512 forestry Quarterly. 



The cause of this strict conformity of type to topography is the 

 problem with which this paper is concerned. 



It has been argued that a difference in the soil accounts for 

 the change in species. There is nothing, however, to show that 

 the soils are essentially different, except in depth, since they are 

 all from the same source and there has been nothing to change 

 them. But, if it is granted that the soils may be different, it does 

 not follow that Lodgepole Pine should be found on one site and 

 Yellow Pine on the other, since both species have practically the 

 same soil requirements. 



Another theory is that Yellow Pine formerly covered much 

 of the pumice flats, as well as on the buttes, and that the area 

 has been reduced to its present limits by fire. Just why the Yel- 

 low Pine should have been left on the buttes and swept clean 

 from the flats, does not seem clear but it has been suggested that 

 a difference in air currents or the amount of ground litter might 

 account for it. Various other theories have been offered, differ- 

 ing as widely as those just given and very likely quite as correct. 



There is one idea, however, that has been suggested which 

 seems to meet quite satisfactorily any points that have been raised 

 against it. This theory is based on the difference in moisture 

 requirements of the Yellow Pine and Lodgepole Pine. 



On the flats, it is evident that the water table must be nearer 

 the surface than it is on the slopes. Capillary action holds the 

 ground water close to the top, and the fine, closely packed pumice 

 acts as a mulch which reduces the rate of evaporation. On the 

 buttes, knolls, rock outcrops, etc., the water table is lower, not 

 only on account of gravity, but because of the difference in the 

 structure of the sub-strata, the more porous surface layers, the 

 more exposel situation and, the consequently greater evaporation. 

 Just how much difference there is in the soil moisture content 

 of the two sites or to what extent it affects the distribution of the 

 tree species can only be determined by detailed study, but that 

 there is a dift"erence in moisture conditions and that the tree dis- 

 tribution is very closely governed by some agent, there can be no 

 question. 



Knowing that Yellow Pine will not grow in any but well 

 drained soil it may not be straining the point too far to assume 

 that the soil of pumice flats is not zvell drained and that for this 

 reason Yellow Pine is not found growing there. On the slopes, 



