1914] VESTAL— PRAIRIE VEGETATION 381 



mountain 



are extremely scanty. The mean 

 18.46 inches. At Brighton (see m, 



mesa 



months 



Evapora- 



tion is on the whole rapid, less so at the foothills. The mean annual 

 temperature at Boulder is 5 1° F. Temperature variations are more 

 extreme on the mesa- terrace than on the higher mesas, the plains 

 having a still more severe climate. Temperature inversion is a 

 local factor in reducing diurnal temperature variations in the mesas. 

 The growing season is thus longer at the mountain-front; spring 

 plants appear here several weeks earlier than on the plains some 



miles 



Diverse character of the vegetation 



In the open plains, as discussed by Shantz (18), the plant 

 associations are well defined, and their relations to soil and topog- 

 raphy have been clearly distinguished. Near the mountains, how- 

 ever, associations are more numerous, are variable, mixed in 

 character, not definite. The following conditions are suggested as 

 tending to complicate the character of the vegetation: 



1. Climatic conditions undergo a transition at the meeting line of plains 

 and mountain regions. The average climate for the mesa zone, if such a term 

 can be used, is thus intermediate in character. 



2. Annual fluctuations in climate are here unusually great, favoring alter- 

 nately mountain and plains vegetation. 



3. Rainfall during April and May is adequate for mesophytic prairie-grass 

 vegetation, while the latter part of the summer is usually dry. Less xerophytic 

 foothill plants are conspicuous in the mesa areas during the vernal period; 

 later, xerophytes of plains and dry prairie-grass predominate. 



4. Variability of soil conditions (soil texture, moisture content, run-off, and 

 absorption) is considerable and is extremely local, the usual mesa soil being 

 composed of unsorted rock fragments of all sizes. Mesophytes can grow imme- 

 diately beside and from under surface bowlders. Local and extreme varia- 

 bility in character of vegetation is thus possible. Radically different plants, 

 as regards growth-form and physiological constitution, occur frequently in the 

 same growth. 



5. Topography is considerably and very locally diversified. 



6. Opportunity for invasion by mountain plants is very good. 



