitude (Figure 1-13). For example, in alpine regions in Alaska 

 and Norway the lichen Cladonia alpestris is dominant and sup- 

 presses other lichens, mosses, and even low shrubs when it is 

 growing under optimum conditions; but under less favorable ones 

 it competes poorly. In Alaska Vaccinium uliginosum has a moder- 

 ately wide range of ecological amplitude, but apparently it is 

 dominant only within its optimum range. Species with wide 

 ecological amplitude, such as Vaccinium vitis-idaea in Alaska and 

 Norway, and Agropyron smithii and Bouteloua curtipendula in the 

 Great Plains and adjacent foothills of the Rocky Mountains, 

 occur in many communities, but as dominants they occur only 

 in sites where their competitive capacities can be well developed. 

 Root systems of plants play a very important part in competi- 

 tion. When the root systems of diflFerent species are in the same 

 horizons, competition is often severe for limited supplies of mois- 

 ture, nitrates, and other resources. Bromus tectorum, a winter annual 

 in many parts of the western United States, by its early spring 



Figure 1-13. A stand of Ladino clover (Trifolium repens 

 var. Ladino), showing maximum competitive capacity, is 

 growing here on slightly wet, silty clay soil, well within its 

 range of optimum ecological amplitude. A plentiful supply 

 of readily exchangeable potassium in the soil is needed to 

 enable legumes to compete successfully with grasses. Arkan- 

 sas. (U.S.D.A. Soil Conservation Service.) 



■Mt- 



^d 



