REPRODUCTION OF WESTERN YELLOW PINE. 57 



years old. The result is that in stands where reproduction is charac- 

 terized as complete, blank spots remain around the tree groups, 

 their outlines coinciding- with those of the grassless areas described 

 above. (PI. XI.) As in the case of grass, pine seedlings grow 

 almost up to the base of the tree trunks if the crowns are 

 high above the ground. This points to isolation rather than root 

 competition as the dominant factor, although root competition is 

 no doubt important. This conclusion is strengthened by the fact 

 that the relations described above are brought out most strongly 

 in stands where the group arrangements is most highly developed, 

 and are scarcely apparent where the trees are evenly distributed. 



IN OPE>7INGS. 



Under the present limitations of silvicultural practice, western 

 yellow pine reproduction must be secured practically without the 

 direct shelter of older trees; that is, in the openings. In virgin 

 stands the trees exert an appreciable influence upon atmospheric 

 conditions, even in the treeless spaces from 50 to 100 feet or more 

 away. Investigations at the Fort Valley Experiment Station have 

 shown that in openings in a virgin stand, 50 to 75 feet from the 

 nearest trees, temperature, wind, and evaporation were appreciably 

 modified, as compared with conditions in an open park a quarter of 

 a mile distant. The results of this investigation (17) are briefly 

 reviewed here. 



Maximum air temperatures average 0.9° F. lower, and minimum 

 temperatures 6.4° F. higher, in the forest than in the open. The 

 great difference in minimum temperature is in this instance due 

 partly to air drainage, since the park lies slightly lower than the 

 forest, both receiving the drainage from the San Francisco Moun- 

 tains which rise about 5,500 feet above the instrument stations. It 

 is probable, however that the forest, in addition to checking radia- 

 tion from the ground, also tends to deflect the cold air currents de- 

 scending from the mountains. In another locality removed about 

 12 miles from the immediate influence of the mountains, a clear-cut 

 area showed minimum temperatures averaging 2.1° F. lower than 

 an adjoining virgin stand. In this case the two areas were practi- 

 cally identical as to topography. During the growing season the 

 soil temperature at a depth of 2 feet was about 5° F. lower on the 

 north side of a group of trees in the forest than in the park. Annual 

 wind movement in the forest was only 51 per cent of that in the 

 park. Evaporation from a free water surface during the growing 

 season was 30 per cent less in the forest than in the park. The 

 above conclusions are in general confirmed by European investi- 

 gations. 



