STORMS. 539 



and sandy soils ; soils above a subsoil of stiff clay, into which 

 the trees' roots do not penetrate, are also very subject to wind- 

 fall. It has been observed that windfall in the Schwarzwald is 

 more frequent on the red sandstone formation than on gneiss, 

 granite, basalt or porphyry. In woods that have been planted 

 on the sites of old fields, windfall is frequent, owing to the 

 looseness and comparative poverty of the soil in mineral 

 matter. Soils iu which root-rot frequently occurs, such as 

 calcareous soil, or land with a wet subsoil, are liable to windfall. 



A high soil-covering, such as heather, broom, thorny or 

 shrubl^y undergrowth, is beneficial, a^ these plants bind 

 together the particles of soil, and also exercise considerable 

 friction on the air in motion. 



The state of the weather before and at the commencement 

 of the storm also exercises considerable influence on the 

 damage done. 



Heavy rainfall accompanying a storm greatly diminishes 

 the coherence of the soil, and increases the danger of up- 

 rooting, its effects vary of course with the nature of the soil. 

 This was the case on the occasion of the terrible storms in 

 1872 and 1874. Frost, on the contrary, greatly increases the 

 coherence of the soil, especially when the ground is covered 

 with snow. 



(./') Density of Crop. — Trees grown in the open, owing to 

 their well-developed root-systems, and low pyramidal crowns, 

 withstand storms much better than stems which have been 

 drawn up in dense woods. Trees recently exposed in thin- 

 nings and regeneration fellings suffer most. The latter 

 improve in their powers of resistance, as they get accus- 

 tomed to their open position. Their root-systems become 

 enlarged ; in crowded woods, however, the stems afford one 

 another mutual protection against the wind. 



Damage is considerably greater in isolated woodlands than 

 in extensive forests, as the various crops in tlie latter afford 

 one another mutual support. 



{(j) Season. — Storms may occur at any season of the year, but 

 the most destructive storms are during the six months between 

 the autumnal and vernal equinoxes. Storms in spring are more 

 dangerous than in autumn, as the soil is tlien generally less 



