208 THE PRINCIPLES OF HANDLING WOODLANDS 



The critical period would be later if the aim of man- 

 agement were the production of wood of high quality. 

 In that case it might depend on a crown-ratio of 20 to 

 30 per cent., according to the species and site conditions. 



The experience in thinnings in American species is 

 so limited, and the markets are so poor for the products 

 obtained from them, that no data can be given as to the 

 time of beginning the work of thinnings with the various 

 forest types. 



Repetition of Thinnings. Thinnings make small, 

 temporary openings in the canopy which grow together 

 in a few years. Theoretically, the thinning is repeated 

 as soon as the crowns close over these openings. The 

 interval between thinnings would depend, then, on the 

 severity of the cutting and size of the openings. The old 

 rule in Germany was to thin early, moderately, and often. 

 Frequently in Europe thinnings are made at intervals 

 of from 3 to 5 years. That is, of course, a very in- 

 tensive application of thinnings. More often from 5 

 to 10 years represents the interval between thinnings. 

 The interval is shorter in youth and middle life, when 

 the growth of the crowns is rapid, than later, when it 

 takes longer for the crowns to meet. 



Application in Europe. The principles outlined in 

 the preceding pages may be illustrated by the policy of 

 thinnings recommended by Dr. Heinrich Mayr, of Mu- 

 nich, Germany, and by Dr. Flury, of Zurich, Switzerland. 

 Dr. Mayr's program for the treatment of the average 

 even-aged stand in Germany is as follows: 



