TIME TO START THIxYNINGS 159 



weak tree cannot quickly be converted into a resistant indi- 

 vidual by one cutting. The change must come gradually. 

 If a weak tree is too suddenly left unsupported the thinning 

 may result in its being broken or overthrown. 



5. Thinnings keep the stand free of unhealthy and dying 

 trees in which insects and fungi find the best opportunities 

 for development. The trees remaining in a stand systemati- 

 cally thinned, being exceptionally thrifty, do not succumb to 

 the attacks of insects and fungi as easily as do the weaker 

 trees in un thinned stands. 



Time to Start Thinnings. — Theoretically thinnings are 

 needed just as soon as the struggle for existence between the 

 trees in a stand has become serious. This condition may 

 arise within a few years after establishment in a densely 

 stocked stand and is rarely deferred later than the tenth to 

 fifteenth years. The density of stocking, productive power of 

 the site and the spreading habit of the species determine the 

 time for making the first thinning. The item of expense also 

 should be considered. In young stands the material removed 

 may not be valuable enough to pay the cost of the operation. 

 A general rule is not to make the first thinning until the re- 

 ceipts will at least pay the expenses. 



The life history of an evenaged stand may be divided into 

 three stages from the establishment of the crop to the begin- 

 ning of the regeneration period. 



The first lasts so long as there is not a marked differentia- 

 tion between individual trees. Its duration is brief and no 

 intermediate cuttings, with the possible exception of a libera- 

 tion cutting, are needed. The second begins with the differ- 

 entiation into crown classes and is characterized as the period 

 of height growth. This lasts for one to two decades during 

 the early part of which cleanings to regulate the composition 

 are required. In the latter part of the period an early thin- 



