CHAP, in.] Characteristics of Trees 61 



Growth in girth is in all species of forest trees practically 

 proportional to the growth in height, as they are each the 

 expression of the vigour of the individual tree. In most of the 

 light-demanding species of trees this begins early, often attains 

 its maximum vigour between the twentieth to thirtieth year, 

 maintains itself till about the fiftieth to sixtieth year, and then 

 gradually declines. It begins later with Oaks and shade-bearing 

 species, but is often vigorously maintained till the seventieth 

 to ninetieth year before beginning to sink gradually. In 

 crowded woods the development in girth is more prejudiced 

 than the energy of growth in height ; but whenever the growing- 

 space allowed is more than sufficient for the normal require- 

 ments of any given species, growth in girth takes place at the 

 expense of growth in length. When trees still in vigorous growth 

 are heavily thinned after being accustomed to but a limited 

 growing-space, they rapidly thicken in the bole in consequence 

 of the greater exposure to light and air, and of the increase 

 in foliage and assimilative power due thereto. The largest 

 girths are attainable by Douglas and Silver Firs, Spruces, Wey- 

 mouth and Scots Pines among conifers, and by Oak, Elm, 

 Beech, and Black Poplar among broad-leaved species of trees. 



Growth in cubic contents, or total increment, is the final 

 expression of the resultants of growth in height and growth 

 in girth, and is a convenient measure of the general energy of 

 growth. Owing to the reduction in the number of individual 

 stems per acre, consequent on increased demands for growing- 

 space, increment culminates earlier in light-demanding than in 

 shade-bearing species. So far as whole crops are concerned, 

 Gayer classifies their energy in total increment as follows : 



Spruce and Silver Fir woods. 



Larch, Weymouth and Scots Pine woods. 



Beech woods. 



Oak, Ash, and Hornbeam woods. 



Birch woods. 



But he specially characterizes the energy of the Firs as nearly 



