56 BULLETIN 1105, U. S. DEPAETMENT OF AGEICULTURE. 



CUTTING. 



Forest cover, as the term is understood by foresters, involves 

 many factors. In studying the effects of cutting or other 

 disturbances affecting the density of the crown canopy, we have to 

 consider on one hand the shelter to young seedlings and on the 

 other the possible detrimental effects of too much shade and root 

 competition. 



SHELTER AFFORDED BY TREES. 



NEAR THE TEEES. 



That seedlings receive shelter from older trees in the forest has 

 been recognized since the beginning of forestry. Forest cover cuts 

 off isolation and thereby reduces day temperatures; but by re-, 

 tarcling losses through radiation, it tends to raise night tempera- 

 tures. Soil temperatures is lower beneath the crown canopy, with 

 the exception of surface temperature, which may be slightly higher 

 at night. Eecluced insolation and wind movement decreases evapo- 

 ration from the soil and the transpiration of plants. This tends to 

 conserve soil moisture, particularly at the surface. Leaf litter, by 

 by forming a soil mulch, adds to the above effects. Undergrowth, 

 which normally draws considerable moisture from the soil, is held 

 in check by the shade of the trees. 



It is a matter of common observation that western yellow pine 

 germinates most abundantly in the immediate vicinity of older 

 trees. Seed supply is only one of the factors here involved. Young 

 seedlings are most numerous in a zone which corresponds roughly 

 to the area shaded in summer by the tree crowns from 11 a. m. to 

 3 p. m., modified to some extent by the distribution of leaf litter. 

 This area, which may be characterized as the zone of maximum in- 

 fluence, extends farther to the north and east than in other direc- 

 tions; on the south side it almost coincides with the outer edge of 

 the tree crowns. Solar radiation apparently is the dominant factor, 

 although wind, which blows prevailingly from the southwest, is 

 probably also important. A prominent feature which marks the 

 zone of maximum influence is the scarcity of typical sun vegetation. 

 Under the crown canopy, and from 20 to 40 feet to the north and 

 shorter distances in other directions, the grasses which predominate 

 in the open spaces are to a large extent supplanted by broad-leaved 

 plants. Under single trees or small, high-crowned groups, however, 

 the grasses often come almost up to the trunk. 



Notwithstanding the abundant initial establishment of seedlings 

 in the zone of maximum shelter, sui-vival is poor. After the sec- 

 ond or third year they begin to pine away, and few live to be 5 



