CHAP. XIL] Protection of the Soil 263 



of trees that may otherwise be best suited for the given soil 

 and situation. 



On hillsides, therefore, the felling and regeneration should 

 take place from above downwards. On being cooled by 

 radiation of warmth from the surface of the soil, the air 

 becomes heavier and seeks a lower level ; and if its progress 

 down towards the valleys be hemmed or made to stagnate by 

 young thickets, the plants forming these are unduly exposed 

 to danger from late frosts in spring and early frosts in autumn. 

 On level stretches of land it suffices if the young crops are 

 fringed on one side at least by a lofty crop of trees ; for then 

 the cold air can circulate freely among the lofty stems in 

 that direction in place of being penned in by older, densely 

 foliaged thickets of somewhat higher growth. Hence, in natural 

 regenerations, it is an error to await the return of another seed- 

 year when patches here and there are deficient in seedling 

 growth. It is much more advisable to sow artificially, or to 

 plant, than to expose the ultimate seedling growth on such 

 patches here and there to subsequent danger from late and 

 early frosts on account of their being somewhat less in height 

 than the seedling growth around them. Similarly, when groups 

 of trees whose seedling growth is likely to suffer from frost, 

 such as Oak, Ash, Beech, Maples, Silver Fir, and Spruce, 

 occur of nearly mature age scattered among similar crops that 

 are fully mature, it is better to regenerate them along with the 

 latter, than to run the risk later on of developing frost-holes, 

 where only the hardier species of trees like Birch, Elm, 

 Aspen, Alder. Larch, and Pines will be able to outlive the 

 dangers constantly recurring for several springs and autumns 

 until the new crops of young plants get their head well above 

 the frost-line, below the level of which the chill night air 

 stagnates. And the quicker such spots are planted up, of 

 course, the less danger is there of frost-holes being formed. 



In order that patches of this description may be utilized to 

 their full productivity, it is requisite that, if they are apt to be 



