DRY GRASSLAND OF A MOUNTAIN PARK 263 



Aragallus society, which may be taken as an example of average 

 xerophytism and that of the M uhlenhergia-Danthonia which 

 is rather hypoxerophytic, determinations were made with 

 porous-cup atmometers. It was found that during the period 

 in which the atmometers were in operation the evaporation in the 

 Muhlenberg ia-Danthonia association was 69 per cent of that in 

 the other society. The one cup was located on an east-facing 

 slope and therefore slightly protected from west winds while 

 that in the more xerophytic community was on level ground. 



Soil moisture and evaporation are often of greater consequence 

 in determining floras than is temperature.^-^ But temperature 

 is here very important, for with such cold soil the number of 

 species capable of growing and reproducing is relatively small. 

 It is hoped to extend the study of these various factors in the 

 future. 



Some of the plants of the dry grassland would scarcely be 

 abundant save for the activities of ground squirrels, Callosper- 

 mophilus lateralis. These animals are present in great numbers 

 in many parts of the park and form mounds of loose earth where 

 seedlings are easily started. The soil which they bring up from 

 their burrows while coarse-grained does not have the larger 

 pebbles and boulders usually present on the surface. Short- 

 li\'ed plants such as Polygonum englemannii, Chenopodium oblong- 

 ifolium, Gayophytum ramosissimum, and Orthocarpiis luteus are 

 likely to find place in this soil. But the plants that most 

 often become established among ground-squirrel diggings are 

 Artemisia frigida, A. Jorwoodii and A. aromatica. They not only 

 obtain a foothold in new soil as suggested, but if present under 

 the freshly spread gi'ound they may grow up through it while 

 other plants with them are not so able to do this and hence are 

 killed. Any fairly luxuriant growth of these species of Artemisia 

 is usually in soil stirred up by ground squirrels. The shrubby 

 cinquefoil, Dasiphora fruticosa, when found in dry grassland 

 probably owes its establishment in the area to opportunity for 

 growth of seedlings on squirrel mounds. 



'* Briggs, L. J., and Shantz, H. L., 1912, l.c 



