CHAP. Lii DWARF-SHRUB HEATH 213 



and it also thrives upon humus-laden soil. Though capable of growing 

 upon moderately good soil it seldom has the opportunity, because it is 

 expelled by other plants. These latter, which are more exacting in their 

 demands, and are more active in anabolism, avoid the more sterile and 

 acid soil of heath and resign it to ling. Here ling becomes a social 

 species and reigns alone for miles. On chalk and marl it rarely grows 

 on good soil, being excluded by competition, but sometimes occurs 

 upon poor calcareous soil, such as on muschel-kalk.^ In a climate of high 

 atmospheric humidity it demands sunlight and open country (in continental 

 sites it grows in the lowlands only within thinly wooded, well-lighted 

 forest), and does not withstand extreme winter-cold combined with 

 drought, for it is sensitive to extreme dryness in any form. One some- 

 times sees the Calluna-vegetation suddenly disappear from large tracts, 

 probably because the plants concerned have attained their limit of age 

 (which is certainly estimated very high at 20-30 years) and are replaced by 

 voung plants, which usually spring up in great numbers. 



It has already been mentioned- that heath-moor often gradually 



passes over into ling-heath. In the moister spots of dwarf -shrub 



heath we often find Erica Tetrahx abundantly mixed with ling. Not 



I infrequently the former species produces associations, and with it often 



appears the majority of those plants characteristic of high-moor. If 



I the climate be sufficiently moist, vSphagnum may grow luxuriantly, and 



I the community easily changes into high-moor. On the other hand, 



I when for any reason Sphagnum-moor becomes drier, then ling gains 



the upper hand. When the dryness becomes excessive ling disappears, 



and only a few easily satisfied mosses, lichens, and Spermophyta are 



dotted over the bare soil. 



' In other places, for instance in northern Norway, communities of 



this kind are encountered. Hult ^ mentions a peculiar ' formation ' 



that is ' perfectly intermediate between heath and moor ' ; its vegetation 



consists mainly of dwarf willows Salix reticulata, S. herbacea, and S. 



polaris also numerous perennial herbs and dwarf -shrubs, such as Dryas, 



Arctostaphylos alpina, Loiseleuria, and Phyllodoce. 



All these occurrences demonstrate that heath and heath-moor belong 

 to the same class of formations. 



On the Alps there are considerable dwarf -shrub heaths, in which several 

 associations are to be distinguished, according to the dominant species : 

 On calcareous Alps, heath formed by Erica camea occurs both in and 

 above the forest-belt ; in the subalpine elfin-wood region of the eastern 

 calcareous Alps is found heath composed of Daphne striata with Polygala 

 Chamaebuxus and Globularia nudicaulis ; at a greater altitude Azalea 

 procumbens produces heath ; in Azalea-heath the layer of raw humus 

 may attain a thickness of half a metre.^ 



In south-eastern Europe, in place of the absent Calluna, appears the 

 ericaceous Bruckenthalia spiculifolia which gives rise to heath.^ 



In arctic dwarf-shrub heath Calluna and Erica play little or no part, 

 but among dwarf-shrubs present are Cassiope tetragona, Vaccinium 



' de Candolle, 1874. ' Chap. XLIX. ' Hult, 1887. 



' See Kerncr, 1863; Christ, 1879; Krasan, 1883; Hayek, 1907; Brockmann 

 Jerosch, 1907, p. 278 ; Schroter, 1904-8 ; Engler, 1901. 

 ' Adamovicz, 1898. 



