118 TRANSACTIONS OF ROFAL SCOTTISH ARBORICULTURAL SOCIETY. 



re-establisli them is the result of errors in cultivation, of which the 

 chief is the destruction of seedling trees consequent on the grazing 

 of forest land by cattle and sheep. Graebner differs from most of 

 his contemporaries. He regards the above view as too narrow, 

 and considers that the climate and impoverishment of the soil are 

 the chief causes. His arguments are here summarised. The rain- 

 fall of the " Heide," as already stated, is higher than the adjacent 

 Steppe area of Northern Germany, and the soils are generally 

 sandy. Rainfall on a heavy clay, or other soil of close texture, 

 results chiefly in erosion, because the rain-water flows over the 

 surface ; on a sandy soil of loose texture there is more percolation. 

 Rain-water sinking through the soil carries with it dissolved sub- 

 stances, which are either absorbed by the roots of plants or pass 

 below into the subsoil. This action will in time reduce a sandy 

 soil, moderately rich in soluble matter, to a condition when little 

 soluble matter remains. According to Graebner, this condition is 

 indicated by the spontaneous appearance of a heath vegetation. 

 If the area be under forest, the trees assist in the removal of 

 mineral food, but this will be returned to the soil if the trees fall 

 and rot in situ. The exploiting of timber will reduce the material 

 returned, and it is generally assumed that one-third of the mineral 

 matier taken up by the tree is thus removed. Tables are given to 

 show that there is a loss by removal of timber, and analyses by 

 Ramann are quoted to show that fallen leaves do not replenish the 

 soil for tree-growth so much as is frequently stated. Graebner 

 ascribes the disappearance of forest to impoverishment of a 

 naturally open soil by rain, assisted by removal of timber, so that 

 the materials necessary for vigorous growth are no longer available. 

 Thus beech forest, with its subordinate vegetation of plants requir- 

 ing a rich soil, gives place to a heath vegetation of dwarf growth 

 and meagre needs. In the transition it is not uncommon to find 

 a phase with Scots pine or spruce forest, since either of these is 

 less demanding than beech. Graebner (p. 70) admits that the 

 process of impoverishment in the way just described must be slow, 

 and is limited to open soils poor in plant-food to begin with. The 

 development of heath from forest may be accelerated by other 

 changes which produce a condition of poverty in the part of the 

 soil available for plant growth; the most important changes of 

 this kind are accumulation of humus and the formation of moor- 

 pan. 



Humus consists of the debris of plants, which being returned to 



