PURE AND MIXED WOODS 45 



fifteen years of age. The most suitable species for under- 

 planting the larch are found to be Picea sitchensis (Sitka 

 spruce), Douglas fir, Tsuga mertcnsiana^ Thuya plicata, 

 Lawson's cypress, and Abies grandis. Here these plants are 

 put in cheaply as two-year seedlings by dibbling them in ; the 

 ground being clear of weeds, it is not necessary to use large 

 plants. The underwood is said to form a thicket covering 

 the ground in from six to ten years, and the rate of growth 

 of the underwood averages about one foot and a half annually 

 when once the plants are established. 



In the course of time an underwood thus formed will catch 

 up the overwood and from that time onwards the crowns of 

 the two species are fairly intimately mixed. 



It is not usually good to mix light- demanders only, without 

 any shade-bearers, as the soil deteriorates under them, and 

 one species very often outgrows and suppresses the other. 

 Such mixtures as the following are often found in existing 

 woods : 



Oak with ash, elm, or sweet chestnut. 



Oak with alder or birch. 



Oak with Scotch pine or larch. 



Scotch pine with birch. 



Scotch pine with larch. 



Scotch pme with sweet chestnut. 



Any intimate mixture of such species should, however, be 

 avoided, as they almost always lead to difficulties. Mixtures 

 of larch with sweet chestnut ; Scotch pine and Corsican pine ; 

 or Scotch pine and Weymouth pine are good mixtures sylvi- 

 culturally if the crop is to be cut down fairly early, say at 

 about fifty years of age, but Weymouth pine is not a very 

 valuable species to plant, and is very subject to disease. 



Oak has often been planted in alternate lines with conifers, 

 the latter being nurses to the oak which was to form the 

 permanent crop. This method seldom gives good results. If 

 the conifers are cut out when they begin to top the oaks, the 



