3° 



UNIVERSITY OF COLORADO STUDIES 



too much. At the city of Longmont which is on the plains about fifteen 

 miles from Boulder the temperature on cold winter nights is sometimes ten 

 or twenty degrees lower than at Boulder. On the other hand, in warm 

 weather of winter or spring the plains have a higher temperature than 

 the mesas, and plants may begin their spring growth so early that they 

 are injured by late frosts. Denver, situated about twenty miles east of 



the foothills, has a climate 

 much more severe than that 

 of Boulder. The winters 

 are colder, the summers 

 warmer and the annuel 

 precipitation less. 



Floras of Mesa and 

 Plain. — Differences in both 

 soil and climate work to- 

 gether to give different floras 

 to mesa and plain. The 

 coniferous trees, so conspic- 

 uous a feature of the foot- 

 hills, become sparse on the 

 mesas and are entirely ab- 

 sent from the plains. Other 

 plants as well — shrubs and 

 herbs — make here their last 

 stand in a fight against 

 unfavorable conditions. 

 From the study of the climate as well as from observation of the flora 

 it appears that plains plants have no difficulty in living on the mesas 

 but that foothill plants coming down to the mesas do not readily pass 

 beyond to the plains. 



Good and Bad Years for Seedlings. — During some seasons and even 

 for a series of years the climate may be especially mild and moist. This 

 results in the establishment of trees on the mesas far out beyond their 

 ordinary range. A series of dry and cold winters may follow when only 

 the strongest of the recently established seedlings are able to live. No 



Fig. 4. — Maximum and Minimum Tempera- 

 tures of Campus and Mesa for the Week May 

 17 to May 23, 1908. — During this week the tem- 

 perature was lower every night on the campus than 

 on the mesa. On May 19 there was a difference of 

 ten degrees. On May 21 there was frost on the 

 campus (twenty-eight degrees F.) while the latest 

 frost on the mesa came May 5. 



