CHAP. LII DWARF-SHRUB HEATH 211 



3. Excess of water usually combined with 



4. Low temperature. 



5. Lack of water (dryness in the warmer season). 



Of these, want of nutrient substances and lowness of temperature in 

 summer are particularly important. Dwarf-shrub heath occurs only in 

 arctic and alpine sites, also in the oceanic west coasts of the cold-temperate 

 zone that are characterized by cool summers. The significance of the 

 nutrient substances contained in soil has been expounded by Grabner l 

 in particular. When soil is relatively fertile, heath passes over into 

 meadow or pasture, as is especially obvious in mountainous places. 

 Extreme drought excludes dwarf-shrub heath ; in the driest spots it 

 is replaced by grass-fields (Weingaertneria) or by lichen-heath. 



ADAPTATIONS AND FLORA 



In many cases the dwarf-shrubs assume a decumbent form such 

 is the case with Betula nana, Salix and Juniperus 2 in arctic heath in 

 other cases this form of growth is normal, for instance, to species of 

 Arctostaphylos. A heath is often a fell-field with a denser vegetation 

 of dominant dwarf-shrubs, and with at least two storeys of vegetation ; 

 but just as in fell-field, moss-heath, and lichen-heath, one finds here 

 also herbs, grasses, mosses, and lichens, and the last two groups often 

 fill the interspaces beneath the dwarf-shrubs. Moss-heath and lichen- 

 heath are thus interspersed in this heath. 



The dwarf -shrubs possess bent, curved, brittle shoots. The majority 

 of them are evergreen, and this is particularly true of the prominent 

 forms such as Calluna, Empetrum, Juniperus, Arctostaphylos Uva-Ursi, 

 and Lycopodium ; but their tint is always dark and brownish green, and 

 more so in winter than in summer. The leaves are closely set, very 

 numerous, small, mostly sclerophyllous, linear and often ericoid. 3 



The shrubs in northern Europe are mainly represented by the following 

 evergreen dwarf-shrubs : 



Ericoid type : Calluna and Empetrum, also in moister places Erica 

 Tetralix. 



Broader coriaceous, flat, entire leaves : Arctostaphylos Uva-Ursi 

 (which occurs particularly in more open spots in the vegetation), Vaccinium 

 Vitis-Idaea, Thymus Serpyllum. 



Pinoid type : Juniperus communis. 



By dwarf-shrubs with thin leaves falling or fading in autumn : Salix 

 repens, and Vaccinium Myrtillus (which, however, occurs rather in forest- 

 soil), also in the northernmost parts, Arctostaphylos alpina, Betula nana, 

 Salix herbacea, S. polaris, S. reticulata, and several others, including, in 

 more southern spots, such representations of switch-plants as Sarothamnus 

 and species of Genista. A number of these deciduous, or at least not 

 evergreen, plants are guarded against excessive transpiration by coatings 

 of grey or white hairs, or of wax. 



By thorny -plants : Species of Genista and Ulex. 



The dominant dwarf-shrubs Calluna, Erica, and Empetrum form 

 tufts and cushions, and possess long-lived primary roots and prostrate 

 shoots that emit roots. 



1 Grabner, 1895, 1901, 1909. * See pp. 26, 38. * See p. no. 



P 2 



