CHAP, iv.] The Food of Trees 89 



two other systems, as being less exhausting than coppice, 

 though more exacting than high-forest. 



High-forests of timber can never lead to exhaustion or 

 deterioration of the soil, so long as this remains protected by 

 a sufficient leaf-canopy, and is not robbed by wind, or other- 

 wise, of the natural manure obtainable through the decom- 

 position of the dead foliage. On the contrary, and more 

 especially in the case of trees like Scots Pine and Spruce, 

 which make lower demands for potash and lime than other 

 species, it is almost always the case that the nutrient salts 

 contained in an available form within the soil at the end of 

 the period of rotation, i. e. on the fall of the mature timber, 

 are considerably in excess of the quantity that was present at 

 the time of such crop being formed provided always that 

 the fertility or productive capacity of the soil has been duly 

 safeguarded by the maintenance of a good, unbroken canopy 

 of foliage throughout all the life-periods of the crop. For this 

 reason, woods formed of trees like Pines, Spruces, and Firs, 

 recommend themselves for the recuperation of the productive 

 energy of soils that have been allowed to deteriorate by being 

 kept long under lightly-foliaged kinds of trees, like Oak, Ash, 

 Elm, or Larch ; because the broken canopy formed by these 

 light-demanding species is unable, without the aid of under- 

 growth, to protect the surface-soil from the exhausting influ- 

 ences of sun and wind, and from having the accumulated 

 supplies of humus consumed unprofitably by a more or less 

 rank and unremunerative growth of weeds. 



