Wind Damage 71 



3. The lowering of the soil fertility by dry winds, which 

 cause drought, and also by blowing away the surface litter. 



Winds blowing uphill are comparatively harmless, because 

 their force is moderated by the friction of their uphill course ; also 

 the root-system and the crown are more developed on the lower side 

 of the slope, so that far more force is required to uproot a tree when 

 the wind is blowing uphill than when it is blowing down the slope. 



Species resistant to wind. Certain species are " more or less 

 capable of resisting wind, owing to the natural formation of their 

 root-system. But, on the other hand, there are other factors which 

 increase or diminish the danger. The oak is the most storm-firm 

 of our broad-leaved trees, then elm, ash and sycamore. Silver fir 

 and Scotch pine are the best of our conifers ; but if these species are 

 planted in an impervious subsoil, they are equally liable to damage. 



Shelter belts. When an area is planted on an exposed site, 

 shelter belts should invariably be placed on those sides from which 

 the prevailing winds of the district may be expected. 



The species forming the belt must be storm-firm and well suited 

 to the soil. Sycamore, elm, mountain pine, Scotch pine, Austrian 

 pine, Corsican pine, are suitable trees for this purpose on poorer soil. 

 These should all be planted about 10 to 12 feet apart, and any large 

 limbs should be removed. It must be remembered that height- 

 growth and clean timber are not the chief requirements for a 

 shelter-belt. 



Uneven-aged and mixed woods. Uneven-aged woods are less 

 liable to damage than even-aged mature woods ; for the younger 

 trees tend to prevent the swaying of the more mature stems. Under- 

 planting is often advisable, therefore. The mixing of storm-firm 

 species with those liable to damage, renders the latter more immune. 



Thinning. Light thinning,^ often repeated, is perhaps the 

 most important precaution to be observed. In an exposed situation, 



^ " Light " in this case means that only a small number of trees are removed. 

 Not in the sense meant when the term "light-thinning" is used in Natural 

 Regeneration. (See p. 27, and par. 2 of foot-note on p. 38.) 



