36 UNIVERSITY OF COLORADO STUDIES 



the eastward spread of the pine. It is very probable that the seedlings 

 are killed by late frosts. 1 



Douglas Spruce. Pseudotsuga inner onata (Rap.) Sudw. — This is a 

 tree of the higher foothill region, not extending down to the lower limit of 

 Rock Pine. In the higher altitudes it grows upon hillsides with north 

 exposure, but in the lower foothill region it is confined to the deep, 

 moist gulches. The map (see Fig. 1) indicates its distribution on the 

 mesas. It will be seen that it grows only on the north slope and deeper 

 parts of Pole and Bluebell Canyons. 



Mountain Juniper. Juniper us sibirica Burgsd. — According to 

 Rydberg, 2 this tree or shrub has its lower limit of altitudinal distribution 

 at about 6,500 feet. On these mesas it extends down to about 6,000 

 feet. A few isolated individuals are found upon the higher parts of the 

 mesas. 



Rocky Mountain Red Cedar. Sabina scopulorum (Sarg.) Rydb. — 

 A few individuals of this species are found scattered here and there on 

 the mesa top among the pines. This is a tree which commonly grows 

 in dry situations extending into both lower and higher altitudes. 



4. Distribution of the Deciduous Trees and Shrubs on the Mesas 



By W. W. Robbins 



The mesa region is an area of transition between plains and foothill 

 forms of vegetation. Plains forms here have their upper limit of distri- 

 bution and foothill forms their lower limit. A general view of the mesa 

 region and its relation to the plains and foothills is shown in Fig. 1, Plate 

 I. The mesa with an altitudinal range of eight hundred feet, its various 

 exposures, its two bounding gulches of different ages, its lateral ravines, 

 depressions and amphitheaters with their accompanying differences in 

 physical factors, offers a variety of habitats. The vegetation shows a 

 response to these differences in the topography. 



The shrubs and deciduous trees on Long Mesa have been mapped 



'Since the above was written, there has appeared a paper by Leroy Harris Harvey, "Floral 

 Succession in the Prairie-Grass Formation of Southeastern South Dakota," Bot. Gaz., 46:81-109, August, 

 1908, in which are mentioned the sance factors as are considered here, as preventing the invasion of the grass- 

 land by the trees. 



1 Flora of Colo., Bull, ioo, Colo. Agr. Exp. Sta. 



