144 BULLETIN 1233. U. S. DEPARTMENT OF AGRICULTURE. 



that is, of high sap density. Yet deep rooting is essential in order 

 that any adequate supply of water may be had, and later extensive 

 rooting must be developed in order to insure the supply of water from 

 a soil of such meager capacity. 



Either yellow pine or limber pine fits these conditions. The large 

 seed of either species is a fairly good guarantee of the vigorous initial 

 growth that is essential. Later, when the pines make shade, an 

 occasional Douglas fir may become established in the most shaded 

 spots. Limber pine will disappear if the stand becomes at all close, 

 because it can not compete with yellow pine in growth rate or ultimate 

 stature. Any species, with the possible exception of spruce, could be 

 planted here if sturdy young trees were used, for the only really 

 dangerous factor is excessive heat. 



(2) A flat ridge. — The insolation is about 0.80 or 0.85. The highest 

 surface temperatures reach 120° to 130°, but they are not prohibi- 

 tive to tree establishment except as they induce rapid transpiration 

 and soil-drying. The wind exposure is greater than on the south 

 slope, and the evaporation is almost as high. Greater difficulty is 

 experienced in forming any soil; all the humus is moved away and 

 there is little chance for any to be deposited. The same is true of 

 the finer particles of soil as the rocks break down ; consequently there 

 is practically no soil over the rock. Very little water can be retained. 

 The forest must always be very open. The difficulties of initiation, 

 however, are not so great as on the south slope. Limber and yellow 

 pines predominate, but the yellow pines are lacking in vigor, being 

 evidently exposed to some desiccating force they can not cope with. 

 This is probably the winter wind. The insolation is not enough to 

 keep the soil from freezing deeply, and the wind exposure is worse 

 than on any other site except a westerly slope. Some Douglas fir 

 seedlings appear, with a little protection, and they do not suffer 

 from the winter conditions. 



(3) The middle of a north slope. — In heating effect the insolation 

 can not be rated over 0.50 at midsummer, and may dwindle almost 

 to zero in winter. Hence the striking characteristics of the slope 

 are ability to retain moisture and a long unbroken period of freezing 

 during the winter. As measured just above the surface on a denuded 

 site, the insolation and evaporation may at times be nearly of unit 

 weight; but of course the evaporation may become negligible when 

 the sun is very low. The conditions for soil building are almost 

 ideal. On this kind of a site there is adequate heat and moisture 

 for the early development of any species. The retention of a snow 

 blanket in winter practically eliminates exposure and further encour- 

 ages a mixture of all the species. Only spruce may at first be pro- 

 hibited, and until other vegetation shades the ground and modifies 

 the superficial drying of the soil. 



The large quantity of moisture present encourages a heavy vege- 

 tation of all sorts and quickly changes the denuded site to a thicket. 

 The later conditions are radically different from those which initiate 

 the succession. Direct light, while not entirely wanting during the 

 growing season, is at a premium. Only the species which are more 

 effective in photosynthesis can develop properly. At first Douglas 

 fir will predominate, simply because spruce was handicapped at the 

 ©utset. In another generation without disturbance, the spruce will 



