l8 UNIVERSITY OF COLORADO STUDIES 



the past year would not account for the present distribution of vegetation 

 on the mesas. Soil samples have been taken at frequent intervals from 

 different exposures and plant habitats and moisture content determined. 

 From these data a few general facts have been ascertained, but no 

 statement can be made of detailed differences because of the abnormality 

 of the seasons. 



Seasonal Distribution of Soil Moisture. — Observations indicate the 

 following general seasonal distribution of soil moisture. At the beginning 

 of the growing season, say about March i, the moisture content is low 

 in almost all of the stations. From this time on there is a gradual increase 

 until about the last of May, when it is at a maximum. During the rest 

 of the growing season there is a decrease. The time of the highest 

 moisture content will of course vary in different years. Also there is 

 no doubt that in a season of normal precipitation the maximum will 

 occur at quite different times on different exposures. For example, it 

 will occur later in seepage areas than on dry ridges. This may be impor- 

 tant in its bearing on plant distribution. 



Local Distribution of Soil Moisture. — The geographical distribution 

 of soil moisture on the mesas has been found to bear a very important 

 relation to plant distribution. The soil of the top of the mesa, as has 

 been noted, is of coarse texture and covers the underlying shales to a 

 depth of several feet. It takes up water from rains and melting snow 

 very readily and there is little run-off from this part of the mesa. The 

 water percolates rapidly so that after a rain the surface layer rapidly 

 becomes dry again while the deeper layers remain wet for a longer 

 time. 



On the slopes the run-off is great. The soil is dry except in the 

 small incipient ravines which are more moist ; the deeper the ravine the 

 greater the moisture content of its soil. In general the south slope is 

 dryer than the north because of the more rapid evaporation. 



On the slope of the mesa one feature of the distribution of moisture 

 is especially noticeable and important in relation to plant distribution. 

 A little distance from the crest of the mesa, just below the zone of large 

 boulders, there is a very well-marked zone much more moist than the 

 rest of the slope. This is especially marked in the amphitheater-like 



