BRITAIN. 



259 



Orr (1912 : 209) has described the sere on sand-dunes in Wales: 



The socies are: (a) Ammophila arenaria on shifting and partially fixed 

 dunes; (b) Salix repens on shifting dunes and in dune hollows: (c) Pteris 

 aquilina occupying a large area of fixed dunes. In connection with (a) it is 

 noted that Elymus arenarius, which in Norfolk gives rise to low dunes, is 

 entirely absent from this coast, while the Agropyretum juncei (sea. couch- 

 grass association) of the Somerset, Lancashire, and other dunes is also prac- 

 tically absent. The Salix forms a carpet of low-growing scrub in the sandy 

 dune valleys, but in more exposed places it collects arornid it the blown sand 

 and gives rise to hummocks and smaU dunes, while in the damp hoUows it 

 forms a fringe around the marshy ground and the dependent species vary 

 accordingly: Salix repens represents the second stage in succession on the 

 dunes (as Ammophila represents the first) ; it occurs on the Lancashire dunes, 

 but according to Moss is rare on the Somerset sand-hills. The Pteris, which 

 according to Massart is absent or rare on the Belgian dunes, has here adapted 

 itself to life on the dunes and grows luxuriantly; its rhizomic habit makes it a 

 successful sand-binder, and it forms the last stage in succession observed in 

 this locahty. Mosses and lichens are common on the innermost margin of 

 the fixed dunes and in the transitional associations, and play an important 

 part in preparing the ground for the growth of fiowering plants. 



Smith (1912 : 81) has traced the colonization of "snow-flushes" (bare areas 

 due to deposit by melting snow) : 



On Ben Lawers in Scotland, as in the Alps, the pioneer is the liverwort, 

 Anthelia, though it is probably preceded by algae. The hmnus turf thus 

 formed is invaded by species of Polytrichum, which replace Anthelia to some 

 degree. The mosses are succeeded by Salix herbacea, Alchemilla alpina, etc., 

 and the alpine climax is finally reached. 



Crampton and Macgregor (1913 : 169) have discussed the stable and migrar 

 tory communities of Ben Armine in Sutherlandshire. The moorland plant 

 associations show the following relations: 



A. CUmax types but recessive and reUct . . ii' -^P""^ P^* mosses. 



\Z. Sphagnum aiireoles. 



B. Retrogressive types: 



(a) Badly drained /^ muwomUrium bogs. 



[4. bcirpus-Erio'pkorum bogs. 



(b) WeU drained /^ Cfunamoor. 



[b, Alpme CaUuna moor. 



f7. Grass heaths. 

 C. Initiative types, progressive but local . .{8. Alpine moss heaths. 



Stable moorland . 



I, v9. Sphagneta. 



Migratory types . . D. Moorland flushes. 



The successive zones of the summit are the following: 



(1) Alpine peat mosses with Sphagnum, etc. 



(2) Juncus sguarrosua dominant, CaUuna mat-Uke (closed), Rkacomiirium, Cladina. 



(3) CaUuna mat-like (closed), Carex rigida (abundant), Rhacomilrium, Cladina. 



(4) CaUuna mat-like (closed), Cladina, Empetrum, Lycopodium alpinum. 



(5) CaUuna mat-like in patches or wave-hke from wind erosion, Azalea procumhens, 



ArUennaria dioica, Lycopodium selago, Carex rigida, Lotus comicidatus, Aira 

 flexuosa, Cladina (in CaUuna mat), Hypnum schreberi (in CaUuna mat).' 



