46 



joch (yoke),' which means an increase of 15 f.m. 

 per year and hectare ; the average height of the 

 trees being 27 metres, average diameter 26 centi- 

 metres. Here we have the culmination of the 

 increment at 20 years, and if we go on the basis 

 of the poorest quality, the full increment came to 

 3*6 f.m. per year and hectare. From what 

 Eberts ^ says the robinia yielded 760 f m. under 

 50 years' rotation. Under coppice the rotation 

 is fixed at from 15 to 20 years, and in high 

 forest at from 50 to 60 years. 



Acacia wood enjoys the best reputation every- 

 where, and in point of durability, hardness and 

 resistance is not far short of oak itself. It is 

 liable to be broken by the wind and it suffers 

 from early frost. Hares, Coccus cacti,^ Lecanium 

 robiniarum also diminish its growth. 



26. Ulmus americana, L., American Elm, White 



Elm. 



This is known all over the world as a tree 

 for parks, but has only just been taken up for 

 forest culture. It is, however, strongly recom- 

 mended on account of its rapid growth. 



' Joch = about one acre. 



= Der Akazienniederwald, "A. F. u. J.," 1899, p. i68. 

 3 Prof. Sajo, " Die Akazienschildlaus," " Forstl.-naturw. Z.," 

 1896, p. 81. 



