CHAP. VIL] Formation of Mixed Woods 141 



to the ravines on the hillsides and the lower-lying tracts where 

 the deeper and richer soil lay, and where the abundance of 

 soil-moisture rendered a loose, broken canopy and a light 

 annual fall of leaves matters of no particular moment where 

 often, in fact, superabundance of soil-moisture made the 

 evaporating action of sun and wind beneficial rather than 

 prejudicial. The wet soil, the severe winters, and the frequent 

 late and early frosts in such humid localities safeguarded 

 their domain from being encroached on by the Oak or the 

 Beech. That most light-demanding, but hardiest, and most 

 graceful, of all our indigenous trees, the Birch, was forced to 

 seek its permanent home wherever no other genera of trees 

 could assert a foothold and maintain themselves in growth. 



Rules for the Formation of Pure Forests. 



Whenever % it is deemed advisable to form pure forests 

 artificially, the following rules laid down by Heyer l should be 

 noted as having proved themselves sound both in principle 

 and in practice : 



I. As a rule, only those species should be grown in pure forests, 

 which retain or increase the quality or productive capacity of the 

 soil. Such are : 



1. Those which are thickly foliaged, and grow in close 

 canopy. These qualities are pre-eminently exhibited by the 

 Beech, Spruces, Douglas Fir, and Silver Firs, and in a less 

 degree by the Hornbeam, Lime, and Chestnut. Whether or 

 not the Sycamore and the Maple may also be included in this 

 class for Britain, depends to a great extent on the mineral 

 strength and freshness of the soil in each concrete case. 



2. Those lightly-foliaged conifers which are evergreen. The 

 thick, spongy growth of Hypnum mosses that springs up when 

 such woods begin the natural process of thinning themselves, 



1 Der Waldbau, 4th edit. 1891, pp. 30-32- 



