32 UNIVERSITY OF COLORADO STUDIES 



between these three sizes are few and scattered. The oldest and largest 

 trees, one hundred and fifty to two hundred years old, are distributed 

 on the higher parts of the mesa top; they are relatively few in number 

 and quite far apart. Those of the next size are eighty to one hundred 

 years old. They are more numerous than the preceding. They occur 

 among the oldest pines and are found growing farther out toward the 

 mesa end, especially along the north crest, and farther down on the north 

 slopes. The smallest pines are between twenty and thirty years of age. 

 There are large stands of these trees in many places. They grow still 

 farther out on the mesa than the preceding groups but they do not extend 

 entirely to the end. The outermost parts of the mesas have sparse 

 growth of small trees of various sizes. On other mesas also the pines 

 fall into well-defined groups, with few trees intermediate in size. Plate 

 II shows two well-marked sizes of pines on a mesa some miles south of 

 the mesas under study. In the mountains higher up, nearer the center 

 of distribution for the Rock Pine, we do not find such well-marked 

 and clear-cut groups of trees but there are trees of all sizes and ages 

 growing together. These clear-cut groups occur only on the mesas 

 where the pine reaches its lower limit of distribution and these groups 

 indicate that the trees there are meeting severer conditions than those 

 farther up; in other words, they are on a tension line and favorable 

 conditions are not met with every growing season. 



The distribution of Rock Pine depends not upon the conditions 

 affecting the large, well-established trees, but upon the conditions which 

 the germinating seeds and seedlings encounter. Good cone-producing 

 years are not common. Furthermore, the seed by no means always 

 falls in a place suitable for germination and even if the seed germinates 

 the seedling requires a series of seasons favorable for its growth, or 

 else it will fail to develop to a size that will make it independent of 

 slight changes in the environmental conditions. Seedlings are able to 

 grow better when shaded; however, shade is not an absolute require- 

 ment. Thus at long intervals will there be a year in which all of the 

 above conditions will come together. A large stand of pines of the 

 same size and age evidently means that there was a good cone year, fol- 

 lowed by a series of years favorable to the germination and growth of 



