MIXED CROP OF YAIUOUS AGES. 85 



I. INNATE VIGOUR. The comparative innate vigour of a plant 

 will obviously depend to a very groat extent on its age. The self- 

 sown sal seedling makes little head in the forest until its gradually 

 increasing vigour enables it, when established, to make a sudden 

 start when it is several years old ; whereas its companions, the 

 Tetrantheras, Kydia calycina, Miliusa velutina, Eugenia ojwrculata, 

 &c., push up rapidly soon after they germinate. In the same way 

 the two or three-year-old Pinus excelsa will grow up as vigorously 

 as a deodar several years older. In many forests of the Central 

 Provinces teak will remain onlv about a foot or two hio;h until it is 



\i O 



often more than twelve years old, when it will at once make a 

 rapid start upwards. And similarly at a later age the vigour of one 

 species will diminish earlier than that of another. In other and 

 more general terms, although the innate vigour of a tree will de- 

 pend very much on its age, nevertheless greater age will not al- 

 ways be an advantage in its struggle with other species. 



II. GREATER SUITABILILY OF SOIL AND SUBSOIL. It has 

 already been said (page 25) that for one and the same species the 

 uusuitability of a given soil or subsoil, as the case may be, may 

 increase with advancing age. Hence the older individual may, 

 because it belongs to a species for which the soil or subsoil is un- 

 favourable, be really weaker than a much younger one of another 

 species for which they are better suited, especially if the suitability 

 of the soil and subsoil for thu latter species increases with the 

 advancing age of the individual. 



III. DEATH, DISEASE, UNHEALTHY STATE, OR RETARDATION OF 



GROWTH BROUGHT ON BY CAUSES EXTRANEOUS TO THE FOREST, 

 vi.:. 



(a) Attacks of insects and other animals. The presence of old 

 and decaying trees will cause a multiplication of insects, which 

 may also injure to a less or greater extent the younger trees, and 

 also individuals of other species. But, on the other hand, in a 

 properly treated forest, there is no reason why trees should be 

 preserved until they have begun to decay and serve as nests and 

 spreading centres for injurious insects. The timely removal of such 

 trees is always possible in a crop composed of trees of various 

 ages, since overtopped younger individuals of one species or 

 another will always be there to ready to take their place, a circum- 

 stance, the absence of which in a crop of uniform age throughout not 

 only aggravates the attacks of insects as the proportion of unsound 

 trees increases with advancing age, but also renders it impossible to 

 combat them, except at the risk of making large gaps in the leaf- 



