14 Rydberg: Phytogeographical notes 



about Ulmus americana and Ostrya virginiana. There are still 

 more trees of the Northern Rockies which are not found in the 

 southern part, such as Larix occidentalis, L. Lyallii, Abies grandis, 

 Tsuga heterophylla, T. Mertensiana, Picea albertiana, Pinus monti- 

 cola, P. albicaulis, Thuja plicata, Taxus brevi folia, several species 

 of Betula and Salix, Populus trichocarpa, P. hastata, and P. 

 Besseyana. It is true that most of the conifers mentioned belong 

 principally to the Selkirk-Bitter Root region, but this is not the 

 case with Picea albertia?ia and Pinus albicaulis, which extend as 

 far south as the Yellowstone Park. Abies grandis has also been 

 reported from there, and Pinus monticola a little north thereof. 



SuBALPiNE Zone of the Southern Rockies 

 The characteristic trees of the Subalpine Zone of the Southern 

 Rockies are Picea Engelmannii and Populus tremuloides, with 

 Pinus aristata and Abies lasiocarpa as secondary ones. 



The Engelmann Spruce, or, as it is usually called in the 

 Rockies, the white spruce, has its best development in Colorado 

 at an altitude between 10,000 and 11,500 feet. It extends up to 

 the timberline, 11,500-11,800 feet or rarely up to 12,000 feet. 

 This region from about 10,000 feet to the timberline may there- 

 fore be regarded as the Subalpine Zone. The spruce grows often 

 in pure stands, as in the La Sal Mountains, or mixed with the 

 subalpine fir, Abies lasiocarpa, and the forests usually cover the 

 northern slopes, except where the conditions are such that no 

 trees can grow. In favorable localities it also grows on the 

 southern slopes, there occasionally mixed with Pinus aristata. 

 It is not confined, however, to the Subalpine Zone, but extends 

 far down on the northern slopes into the Montane Zone as low 

 as to 7,000 or 6,500 feet altitude. Here, as well as in the lower 

 portion of the Subalpine Zone, it is often mixed with the Douglas 

 or red fir, Pseudotsuga mucronata, the blue spruce, Picea Parryana, 

 the balsam fir, Abies concolor, the limber pine, Pinus flexilis, 

 and the lodge-pole pine, Pinus Murray ana. Picea Engelmannii 

 extends throughout the northern Rocky Mountain region, but 

 is not there so exclusively the tree characteristic of the Subalpine 

 Zone. 



The Engelmann spruce is a slender tret? with narrow crown, 



