700 JOURNAL OF FORESTRY 



by the wind. Mattoon^* reports considerable damage to shortleaf pine 

 (Pinus echinata Mill.) from wind action and from ice storms in the 

 tornado region of the Middle Western States. 



The site upon which a stand of timber grows has a marked effect 

 upon the chances of windfall. Exposure to air currents by means of 

 high, exposed ridges and by narrow, draughty canyons presents, in 

 amount of wind damage, a marked contrast to such sites upon which 

 the stand is protected from the direct action of the wind. High, ex- 

 posed sites along ridges either disclose considerable windfall and wind 

 breakage or exhibit a stand of dwarfed and eccentrically shaped trees, 

 due to the direct action of the wind. Hess^ shows that a tree growing 

 upon a slope is more easily thrown down hill than up, due to these four 

 factors: (i) Trees are xnore heavily branched on lower side; (2) root 

 development offers greater resistance to wind blowing up hill; (3) as- 

 cending wind encounters greater resistance ; (4) tree is more easily 

 overbalanced and thrown in a downward than in an upward direction. 



One of the principal factors concerned in the windfirmness of indi- 

 vidual trees and stands is the character of the soil in which they are 

 growing. Wind action on trees growing in loose, wet soils causes wind- 

 fall, while in firm, rocky soils the windfall danger is greatly lessened 

 and breakage is the rule. This is evident when windfall areas are 

 studied. The loose moist soils offer little resistance to the pulling out 

 of the root crowns as the wind force tilts the tree and consequently 

 overthrow results.- The presence of impermeable layers in the subsoil 

 often prevents the roots from penetrating to any considerable depth. 

 This condition subjects the tree to windfall. Hess^ states that trees 

 growing on granitic basalt, gneiss, and porphyritic soils are less subject 

 to windfall. Smith and Weitknecht^^ find as a result of observation 

 made on a yellow-pine area that windfalls are as numerous on medium 

 and deep soils as on shallow soils. Hodson and Foster^" find that 

 Engelmann spruce (Picea engelmanni Engelm.) growing on thin soils 

 in exposed places is very subject to windthrow. In some cases the 

 roots were found not more than two feet below the surface. 



Excessive rainfall generally precedes a windfall, and it is found that 

 heavy rains before or during a storm greatly increase the damage. 



The species of trees seem to offer varying degrees of resistance to 

 windfall, and this is no doubt largely due to the root system. Shallow- 

 rooted trees have been found predisposed to windthrow more so than 

 the deep-rooted species, and the wind action upon shallow-rooted spe- 

 cies invariably causes windfall, while in deep-rooted species breakage 

 is more common. An interesting point in connection with the root 



