EXTERNAL DANGERS. 277 



danger fi-oiii late frost is excessive, larcli, owing to its early 

 sprouting, is less well adapted as a nurse, l)ut it does very 

 well in all other localities. In moist localities alder and 

 willows have been similarly used. 



The nurses may be distributed evenly over the area, or 

 placed in alternate lines. They are removed when the tender 

 species can do without them. Frequently some of the nurses 

 are retained so as to form a mixed wood. 



r. Cold Winds. 



The effects of raw, cold winds are often more disastrous 

 than frost produced locally by radiation. Where they are to 

 be feared, lateral as well as vertical shelter is required. This 

 may be given either by adjoining woods of sufticient height 

 and density, or, in their absence, by artificial shelter belts, or 

 wind breaks. These must be dense, and they should be 

 established some time before the area to be protected is placed 

 under cultivation or regeneration. The species of which wind 

 breaks consist should if possible be evergreen, and with dense 

 crowns coming close to the ground, such as spruce. 



All alternative measure consists in mixing a hardy species, 

 such as Scotch pine, with a tender crop. Or the wood is 

 treated under the selection system, when trees of all ages are 

 intermixed on the same area. In that case, the middle aged 

 and younger trees provide lateral shelter for the young growth, 

 while the old trees give vertical shelter. 



In all these cases it is essential that the edges of the wood, 

 towards the side whence cold winds blow, should always be 

 kept as dense as possible. 



What has been said above applies not only to frost, but also 

 to drought, more especially in tropical climates, \yhere hot, 

 dry winds may be even more disastrous than cold winds are 

 in higher latitudes. 



d. Weeds. 



In the case of the shelter- wood systems, under regular and 

 successful management, noxious weeds and objectionable 



