FOREST WEEDS. 395 



v. DENSITY OF FOREST GROWTH. 

 The growth of weeds which have taken possession of the 

 soil after a felling, makes way for a covering of moss, needles 

 or dead leaves, after the forest has been reconstituted. When 

 age again begins to open out the wood, or when, owing to bad 

 management, or to accidents, thin places and blanks appear, 

 weeds reappear in direct proportion to the amount of light 

 admitted to the soil. 



vi. WEATHER. 



Damp warm years are most favourable to a growth of weeds, 

 and during such years tender forest species require little or 

 no shelter. Hence for both these reasons weeds are then 

 most hurtful. 



vii. HABIT OF THE WEEDS. 



Perennial weeds, and especially those which produce root- 

 suckers, are much worse than annuals. Also those with dense 

 foliage and those which are social over extensive areas, or 

 gregarious in patches, injure forest plants more than scantily 

 foliaged and solitary growing weeds. Species such as black- 

 thorn, aspen, and forest-willows soon get the upper hand of 

 other weeds. 



Weeds which by their decomposition yield dry or acid humus 

 are also hurtful, as they produce soil unsuitable for forest 

 vegetation. Dry humus formed of lichens, etc., contains little 

 carbon dioxide, easily crumbles, decomposes with difficulty, 

 and absorbs very little water. Acid humus, on the other 

 hand, formed by sedges and peat-plants, injures by exhaling 

 marsh-gas, and by containing certain organic acids that are 

 detrimental to tree-life. 



5. Protection against Weeds, 

 a. Preventive Measures. 



The following rules for keeping down forest weeds should 

 be observed : 



i. MAINTENANCE OF THE DENSITY OF THE FOREST. 



Great care must be taken in the shelterwood systems, on 

 soil liable to become weedy, that the fellings are not too open. 

 The seeding-fellings must be dark, and the secondary fellings 



