INJURIOUS INFLUENCE OF THE ELEMENTS. 129 



frost causes fissures to appear in the bark and wood of 

 trees, by which they are weakened to such a degree that 

 they succumb as soon as the weather becomes very cold. 

 During early spring, when the running of the sap in the 

 trees has commenced, and a very cold spell follows after 

 some warm days, the sap often freezes, especially at the 

 south-side of the trunk ; then the bark, covering the 

 frozen sap, becomes loosened, thereby exposing the under- 

 lying wood, which in consequence goes to rot. 



To prevent these casualties there is only one means, 

 viz. : the preservation of the close density of the forest, 

 especially at its margin where it borders on an open 

 field or a very young plantation. This means of preven- 

 tion may in case of need be assisted by cultivating " wood- 

 mantels." A wood-mantel is a strip of ground along the 

 margin of a forest upon which hardy trees are planted 

 at such distances that the single trees may be able to 

 send their roots both perpendicularly and horizontally 

 into the ground so as to fully resist the fury of the wind- 

 storms and to develop, far down upon the trunk, strong 

 branches by which cold winds are prevented from enter- 

 ing the forest. 



Alternate freezing and thawing of the surface soil 

 causes often the greatest injury to young plantations, es- 

 pecially when they are produced by seeding. In such 

 case the roots are lifted and separated from their hold 

 upon the soil. The deep roots which penetrate below 

 the reach of frosts are broken off, and from the higher 

 roots the earth is more or less loosened ; finally, the plant 

 is lifted by the expansion of the surface soil, and the re- 

 maining roots are laid bare. The seedling then dies. The 

 only protection against this evil is to allow the natural 

 grasses and weeds to extend moderately over the ground 

 of the plantation so as to furnish a covering for the 

 plants not unlike that afforded by snow. In forest-nur- 

 series where, under no circumstance, grasses and weeds 



