194 Studies in Forestry [CHAP. ix. 



greatest just immediately after the thinning takes place ; and it 

 gradually decreases as the crowns close up again towards the 

 termination of the period at which another thinning becomes 

 necessary. The physiological explanation of this decrease is to 

 be found in the fact that the assimilative functions of the lower 

 foliage again become weakened through the gradual exclusion 

 of the light, air, and warmth requisite for assimilation. 



Experience has shown that the height of timber crops, as 

 well as being practically proportional to the quality of the soil 

 for the species of tree in question, is, in all regular crops grow- 

 ing on soils and situations of similar quality, and under similar 

 conditions as to their development, as a rule proportional to 

 their basal area until they have completed their chief growth 

 in height. But when the conditions of development vary, as 

 must be the case when, ceteris paribus, certain woods are only 

 slightly thinned, and others moderately, or perhaps even heavily 

 thinned, then the natural proportions between height and girth 

 become to a greater or less extent interfered with. The fuller 

 the leaf-canopy remains, the more does the form of the bole 

 approximate to the cylindrical (i.e. the higher does the form- 

 factor become) ; and the freer the growing-space, the greater is 

 the tendency to conical growth of bole (i. e. the lower does the 

 form-factor become) with simultaneous increase of branch de- 

 velopment. Experiments carried out in Beech and Scots Pine 

 woods by Dr. Behringer, under the direction of the experi- 

 mental section of the Forest Branch of Munich University, 

 went to prove l that 



' When thinnings were carried out freely, the development in height was 

 relatively much greater than the diametral increment : and that at any 

 rate during a certain period in the growth of crops, the heaviest degree 

 of thinning produced the loftiest and the cleanest boles.' 



Of course, the correctness of this statement, or of any other 

 dictum with regard to most of the operations of practical 

 sylviculture, depends in each case on the nature and condition 



1 Op. dt., p. 28. 



