PURK WOODS OR MIXED WOODS 'i 197 



with such species and treated in such a manner that the 

 yield-capacity of the locaHty is not reduced. On the contrary, 

 it should in many cases be improved. And thus we arrive at 

 the question whether pure or mixed woods are indicated, and 

 in the latter case, how they should be arranged. 



The beneficial effects of a full crop of trees upon the soil are 

 brought about chiefly by the following two agencies : — 



(1.) A dense leaf-canopy formed by the trees which protects 

 the soil against the effects of the sun and air currents. 



(2.) A layer of humus formed by the fallen leaves and 

 certain plants which grow in the shade of trees, such as 

 mosses, which covers the mineral soil and produces a suitable 

 proportion of organic matter. 



These two agencies secure to the soil fertility and, above 

 all, a permanent supply of moisture, without which no crop 

 of trees can thoroughly flourish. Whenever the above two 

 conditions are fully secured, the yield-capacity of the soil is 

 maintained and in many cases improved. Hence the answer 

 to the question before us runs thus : — 



" Only trees irJtich have a fairUi full foliage (Did preserve a 

 good leaf-canopy to an advanced age are fit to be raised in j;?(re 

 tcoods. Species wJiich do not j^o.vsess tJiese qualities should he 

 mixed with trees of the former kind." 



Accordingly, foresters arrange the trees grown for economic 

 purposes into two classes. To the first class of trees fit to be 

 grown in pure woods belong the beech, hornbeam, silver fir, 

 spruce and in a less degree sycamore, Weymouth pine, and 

 Douglas fir. To the second class of trees belong larch, birch, 

 poplar, ash, oak and sweet chestnut. Half-way between the 

 two classes stand Scotch, Austrian and Corsican pine, 

 inasmuch as they benefit the soil up to a certain age, say, 

 forty or fifty years, after which they begin to thin out and 

 join the second class. As a rule, the trees of the first class 

 are shade-bearing, whereas those of the second are light- 

 demanding, in addition to being thin-crowned. It so happens, 

 however, that the second class comprises most of the valuable 



