REFORESTATION IN NORTHERN IDAHO • 33 



meters from the top of the ridge, and a second atmometer was 

 placed just over the ridge on the north slope. Neither station was 

 occupied by mature trees, although Douglas fir and yellow pine 

 grew on the ridge and north slope on both sides of the prairie 

 in which data were obtained. Coniferous seeds were avail- 

 able at both stations and pine cones were actually lying scat- 

 tered about on the ground. On the sheltered slope a few 

 pine and fir seedlings had established themsleves, while none 

 occurred near the station on the south slope. The average 

 daily evaporation in the area occupied by these small seedlings, 

 from May 10 to September 5 was 19 cc, while the exposed 

 side of the ridge gave an average daily evaporation of 27.7 cc. 

 The greatest stress on the south side for any seven-day period 

 occurred during the week ending August 29, when the average 

 daily evaporation reached 50.8 cc. On the north side during the 

 same interval it averaged only 34 cc. per day. 



Figure 1 shows a sharp ecotone between prairie and forest. 

 This spur of Tekoa Mountain is illustrative of many similar 

 situations of transition from grassland to forest. The shel- 

 tered northeast slope supports a dense growth of yellow pine, 

 Douglas fir, and some tamarack, but these invade the prairie 

 of the windward slope with great difficulty. Figure 2 shows a 

 similar situation and illustrates the effect of exposure to wind 

 upon the few trees that have succeeded in growing upon the 

 ridge. 



Likewise the destruction by fire of the accumulated humus 

 constituting that all-important part of the forest floor may re- 

 tard for many years the reappearance of young trees of sufficient 

 vigor to endure. 



A study of the physiology of reforestation without taking into 

 account the intimate relation of other plants to the reappearance 

 and permanent establishment of the forest growth would be 

 incomplete; for there can be no doubt concerning the vital impor- 

 tance of these plants as agents modifying the factors influencing 

 the restoration of that physiological balance so essential to the 

 incipient growth of coniferous seedlings. 



The Thatuna Hills, a western spur of the Bitter Root Moun- 



