SiO LOCALITY AND FOREST VEGETATION. 



Lir!;ht and sliade in relation to tree growth are of the 

 greatest importance in practical sylviculture, especially in 

 the regeneration and tending of woods, the composition of 

 mixed woods, etc. In each of these cases the light require- 

 ment of the species must be fully considered, or serious 

 mistakes may be made. The most important period in this 

 respect is early youth, because at that time several species 

 require some shelter, either against heat or frost. If that 

 shelter, on the other hand, is excessive the young trees may 

 be permanently injured, or even die. When a plant has stood 

 in shade for some time, the activity of its leaves is reduced, 

 and it takes some time, after the removal of the shelter, before 

 the increased light produces increased assimilation and visible 

 activity, because fresh organs, fitted for the altered conditions, 

 must be produced. In all such cases it is best to remove the 

 cover gradually and not all at once. If the plant has stood in 

 shade for a considerable period, it may be no longer capable of 

 developing into a tree of normal size. As long as several 

 strong buds are found, especially near the top, this is not to 

 be feared, but the recovery of plants witli a few miserable 

 buds may be considered as hopeless. 



8. Moisture. 



The Ih-st question for consideration is, whether moisture in 

 the air is directly necessary or beneficial to plant life. The 

 investigation of this problem meets M'ith great difiiculty, 

 because it is often difficult to separate the effect of air 

 moisture from that of' soil moisture. As a matter of fact, 

 only meagre data are available as far as trees are concerned. 

 It is known that some species, such as spruce, alder, maple, 

 ash, and next to these, silver fir, beech and birch, thrive 

 generally better in moist than in dry air. It is also a fact, 

 that spruce appears naturally over extensive areas in high 

 situations and near the sea shores of northern Europe, that 

 is to say, in localities with a comparatively moist air, while 



