26 5 



CHAPTER LXX 



STATIONARY, OR GREY, SAND-DUNE. SAND-FIELDS. DUNE- 

 HEATH. DUNE-BUSHLAND. DUNE-FOREST. OTHER 

 EXAMPLES OF PSAMMOPHILOUS VEGETATION 



Grey Dune and Dry Sand-field in North Europe. 



WHEN wind does not disturb the dune other plants may settle between 

 the shoots of Psamma arenaria and Elymus arenarius ; the more effec- 

 tively do these two species maintain the sand in a stationary condition 

 the more do they prepare the soil for other species and for their own 

 extinction. There now appear shorter plants which have less vigorous 

 subterranean organs, are more confined to their point of fixation, are 

 annuals or perennials, and cannot endure being buried by much sand. 

 The vegetation continues to become closer, as mosses (Polytrichum, 

 Ceratodon purpureus, Grimmia and others), lichens, and some Schizo- 

 phyceae, establish themselves and permeate the sand with their rhizoids 

 or thalli : x the soil thus becomes firmer and more densely clothed with 

 vegetation. Perennial species of caespitose habit or possessing a multi- 

 cipital primary root can at this stage establish themselves here, so that 

 the soil ultimately is covered with a low, dense, greyish-green carpet. 2 

 The grey dune has arisen. The space thus becomes too confined for the 

 two tall species of dune-grasses, whose long vegetative shoots carry on 

 a prolonged struggle for life and this is particularly true of the 

 sea-marram only to succumb in the end. 



Next to dunes, and often in the interior of countries, for example 

 in Jutland and North Germany, are found extensive flat or undulating 

 expanses whose soil is dune-sand or glacial sand. On such soil ' heather- 

 less sand-field ' 3 arises. Such dry sand-fields on inland spots in 

 northern and central Europe are, for the most part, a product of cultiva- 

 tion. Sand-field arises on old heath-soil poor in nutriment and is re- 

 converted into heath, unless such a change is prevented by human agency. 

 It includes essentially the same species as grey dune, as well as some 

 species common to dwarf-shrub heath ; the species are accommodating 

 and generally fitted to endure prolonged drought. As the growth-forms 

 present seem to be like those on grey dune, sand-field may be combined 

 in the same formation with grey dune. 4 



OECOLOGICAL FACTORS 



The vegetation is xerophytic in character. The necessity for this 

 xerophytism has already been explained on p. 59 in connexion with 

 the account then given of sandy soil ; but additional factors contributing 

 to the same result are the intense light and heat, and strong winds, all of 

 which tend to accelerate transpiration. 



1 Grabner, 1 895 a. 



1 In regard to the Flora in northern Europe see Warming, 1891, 1907-9 ; 

 Buchenau, 18890, 18896. 



3 Grabner, 1895 a, I 9 I J Massart, 1908. 



4 Regarding sand-field: in Denmark, see Warming, 1891, 1907 ; in Bohemia, 

 see Domin, 1904, 1905 ; in Servia, see Adamovicz, 1904. 



