352 GROWTH OF TREES IN DIAMETER 



That of western conifers is double the cycle characteristic of those 

 in the East, while that for redwoods and Sequoia is fully five times 

 as great as for most of the remaining western conifers. 



The life cycle of any individual tree is governed by the average for 

 the species but appears to depend on size and not age. A tree is mature 

 when it has reached the maximum size permitted by its site and vigor 

 of crown, whether this is secured by continuous rapid growth as a 

 dominant tree or is delayed by a period of suppression. Trees character- 

 istically intolerant and dominant, and accidentally suppressed in youth, 

 if they recover from this suppression, will add the period of suppression 

 to the average age which they attain and continue to grow until they 

 reach the usual size. Trees naturally undergoing and recovering from 

 a period of suppression, such as spruce and balsam, may attain maturity 

 under these conditions 100 years later than trees of the same species 

 growing in the open, and their life cycle will be that much longer. This 

 law was also found to hold true for the Sequoia gigantea.^ 



272. Effect of Quality of Site. The greater productive capacity 

 of better sites is reflected in the increased rate of growth in diameter 

 of the species on these sites. Either deficiency or continuous excess 

 of moisture greatly reduces the site quality and slows down diameter 

 growth. The final expression of site quality is found in terms of total 

 volume or rate of growth per year, of which this average diameter 

 growth is one of the l^est indications. 



273. Effect of Density of Stand. The rate of growth of the individ- 

 ual or average tree is profoundly influenced by the number of trees 

 in the stand. The original number of trees germinating and becoming 

 established on a site bears no relation to the number which may grow 

 to maturity. The reduction of numbers with increased size and crown 

 spread is accomplished by competition between individuals, resulting 

 in the death of the weaker trees. With species which become estab- 

 lished in dense stands in a single year and maintain an even height 

 growth, the inability of the stand to differentiate itself and destroy 

 the necessary proportion of the weaker trees is reflected in a great 

 reduction in diameter growth on all of the trees. Of this tendency, 

 lodgepole pine gives the best examples. In almost all species of conifers 

 and many hardwoods, dense, even stocking, unless artificially corrected 

 by thinning, gives a much lower rate of diameter growth than the aver- 

 age which may and should be secured by the species. Diameter growth 

 is therefore apt to be greatly reduced by increased number of trees 

 per acre in the stand, or overstocking. 



" Ellsworth Huntingdon, The Climatic Factor, as Illustrated in Arid America, 

 Carnegie Institution of Wash., D. C, 1914, Chap. XII. 



