FORF.ST DISTRIBUTION 



trees. One of these belts is not more than !"> feet wide and 

 extends down the slope 1'or some hundreds of yards. 



Tin- same history of the slid'- rock has been observed in 

 many places and the character o'' its vegetation man be corre- 

 lated with that prevailing ii? the locality and with the direction 

 of exposure. The identity of its vegetation varies with the alti- 

 tude and geographic position but the course of events is usually 

 the same, the lichen crust, the mosses of the (Jrimmia type, the 

 herbs, the shrubby vegetation and eventually the forest or the 

 prairie, according to exposure and climatic conditions. 



THE WESTERN VALLEYS. 



Whit ford has described the mesophytic type of forest 

 of the Flathead Valley as characterized mainly by the pres- 

 ence of Douglas spruce and western larch but including also 

 J'inns roiitftrta, P. monticola, Abies grancUs, Thuja plicata 

 and ricea Engelmannii. It would seem, however, as indic- 

 ative of the purely mesophytic type, that the species which 

 should be taken as characteristic are Pinits monticola, Abies 

 H rn mils. Tsuga heterophyUa, and Thuja plicata. These species 

 are almost invariably, associated with one another in this region 

 and forests containing them are not only of restricted distri- 

 bution in Montana, but also are marked by other more or less 

 distinctive species among the lesser forest plants. This group 

 will be considered later. 



While the Douglas spruce and yellow pine are frequent 

 companions on the southward mountain slopes, the former <m 

 the northern slopes in western Montana is frequently associated 

 with the western larch. The mixture of the two species obtains 

 only under the better watered conditions and diminishing the 

 quantity <f larch and increasing that of Douglas spruce even 

 to a pun- stand, in the direction of the drier situations. On the 

 other hand tin- lan-h sometimes attains pure stands on the cooler 

 and better watered slopes. 



The western larch f L. Occident alis) has several distinctive 

 haraeteristics. Certain positive (jualities stand out conspicii- 

 and certain ne._r;,tj V e ,,ne S an- no less prominent. The posi- 

 tive facton in the or-jani/ation of thi> gpeciefl may be summed 



