dominant in another stand; for example, Agropyron spicatum in 

 western North Dakota, where it is near the eastern Hmits of its 

 geographic range, is associated with Muhlenbergia cuspidata, Carex 

 Jilifolia, Bouteloua gracilis, and Eurotia lanata, none of which grow 

 with it in British Columbia where it is dominant. The presence 

 or absence of certain associated species has important indicator 

 value in pointing to significant conditions such as severe compe- 

 tition, presence of disease, or prevalence of one or more unfavor- 

 able environmental factors. Association of species is also important 

 in testing new strains of forage plants, for usually they are grown 

 in mixtures in meadows and pastures. Therefore the new strains 

 need to be tested, not only in pure cultures to determine their suit- 

 ability, but also in mixtures for their associative capacity. 



A number of methods have been used to measure the degree 

 of interspecific association.^^ The association index, one measure 

 of ecologic association between plants, is secured by dividing the 

 number {a) of random samples of a given stand in which species 

 A occurs into the number [h) of samples in the same stand in 

 which species A and B occur together. For example, if species A 

 occurs in 40 sample areas {a = 40), and species B occurs to- 

 gether with A in 30 of these sample areas (A = 30), then the 

 association index of species A is B/A, or 30/40, or 0.75; i.e., on 

 three-fourths of the sample areas in which species A occurred it 

 was associated with species B. But if species B is used as the base, 

 the association index is 1.0. At least 100 samples should be taken 

 for these calculations. 



Another, probably more meaningful method uses the index of 

 similarity of Sorensen,^2io3 ^hich was developed originally on 

 the basis of species presence. If a represents the total number of 

 species in type A, b the number in type B, and there are c species 

 present in both types, then the index of similarity = 2c X 100/ 

 a + b. Instead of using the number of species, the index can be 

 calculated on the basis of the average cover estimates or abun- 

 dance. In this procedure a represents the sum of all averages of 

 estimates of species found in vegetation type A, b the similar figure 

 for type B, and c the sum of the various estimates of species shared 

 by both types. 



Axmalytic CHairacterisfticrs of the Comxnvazmity • 99 



