Degeneration of Woodland on Sandy Soil 91 



A brief account of the main features of this process 

 . may be quoted. According to Graebner, 

 of woodland ""when there is a rainfall of 70 cm. 

 (28 inches) or more . . . the surface-layers 

 are being continually impoverished in mineral salts by 

 washing out or 'leaching,' and the typical plants of 

 the forest-floor are thus starved and give way before the 

 invasion of mosses and shade-bearing heath plants. The 

 matting together of the surface-layers of soil by the 

 rhizoids and rootlets of the invaders prevents the access 

 of oxygen to the soil, and leads to the accumulation of 

 ' acid humus ' or ' dry peat ' (Bohhumus) in place of the 

 original mild humus of the woodland soil. Thus we have 

 the formation of a type of wood with a heathy vegeta- 

 tion^ poor in species, on a soil composed of a mixture of 

 sand and dry acid humus or peat. Finally, according 

 to G-raebner, either by the leaching of the soil to such a 

 depth that the roots of the trees can no longer obtain 

 enough food, or by the formation below the surface of 

 a layer of ' Ortstein' (moor pan), i.e. a hard layer of sand 

 bound together by humous compounds, which the roots 

 of the trees cannot penetrate, the rejuvenation of the 

 wood is rendered impossible, the gaps formed by the 

 dying of the old trees are not filled up, and the forest 

 is eventually replaced by heath\" 



If we assume that this secular process of the im- 

 poverishment of sandy soils has been going on since 

 the glacial period, it is easy to see that, acting on a 

 series of soils of varying original constitution, it will 

 produce a vegetation showing many local differences at 

 any given period. 



While some very barren sands may have been origin- 

 ally colonised by heath, others which originally bore 

 woodland may have become occupied by heath before 

 the beginning of the historical period ; others again have 

 1 Mobs, Eankin and Tansley, 1910 : pp. 132, 133. 



