TEEES FOE WATEE CATCHMENT AEEAS 133 



per acre; annual increment, 17 cubic feet per 



acre. 

 Tortworth, Gloucester, 2*26 acres, 29 years planted; 



average height, 66 feet; volume, 3690 cubic feet 



per acre; annual increment, 127 cubic feet per 



acre. 

 Dunster, Somerset, 0-84 acres, 33 years planted; 



average height, 74 feet; volume, 4985 cubic feet 



per acre ; annual increment, 151 cubic feet per 



acre. 

 The volume of the Taymount plantation (Fig. 21) is 

 much below what it would have been if the trees had not 

 been improperly thinned. In fact, in all these plantations 

 the conditions have not been ideal, as the trees in every 

 case were planted much too far apart or were mixed with 

 larch, which soon disappeared, leaving the number on the 

 ground insufficient. The annual increment is shown to 

 increase after the 30th year, and the tree is in full vigour 

 between the 50 th and 60 th year. For a rotation of 

 50 or 60 years, the yield may be estimated to vary from 

 150 to 200 cubic feet per annum per acre. 



Douglas fir should nearly always be planted pure, when 

 it is used for a main crop, as no other species will keep 

 pace with it. Larch or spruce employed as nurses are 

 speedily suppressed, (their thinnings in that case being of 

 no value. In such mixtures, the lower branches of the 

 Douglas fir are not killed off soon enough, and coarse stems 

 and knotty timber result. When planted pure at 4^ to 

 5 feet apart, the young trees soon cover the ground and 

 suppress the grass ; while their stems are cleaned before 

 the branches become too stout. More vigorous crowns are 

 developed than is the case when closer planting is adopted. 

 At Gairletter, on the Benmore estate in Argyllshire, a 

 plantation, in which Douglas fir was planted alternately 

 with Thuya gigantea, was very successful. The Thuya, 

 though overtopped by its more vigorous companion, was 

 not suppressed, and helped by its dense shade to clean the 

 stems of the Douglas fir, while the branches were still 



