Limestone Scrub 153 



saxicolous lichens and bryophytes, of ephemeral species 

 as Arenaria serpyllifoUa, Draba verna, and Baxifraga 

 tridactylites, and of dwarf perennials like Sedum acre and 

 Thymus Serpyllum. Such a community does not, except 

 in a topographical sense, belong to a woodland association 

 at all, and is to be regarded as an outlier of another 

 association. 



Limestone screes and cliffs also occur in the midst of 

 the ashwoods. These, if damp, become in time clothed 

 with the vegetation of the ashwoods ; and, by comparing 

 several such localities, it is possible to gain some idea of 

 a progressive succession from bare screes and cliffs to a 

 closed ashwood association. Such a succession supplies 

 the reason, an historical one, why plants like the 

 mossy saxifrage (Saxifraga hypnoides) and the limestone 

 polypody (Phegopteris Bobertiana) are sometimes found 

 on old screes in the midst of existing ashwoods. 



Limestone scrub association. The ashwoods merge 

 imperceptibly into calcareous scrub, and many areas 

 occur which it is difficult to classify either as woodland 

 or scrub. An excellent series of such transitional areas 

 may be seen in Monsal Dale and Cressbrook Dale in 

 Derbyshire. As on the siliceous soils, the scrub assumes 

 many different facies. In some cases, the taller woody 

 plants are more or less isolated, and the ground vege- 

 tation quite grassy. In other cases, dense thickets of 

 shrubs occur, and the ground flora contains many shade- 

 loving species. In the great majority of cases, however, 

 the scrub appears to be of a retrogressive nature, though 

 progressive scrub communities occur here and there, as 

 at the foot of cliffs and on screes. As the scrub contains 

 many woodland species as well as many grassland species, 

 it is often extremely rich floristically, as in the case of 

 the calcareous scrub in upper Cressbrook Dale above 

 mentioned ; but very nearly all the plants of scrub occur 

 in the related woodland or grassland (Plate XIII a). 



