FORESTS OF ROCKY MOUNTAINS FOREST RESERVE 17 



INFLUENCE OF ASPECT. 



The influence of aspect on the forest type can be traced only in certain localities. 

 In many places only minor and irregular differences can be observed between the 

 stands on slopes facing in various directions. However, marked contrasts frequently 

 exist between the types on slopes facing in opposite directions, and comparison from 

 place to place shows a fairly consistent effect to be attributed to the differences in 

 aspect. The contrast is usually between these two conditions; on the one hand, a 

 rapid change from the spruce type of the valley to pine on the slope, the pine becom- 

 ing progressively purer as higher elevations are reached and, near the timber line, 

 scrubby in character ; on the other hand, the occurrence of spruce in considerable 

 proportion right to timber line, in mixture with pine, alpine fir (' balsam ') and also 

 Douglas fir, with white-bark pine and Lyall's larch near timber line. The two series 

 of stand-tables previously given to show the altitxidinal variation in type illustrate 

 contrasts which may be observed on different aspects. The predominance of pine is 

 found usually on south and west exposures, and the mixed type on north and east 

 aspects. The south and west exposures are much the driest, not only receiving the 

 sun's rays more directly and for a longer time each day, but also being exposed to the 

 winds from the south and west, which are especially warm and dry on account of 

 the Chinook influence incident to the passing of the air currents over the conti- 

 nental divide to lower elevations. 



The manner in which this influence of aspect directly affects the type of the 

 forest is hard to explain. The occurrence of thrifty trees of both spruce and pine 

 in considerable numbers on both aspects precludes the idea that the growth of adult 

 trees is hindered sufficiently to gradually give one species the advantage on certain 

 aspects. The influence of the aspect on the character of the stand is mainly through 

 its influence on factors which govern the character of reproduction. For example, 

 the drier sites would be more apt to have intense fires and also light ground cover, 

 both of which favour pine reproduction, while the moister sites would be more favour- 

 able to the reproduction of spruce, fir and alpine fir ('balsam'), and more likely to 

 have seed-trees of those species left after a fire. 



SECOXD-GROWTH STAXDS (RESULTING FROM FIRES). 



These occupy three quarters of the forest area and the timber is over ninety 

 per cent pine, the remainder being mainly spruce or, in the foothills, poplar. The 

 reasons for the dominance of pine over spruce are explained in the discussion of 

 the influence of fires. There does not seem to be any limit to the period during 

 which fires have occurred. An extensive fire occurred in the Ghost River valley one 

 hundred and eighty-three years ago, and the resulting stand is now being lumbered. 

 Evidences of fire are found in still older stands and the general even-aged character 

 of the forest indicates the extensive influence of past fires on the present condition 

 of the forest. 



Within the past sixty years, fires have increased greatly in number, judging from 

 the ages of most of the second-growth stands, which lie below that age. An extensive 

 fire occurred in the vicinity of the Sheep River forty-five years ago, and widespread 

 fires have been periodical events there during more recent times. The splendid repro- 

 duction of pine, even after very severe fires, has been a boon to the forest, since prac- 

 tically all the burned-over areas have seeded up to merchantable species instead of, as 

 in many regions of Canada, coming to be occupied by comparatively worthless species. 

 Under conditions unfavourable to reproduction, occupation of the burned areas 

 by grass takes place; and that is the worst result than can come of a fire, or of 

 repeated fires. Of course, on most natural forest areas, except in moist valleys, the 



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