256 MINNESOTA BOTANICAL STUDIES 



Shantz has shown in his Colorado studies (60) that the physical 

 conditions in the "wire-grass association" are such as to "permit the 

 growth of both shallow-rooted and deep-rooted plants. The 

 shallow-rooted species are chiefly short-grasses which obtain most 

 of their water supply from the first foot of soil." Being sandier 

 than the soil within the grama-buffalo-grass asociation, water pene- 

 tration is greater and run-off is doubtless less so that a greater quan- 

 tity of available soil-moisture is to be had at considerably deeper 

 levels within this association. This fact probably accounts for the 

 presence of many of the deep-rooted perennials of this association 

 which are lacking in the grama-buffalo-grass association. Wire- 

 grass itself is a much deeper-rooted plant than the typical short- 

 grasses. Shantz has further pointed out that "the presence of 

 short-grass here indicates a condition unfavorable to the greatest 

 development of the taller, deep-rooted plants. If the water supply 

 were increased the latter would undoubtedly entirely replace the 

 short-grass and the vegetation would probably pass over into the 

 bunch-grass association. The amount of water available below the 

 first foot of soil determines the number of deep-rooted plants which 

 can develop. Since in the wire-grass association this amount is not 

 sufficient to produce a dense growth of such plants, inter-spaces are 

 left which are occupied by the shallow-rooted short-grasses. The 

 presence of the short-grasses further reduces the amount of mois- 

 ture available to the deeper-rooted plants but can not crowd them 

 out entirely. ... A condition of unstable equilibrium there- 

 fore exists, being more favorable for deep-rooted plants during wet 

 years and more favorable for the short-grasses during dry years." 

 It appears from a preliminary study of this association in our state 

 that the above prophecy is probably correct. Here where the precip- 

 itation is considerably higher (toward the east about 10 inches 

 greater) the number and frequency of the deeper-rooted species are 

 apparently greater than in eastern Colorado. As to whether the 

 final stage will be short-grass or some prairie-grass association can 

 not yet be said, although it appears that one of the better-sodded 

 associations of the prairie would be the more likely successor to the 

 wire-grass transition association. 



The Short-Grass Formation 



The short-grass formation is poorly represented in the sand- 

 hill region of Nebraska because of the relative absence of the soil 



