110 FOSEST DI:-TRIBtTION 



to tlie particular species which occur in this region. 



The three species which occur most commonly in the foot- 

 hill regions of ^Montana are the yellow pine, the Douglas spruce 

 and the Eocky ilountain juniper. Taking first the case of 

 the pine certain highly important positive qualities may be 

 pointed out. This species produces abundant seed amply winged 

 for distribution by the wind and with high percentage of via- 

 bility. The moisture reciuirement both as to soil and air is 

 relatively very low, within its range it is subject to a tempera,- 

 ture range of at least ISO degrees, and it wiU grow on soils of 

 widely differing texture and composition. Add to these quali- 

 ties that of immunity to most diseases and. on account of its 

 thick bark, to ground fires of ordinary intensity, and also the 

 general vigor and robust habit of the tree, and it appears that 

 the yellow pine is ecjuipped with an unusual number of impor- 

 tant positive qualities which make for extension and survival. 

 The only serious limiting factor which seems to militate against 

 its success is its intolerance of shade. Owing to this weakness 

 it is largely excluded from moister soils suited to the more vig- 

 orous and rapid development of its bvillj, and eventually suc- 

 cumbs in the gradual march of succession to the more tolerant 

 species which compose the climax forest. From the point of 

 view here advanced "it is this combination of positive qualities 

 which accounts largely for the wide distribution of the species 

 over most of the western half of North America and its common 

 occurrence in most places within its range. 



The Douglas spruce offers a similar case. Its seed bearing 

 capacity is large and the light seeds are readily carried by winds 

 to considerable distances and show a high degree of viability. 

 It is a tree of vigorous growth up to matvirity and has the added 

 advantage of greater tolerance than the pine, in favorable sit- 

 uations forming dense stands to the exclusion of almost every 

 other green plant. This species also resists as wide a range of 

 temperature as the pine, and seems impartial as to most soils. 

 Toward maturity the tree is largely resistant to slight fires. In 

 its moisture demands, however, it is slightly less resistant than the 

 yellow pine. While a closo second to the pine in the tension 

 line between prairie and forest, yet the Douglas spruce more 

 readily shows the effect of adverse condition in diminished 



