142 Studies in Forestry [CHAP. vn. 



and which acts like humus or mould in protecting the soil- 

 moisture against the effects of sun and wind, gradually dis- 

 appears when the canopy commences to become very much 

 interrupted and broken. The utilization and reproduction of 

 such pure forests should therefore not be delayed too long. 

 The species coming under this description are the various 

 members of the Pine genus, although Black (Austrian and 

 Corsican), Weymouth, and Maritime Pines are all more 

 densely foliaged, and maintain better canopy, than our indi- 

 genous Scots Pine. 



II. Such species as do not retain the productive capacity of the 

 soil may, however, be groivn in pure forests : 



1. When worked with loiu rotation. All species of forest 

 trees protect the soil during the earlier stages of their develop- 

 ment, because the crop is denser and the crowns of foliage are 

 nearer the ground. This applies, along with other crops, to 

 osier-holts and coppice-woods, for example. 



2. When the productive capacity of the soil is not endangered 

 by imperfect cover. Under this head are classifiable such cases 

 as marshy soil, where evaporation of the moisture by insolation 

 and by the free play of winds is directly beneficial; or as 

 valleys or low-lying tracts whose soil always remains fresh or 

 damp, and whose depth and porosity would only be injuriously 

 interfered with by an accumulation of humus. Alder and 

 Birch are not infrequently to be found on soils of such 

 description, and Ash, Elm, and Oak on those of a better class 

 formed by rich alluvial deposits. 



It may be noted that in the above no provision seems made 

 for such trees as Larch and Oak in general. Pure forests of 

 these species may of course be formed under II. i ; but, if they 

 are to be maintained as pure forest to be worked at the 

 ordinary periods of rotation for such classes of timber, they 

 should certainly be underplanted with a shade-bearing species 

 to protect the soil. Many of the Larch forests of Scotland 



