where it, has once been expelled by conflagration. What is the chief cause of this in 

 the present case again, if it be the drought, direct exsiccating seeds and seedlings, or 

 others, as changes in the character of the soil, such as destruction of the mould, a 

 reduction of the mossy cover or the like, is not always easy to settle definitely, though 

 the drought in the summer and after-summer may be most likely direct to account 

 for the seedlings exsiccating and being destroyed. F. E. Clements expresses his opinion 

 on this question in his work, Plant Succession (1916) Pag. 70: «With the seedlings of 

 woody plants the cause of the greatest destruction is drouth in midsummer or later. 



Fig. 40. A view in the south side of the Sayansk mountains at an altitude of about^900 m. above 

 sea-level. Coniferous trees in various stages of exsiccation. 



This is the primary factor in limiting the ecesis of many conifers, though the cheaving» 

 action of frost is often great or even predominant. The root-system is often inadequate to 

 supply the water necessary to offset the high transpiration caused by conditions at the 

 surface of the soil. Moreover, it is likewise too short to escape the progressive drying- 

 out of the soil itself. In open places in the Rocky Mountains, such as parks, clearings, 

 etc., the late summer mortality is excessive, often including all seedlings of the year.» 



Experience shows that when the forest for some reason or other is expelled from 

 habitats under extreme conditions, near the boundaries of its area, it does not reappear 

 any more. During a climatic change, however, forests will be able to linger for a time 



68 



