MODIFICATIONS OF THE METHOD 51 



winds which would overturn or break the same trees if stand- 

 ing singly. This is probably the fact where all the trees avail- 

 able are relatively slender and small crowned. Such trees may 

 have greater strength as a group than singly. In opposition 

 to this field observations indicate that sometimes groups offer 

 a greater resistance to winds, 5 which are thus enabled to exert 

 greater force against the group and often cause overturn or 

 break all the trees in the group.* 



The size of the seed group has of course great influence in 

 determining its windfirmness. To be reasonably immune 

 from windthrow a group should have length and breadth at 

 least equal to its height. On exposed ridges or in moist 

 ground groups of this size might be overthrown. Groups 

 with breadth and width equal to their height are impracti- 

 cable, requiring too great an investment for securing natural 

 reproduction. Seed groups as ordinarily employed will con- 

 tain from two to ten trees. 



Except in reducing danger to the seed trees from wind 

 the group seed method has nothing to recommend it in con- 

 trast to scattered seed trees. The distribution of seed to all 

 parts of the area must be less even, unless a larger per cent 

 of the total volume of the stand be reserved in the seed 

 groups. If only the small volume percentage be retained as 

 in the scattered seed tree method, then the distance between 



* As an illustration the following is quoted from a circular letter, SI Pro- 

 tection-weather, Windfall Study, dated February 12, 1915, sent from the 

 Portland office to Supervisors of National Forests in District 6 of the U. S. 

 Forest Service, speaking of windfall in yellow pine on a certain sale area. 



"In groups of five or more trees there is considerably more windfall than 

 where the trees are evenly distributed. Where there are sixteen or more trees 

 in a group the loss is 13.4 per cent greater than where the groups are smaller. 

 A good deal of the loss in groups comes from firm trees being knocked over or 

 broken off by windthrown neighbors, or by their root anchorage being weak- 

 ened by the uprooting of adjacent trees. This is on the principle that groups 

 of trees do not give mutual support, but do give mutual resistance to the wind." 



