PROTECTION FROM OTHER AGENCIES 319 



the mature trees. There are many instances in the West- 

 ern forests where nearly all trees of a given species are 

 affected by disease, including not only the old but the 

 middle-sized trees as well. An illustration is the hem- 

 lock in certain localities in northern Idaho. 



It is one of the aims of forestry to maintain a forest 

 in healthy condition, in order that the trees may be sound 

 when ready for the market. In a forest under intensive 

 management diseased trees are promptly removed as soon 

 as their condition is discovered. In this way the spread 

 of disease is checked. In operating virgin forests it is 

 essential to make the first cuttings in those portions on 

 which the trees are past maturity and are rapidly suc- 

 cumbing to disease. 



The study of the diseases affecting American trees is 

 in its infancy. It is of increasing importance to deter- 

 mine the facts regarding the habits of different fungi, in 

 order to develop practical measures to protect our forests 

 from their injurious effects. 



Protection Against Other Agencies 

 Trees suffer damage from various other agencies. 

 Mention has already been made, in the discussion of the 

 silvicultural systems, of damage by windfall. While this 

 cannot be altogether prevented, it can be very largely 

 reduced through the proper location of cuttings and the 

 application of careful silviculture. 



In addition to the sources of injury already men- 

 tioned, trees are injured by certain animals, by ice, snow. 



