MESA AND FOOTHILL VEGETATION 3 I 



new ones can get a start. So it happens on some mesas that there are 

 old trees of one hundred or two hundred years, then others twenty or 

 thirty years old and none of intervening age. Ability to become suc- 

 cessfully established depends largely on the periodicity of climatic condi- 

 tions. Once a tree is fairly started and as old as five or six years it is 

 able to withstand a number of unfavorable seasons and is likely to con- 

 tinue to grow without interruption. 



Variation in Climatic Conditions. — By reference to the tables pre- 

 viously given it will be seen that there is some variation in temperature 

 and rainfall at Boulder from year to year. While in any series of years 

 the mean temperature and rainfall may be counted on with some certainty 

 yet individual years may show a considerable departure from the normal. 

 Boulder does not, however, show as great variability as Denver and 

 there is reason to believe that the mesas and lower foothills are still less 

 variable in climate from year to year than Boulder. Thus it comes about 

 that the mesas are more favorable for tree growth than the plains and 

 that except with plants which grow best in fine-grained soil the species 

 of adjacent plains also grow on the mesas. On the other hand, as noted 

 above, certain foothill plants although able to grow on the mesas cannot 

 extend out to the plains which in summer are drier and hotter and in 

 winter colder and probably more windy. 



3. Distribution of Conifers on the Mesas 

 By W. W. Robbins and Gideon S. Dodds 



Rock Pine. Pinus scopulorum (Engelm.) Lemmon. — Rock Pine 

 is the dominant conifer of the foothill region. It extends, in fewer num- 

 bers, however, into the montane region, occasionally reaching an altitude 

 of 10,000 feet. Its lower limit of altitudinal distribution is found on 

 the mesas where it meets the grassland formation of the plains zone. 

 A careful study has been made of the distribution of the Rock Pines on 

 Long and Horse Mesas and observations have been made on other 

 mesas lying to the south. In every case its general distribution is the 

 same. 



There are three well-marked and noticeably different sizes of trees, 

 other than the seedlings, growing on these mesas. Trees intermediate 



