SOME VEGETATION TYPES AT HIGH ALTITUDES. 49 



it becomes almost completely closed. This association extends 

 along the upper margin of tree-growth, where it forms the 

 highest forest zone, and ascends to 7000 feet. It occurs 

 typically as almost pure forest on the screes of the middle 

 valley slopes, which are quite incapable of supporting any other 

 species. Like the last association it descends considerably 

 in the wash-out channels, and in these the trees tend to arch 

 over the centre of the channel (PI. III. fig. 3). It also occurs at 

 comparatively low altitudes on moraines and gravel banks. 

 Hence it would appear that soil conditions rather than climatic 

 conditions are the main factors in its distribution. From 

 the growth habit of the species its great value as forming 

 protection-forest may be easily seen (PI. III. fig. 4). As a 

 result of the peculiar trailing habit, many of its branches are 

 buried in snow for the greater part of the year. These are 

 often attacked by the fungus Herpotncha ?iigra, which, by 

 means of its mycelium, holds the trailing branches to the 

 ground and forms a brown felt of hyphas round the needles 

 of erect branches. Practically the only species obtained in 

 admixture in this association is spruce {Ptcea excelsa), and its 

 occurrence is exceedingly rare. 



The chief plant associates of the mountain pine are Erica 

 carnea, Daphne striata, Dryas octopetala, Calluna vulgaris, 

 Arctostaphylos iiva ursi, Jutiiperics nana, Vacciniimi uligifiosum, 

 V. vitis idcea, V. myrtillus, Pirola secunda, Cotoneaster vulgaris, 

 etc. These, however, do not occur in the form of an under- 

 growth, since the low, branching habit of this form of mountain 

 pine does not allow of any such development. They are to be 

 found, therefore, forming the soil-covering of the many gaps 

 which occur in the formation. 



Perhaps one of the most interesting features of this type of 

 forest is the suddenness with which it may give place to quite a 

 new type. The main factors responsible for the change would 

 seem to be a difference in depth and mechanical consistency of 

 the soil. For example, PI. III. fig. 5 shows in the foreground 

 Finns montana (bush form) forest changing abruptly into larch 

 {Larix europcBa) forest at the same altitude. The only explana- 

 tion to be offered is that, where the larch occurs, the soil is 

 deeper and not so loose as that on the scree slopes which 

 bear the mountain pine formation. Another interesting case is 

 seen in PI. III. fig. 7, where the mountain pine (bush form) 



VOL. XXVIII. PART I. D 



