126 FORESTRY 



For the application of yield tables the composition of a 

 wood comes into consideration along with the examination of 

 the locality. One has to decide in how far a wood differs 

 from the perfect, or, as it is called, " normal " state, it being 

 understood in economic forestry that the forest is in a normal 

 condition when the crop is so complete that it preserves all 

 over the area an unbroken cover. In reality, woods never 

 quite attain to this degree of perfection, which, however, 

 is frequently taken as the standard = I, and the divergence 

 from it is expressed in decimal fractions. To be normal, 

 the canopy of the whole wood must be so perfect that 

 absolutely no direct sunlight can reach the forest floor. As 

 such woods at least, of light-demanding species do not 

 exist, the wood's condition may be more precisely deter- 

 mined by a comparison with the " normal " state given in the 

 yield tables. 



If the quality of a locality be known, and the age of the 

 wood, one may, by means of yield tables, estimate what 

 quantity of timber can be produced on such an area, during 

 any given period of years. 



The amount of the increase in volume duly arrived at, it 

 is easy to calculate the mean annual increment as being the 

 volume increment evenly distributed over the period. The rate 

 of volume growth is, however, by no means uniform. Most 

 generally, during a wood's earliest youth, the yearly accretion 

 is slight ; it then becomes accelerated, reaches its maximum, 

 and ultimately decreases at first slowly, but later rapidly. 

 The ordinary course, the so-called current annual increment^ 

 is occasionally interrupted by storms, the occurrence of seed 

 years, and the troubles of insect, fungus, or other attack. 

 The true current annual increment of a tree or wood is very 

 difficult to determine, and it usually suffices to take the average 

 increase over a short period say from five to ten years and 

 consider this as being accurate for each of the years within 

 the period. 



The mean annual increment follows a course very 

 similar to the actual yearly increase. At first it is even 



