GEKERAL FOREST ASPECTS 77 



phylla, Abies grandis and Thuja plicata are important, but these 

 are confined to relatively narrow areas. Among the rarer or 

 less important gymnosperms may be included Larix Lyallii, 

 Tsuga Mertensiana, Jvniperus scopulornm, J. communis, and 

 Taxus hrevifolia. 



East of the main Divide the forest is more open and assumes 

 a more xerophytic aspect. In the more sheltered canyons good 

 forests are developed, but along ridges and slopes, more or less 

 exposed, the trees straggle out in open stands, with lessening 

 frequency as they descend to the foothills and plains. The num- 

 ber of species diminishes also and only the more hardy are rep- 

 resented. The following are the more common trees on the east- 

 ward slope of the Rockies and the outlying ranges. Pinus flexilis, 

 P. contorta, P. ponderosa, Pseudotsnga iaxifoUa, Juniperus scop- 

 ulorum, Picea Engelmannii and Abies lasiocarpa. J. communis 

 and J. Sabiiia also occur, the latter more or less common on 

 prairie slopes. The spruce and fir are confined to the higher 

 elevations ; the most common trees at low elevations and about 

 the foothills are the yellow pine, the Douglas spruce and the 

 Rocky ^Mountain juniper. These are usually present on the 

 bluffs and along the rim-rock at the margins of the valleys of 

 the Yellowstone and the Missouri and their countless tributaries 

 over most of Montana down to the eastern boundary. The higher 

 elevations of land known as the Crazy Mountains, the Snowy 

 Range, the Bear Paws and the Pryor ^Mountains, and others all 

 support forests of some or all of the above species in greater 

 or less numbers and sizes. However, the forests as such are 

 found only on the mountains and become less and less frequent 

 eastward until the Black Hills are reached. The following sum- 

 mary presents the usual distribution of the species and their 

 approximate altitudes in different parts of the state. 



In considering the problem of forest distribution it is essen- 

 tial to determine as far «as possible the relative abundance of the 

 constituent species and the place which each occupies in the 

 composite formations of which it is a part.- It is likewise impor- 

 tant to determine the areas occupied by the several species and 

 to relate the species wherever possible to their controlling factors. 

 In connection with the facts of horizontal extension the altitudinal 

 distribution also claims attention, especially in a region where 



