Figure 1-15. Association of smooth bromegrass and alfalfa 

 apparently benefits both. Density, cover, and vigor are ex- 

 cellent here. Nebraska. (U.S.D.A. Soil Conservation Service.) 



in cool, wet w^eather and for legumes to dominate when warm, dry 

 conditions occur. 



DiflFerences of Species in Capacity of Association 



This is often called interspecific association, and refers to 

 species, mostly in different genera, living together in an area and 

 drawing upon a common pool of resources. Similarity in require- 

 ments and ecological amplitude is often essential, especially in 

 habitats characterized by limiting factors such as soil salinity or 

 the shortness of the growing season. In Spitzbergen some species 

 such as Saxifraga oppositifolia, Cerastium arcticum, Salix polaris, Draba 

 subcapitata, Papaver dahlianum, Phippsia algida, and Poa alpina vivi- 

 para meet one of the first requirements for association because they 



can endure the short growing season at 600 to 800 m above sea- 

 level, i '4 



Species of different life-form are often associated because they 



34 * Species and Popimlatioims 



