272 Studies in Forestry [CHAP. xir. 



to keep the soil fresh or at most slightly moist, and by causing 

 it to respond gradually to actual periodic variations, and not 

 to mere sporadic daily changes of temperature, prevent the 

 commencement of the annual vegetative activity till the general 

 warmth is greater and the assimilative process can be carried 

 on with greater energy, and without risk of the young flush 

 of leaves and shoots being killed by late frosts. But the 

 advantages derivable from delaying the early awakening of 

 vegetative activity in timber crops have previously been con- 

 sidered l ', and it is only necessary to point out here that the 

 relation of soil towards warmth will be most favourable when 

 the timber-crops are of such a nature as to favour the formation 

 of a good layer of humus, and when they are subjected to such 

 methods of treatment as will not interfere with the normal 

 process of humification. Thus soils naturally of a moist descrip- 

 tion can most advantageously be utilized for crops of Ash, 

 Alder, Oak, Elm, Maple, Sycamore, Willow, or Birch, where 

 the more densely foliaged species of trees, with their heavier 

 fall of dead foliage, might be apt to litter the ground with so 

 thick a layer of dead leaves as to interfere with the progress 

 of humification and to impede the entrance of air into the soil, 

 or where, by their greater density of canopy and their lower 

 capacity for transpiration through their foliage, they might tend 

 to keep the soil too moist and inactive. On land of this class 

 the more lightly foliaged trees have not only a better leaf canopy 

 than on dry soils, but the action of the sun may be even 

 beneficial where there is any tendency to excess of moisture. 

 On dry soils, however, there exists a necessity for the mainten- 

 ance of good canopy, for admixture of Beech along with Oak, 

 and of Douglas Fir, Silver Fir, or Black Pines along with Larch 

 and Scots Pine, or for underplanting when the crop of timber 

 trees is unable for itself to maintain the soil against the action 

 Of sun and wind, and unable to prevent the upper layers from 



1 See also chapter on Underplanting. pp. 236, 237. 



