CHAP, ii.] The British Sylva 35 



And, at the same time, from the leaf-canopy formed by the 

 crowns being much more apt to be interrupted here and there, 

 or sometimes quite broken in its continuity, the surface of 

 the soil must be exposed to the action of sun and wind and to 

 the consequent danger of rank growth of weeds, subsequent 

 deterioration, and perhaps final exhaustion. 



This normal density, which should be maintained unbroken 

 or uninterrupted in order that the productive capacity of the 

 soil may be fully utilized, and yet at the same time protected 

 against deterioration, consists of a full or close leaf-canopy. 

 The different species of forest trees are very unequally endowed 

 with capacity to retain close canopy when once they have en- 

 tered on the later stages of development, for they make different 

 demands with regard to individual growing-space, and then the 

 density of the foliage forming the crown also varies with each 

 individual species. To estimate these differences it is only 

 necessary to compare the crown of a Beech or a Silver Fir with 

 that of a Birch or a Larch, although in these cases extremes are 

 purposely chosen in the two classes of broad-leaved trees and 

 conifers. This physical fact, that all trees of forest growth 

 are not alike endowed with the capacity of shading the soil 

 and protecting it against the exhausting influences of sun, 

 wind, and rank growth, is a matter of very great moment from 

 the sylvicultural point of view. Although all our forest trees 

 may be grown in pure woods, if worked with a low rotation, 

 owing to timber crops being naturally dense during the thicket 

 period (that is, till the crop begins to clear itself naturally of dead 

 branches), and during the pole-forest stage of growth (which 

 means till the leading stems have attained a girth of about two 

 feet measured at breast height), yet after that, when once they 

 have entered into the tree-forest or high-forest stage of develop- 

 ment, their natural individual differences with regard to demands 

 for light, i.e. growing-space, and with respect to density of foliage 

 in the interior of the leafy crown, begin to make themselves 

 unmistakably apparent. Thus whilst thickly foliaged kinds of 



D 2 



