96 FORESTRY 



injury is possible only when snowfall takes place unusually 

 early or late. 



Very little can be done in defending woods against snow. 

 Sometimes it will be found desirable to obviate the risk by 

 selecting species not liable to injury ; or, instead of growing 

 them in pure woods, one may mix the species for which there 

 is fear, among resistant sorts. In the case of Scots Pine this 

 latter precaution is specially worthy of attention at elevations 

 of 600 to 1,200 feet. The Spruce may there be mixed with 

 the Pine in order that it may be present to fill vacancies caused 

 by snow-break of the latter. For the Spruce itself, this rule 

 hardly holds good, for it is the only species which can be 

 grown on a large scale in high situations of 1,300 to 3,000 

 feet. In such positions the lessening of damage is possible 

 only by the production of symmetrical, well-developed crowns 

 and strong growth. Careful planting, together with early and 

 frequent thinning, have their influence in saving the trees. 

 Even this treatment, however, does not prevent Spruce and 

 Silver Fir from losing their leaders in localities specially 

 subject to snow. 



Against extremes of HEAT, DROUGHT, and COLD, all rules 

 which tend to prevent or regulate insolation and radiation 

 should receive consideration. The humus covering natural 

 to the forest floor must be carefully conserved. 



Danger from frost and drought is to be feared chiefly 

 during the plants' earliest youth ; natural regeneration may 

 therefore be necessary, for under that system the young crop 

 is gradually introduced under the shelter of the old. The 

 production of a special protection in the shape of a shelter 

 wood of forward growth is also applicable in extensive opera- 

 tions, while for work in the forest nursery, artificial shading 

 and covering sensitive species with branches, leaves, etc., are 

 of service. 



Special forms of injury induced by frost and the sun's 

 heat are frost-crack, frost-lifting, and bark-scorching. 



Frost-cracks are longitudinal fissures in the stem caused by 

 intense cold in winter ; starting at the bark, they reach more 



