152 ARTHUR G. VESTAL. 



It should be pointed out that conditions in the ground stratum 

 of the aspen association are the same in the open treeless parts 

 as among scattered trees. Exposure to sun and wind, as well as 

 the vegetation, are almost identical in the two places. The 

 open aspen forest is then, so far as its ground stratum is concerned, 

 a grassland association. This is in accord with the results of 

 Shelf ord ('i2a: 82), who found that ground stratum conditions 

 in open forests in sand at the lower end of Lake Michigan lag 

 behind in the succession from herbaceous to forest growth. In 

 reality open forests of very scattered trees are usually mixed 

 associations, dominated in places by trees, and in more open 

 places by herbaceous plants. So far as ground conditions are 

 concerned, the association is herbaceous, and the bracken-blue- 

 berry growth is accordingly grouped with dry grassland. Ground 

 conditions are not those of forest until the closed stage of the 

 aspen association is reached. The lichen growth of open aspen 

 areas, affording a locally different environment, may be regarded 

 as a minor division of that association. 



A primary division in the above synopsis, "shrub associations,'' 

 might have been added. The thicket and bramble associations 

 were small in area, were little studied, and as the conditions in 

 the ground stratum in thickets are much the same as those in the 

 hardwood forests, with about the same grasshopper assemblage, 

 this division is omitted for simplification. 



The grasshopper assemblages of these associations are pre- 

 sented in summarized form in the third section of this paper, 

 which here follows. 



III. SUMMARY OF LOCAL DISTRIBUTION OF THE SPECIES. 



The table which follows summarizes the habitat relations of 

 the grasshopper species, and includes an approximate estimate 

 of the numerical status of each species in each of its habitats. 

 The method of numerical estimation is described on p. 165. It 

 is necessary at this point to define the terms used to denote 

 degrees of frequency, or regularity of occurrence, and abundance, 

 or numbers of individuals per unit of area. Dominant species 

 in a habitat, usually only one or two, have a very high abundance, 

 and a high frequency, in that habitat. Abundant species are alsa 



