92 VEGETATION OF THE PEAK DISTRICT [CH. 



on the formation of moor-pan (Ortstein) in sour soils. 

 Graebner pointed out that his explanation does not invalidate 

 Krause's view as a partial explanation. Graebner's theory is 

 a very reasonable one with regard to woods which occur on com- 

 paratively flat plains and plateaux ; but it is scarcely satisfactory 

 with regard to forests on many steep hill slopes, for, in such 

 places, newer and richer soil from below is often exposed by 

 denudation and occasional land slips may bring fresh soils from 

 above. As the great majority of the degenerate woods of this 

 locality are situated on such steep slopes, some additional 

 explanation of forest degeneration must be sought. 



Of course, it is well known that the seedlings of most trees 

 fail to develop under dense shade; and, for this reason, some 

 forests fail to rejuvenate. For example, in the High Engadine, 

 in Switzerland, it has been established by means of long- 

 continued observations that the forests of larch (Larico decidua) 

 which partly cover the slopes and parts of the valleys of this 

 part of the Alps do not everywhere regenerate themselves from 

 seed. The seedlings of larch require abundant light ; and this 

 they do not always find beneath the old forest-growth. But 

 the Arolla pine (Pinus Cembra) finds the conditions of light 

 more favourable to its development. It sows itself abundantly 

 and develops vigorously ; so that under these special and rare 

 conditions, the forest of Arolla pine will succeed the forest 

 of larch without the intervention of man (cf. Flahault and 

 Schroter, 1910; Rubel, 1911). However, no such explanation 

 as this is applicable in the present district. 



A matter which, in my judgment, is not as a rule sufficiently 

 emphasized by plant geographers and foresters is that, in a 

 closed plant community, seedlings, especially seedlings of plants 

 with large seeds such as the oak and beech, are rarely found. 

 On the other hand, open and (to a less extent) intermediate 

 associations, if the general life-conditions are favourable, permit 

 of invasion and rejuvenescence. For example, the elms near 

 Cambridge produced an excessive quantity of fertile seeds in 

 the summer of 1909. Many of these seeds germinated on more 

 or less bare patches of soil, but not on the adjoining closed 

 pasture-land. It follows that a wood whose carpet is fully 

 occupied by closed ground societies does not tend to rejuvenate 

 itself; and, as the more upland ash, oak, and birch woods of 



