FUNGI AS CONTRIBUTORY CAUSES OF WINDFALL 699 



Whenever heavy thinnings are made in dense stands, or when clear 

 cuttings are made on the edges of dense stands, they are very h'kely to 

 result in heavy windfalls, unless other factors enter to protect the ex- 

 posed trees. The removal of trees from a dense stand causes gaps to 

 occur in the crown, which present to the force of the wind unprotected 

 crown surfaces. In a dense stand the trees are usually crowded and 

 tall, producing the crowns high up on the stem. When such a stand is 

 left undisturbed, under ordinary circumstances the dense association 

 of the trees affords mutual protection from wind. If an opening is 

 made by thinnings or clear cuttings certain of the trees become ex- 

 posed, their resistance weakened, and the first high wind is very liable 

 to overthrow them, often resulting in a large windfall area. Indi- 

 vidual seed trees selected from dense stands, when not properly pro- 

 tected from the wind, are more liable to windthrow than groups of such 

 trees selected for seeding purposes. This is evident from the fact that 

 groups of trees supply mutual protection and are therefore more wind- 

 firm. 



Fiirst,'^ Hess,® Nisbet,^*' Fisher,^ and other European writers have 

 laid stress upon the windfall loss due to heavy thinnings and to clear 

 cuttings or "gap-fellings," and recommend care in the selection of trees 

 in selection cuttings and strip cutting in dense stands instead of "gap- 

 fellings." In the latter operation proper regard should be had for the 

 direction of the prevailing winds. Isolated trees having shallow root 

 systems and in exposed situations are recommended to be removed. 



Among the more recent writers of this country Fernow,* Frothing- 

 ham,*' Woolsey,"* Sterrett,-° and Mason" treat upon the windfall dan- 

 ger due to various cutting operations. The consensus of opinion is 

 against drastic thinnings, and in most cases it is recommended not to 

 leave individual seed trees. In the case of lodgepole pine on the Deer- 

 lodge National Forest it was found"'' that selection cuttings, properly 

 handled, worked much better than the seed block system, especially in 

 exposed, dense stands, where windfall trees amounted to as high as 

 90 per cent of the trees left after cutting. Here the tall, slender, top- 

 heavy growth habit of this tree makes it particularly susceptible to 

 windthrow. Woolsey,^* treating of western yellow pine (Pin us pon- 

 derosa Laws), and Sterrett,^" on the scrub pine [Pinus virgiuiana 

 Mill.), state that these trees, when normally growing in dense stands 

 and suddenly exposed to wind, are not found to be windfirm. and con- 

 siderable damage results. In the case of yellow pine, seed trees, un- 

 less left in groups or otherwise protected, are very liable to be thrown 



