36 Studies in Forestry [CHAP. n. 



trees, like Beech, Silver Fir, and Spruce, can be grown in pure 

 forests until they attain their full technical and mercantile 

 maturity, without the productive capacity of the soil being 

 endangered, it would be contrary to one of the leading prin- 

 ciples of sylviculture viz. the conservation of the productive 

 capacity of the soil to cultivate pure high forests of light- 

 demanding genera like Oak, Ash, Maple, Pine, or Larch, 

 except under special circumstances where the productive capa- 

 city of the soil is not endangered by insufficient cover, as, for 

 example, on some classes of marshy land where the evaporation 

 caused by insolation and by the free play of winds is directly 

 beneficial, or on low-lying tracts with fresh soil, whose depth 

 and porosity might perhaps be injuriously affected by any accu- 

 mulation of humus. It is solely with regard to the soil-protecting 

 capabilities of the different forest trees that the classification 

 above given of Ruling and Dependent Species is based; for it 

 may be briefly stated that the productive capacity of woodland 

 soil is only safeguarded to the necessary extent when the 

 timber crop consists either of thickly-foliaged species growing 

 in close canopy, and providing the soil with a good layer of 

 leaf-mould, or else of conifers having evergreen foliage, under 

 which a covering of mosses performs the functions of the humus 

 or mould elsewhere. The species of trees naturally fitted to 

 be grown in pure forests under the first of these conditions 

 are pre-eminently Beech, Spruce, Menzies, Douglas and Silver 

 Firs, and in a less degree Hornbeam, Lime, and Chestnut; 

 whilst those falling under the second condition comprise the 

 several varieties of the Pine genus, so long as they are not 

 worked with too long a period of rotation, i. e. so long as their 

 fall is not delayed too long after the time when they sink so far 

 below the normal density of canopy that the growth of mosses 

 gives place to weeds and berries, and the soil begins to 

 deteriorate through insufficient protection against insolation 

 and exhausting winds. Unfortunately the broad-leaved trees 

 best qualified by nature for the formation of pure forests are 



