210 OXYLOPHYTES SECT, vi 



CHAPTER LII. DWARF-SHRUB HEATH 



BY the term heath, 1 so far as it concerns northern Europe, is meant 

 a treeless tract that is mainly occupied by evergreen, slow-growing, small- 

 leaved, dwarf-shrubs and creeping shrubs which are largely Ericaceae 

 (ericaceous heath). The vegetation varies in height according to the 

 temperature, humidity, and light prevailing, and often rises to thirty 

 centimetres or more, but often only to ten or twenty ; on the one hand 

 it may be so dense that the soil is invisible, but on the other so open 

 that the soil is partially bare, and leaves space between the shrubs or 

 other plants. 



The stunted and xerophilous character of the vegetation is due to 

 climate and soil, but particularly to the latter. 



The vegetative season is usually dry, and transpiration may then 

 be intense, even though the prevalence of heath demands a certain atmo- 

 spheric humidity. In spring (May and June) the atmospheric humidity 

 is at its minimum, at least in Denmark. During summer, periods 

 of drought are frequent, and the air hangs quivering over the hot 

 surface of the heath. During winter in the northernmost sites, cold 

 and drought together with storms play an important part in reference 

 to the evergreen plants. Winds blow with great violence over the dry 

 surface, upon which dwarf-shrub heath is wont to occur. 



The nature of the soil is of far greater import than is the climate. 

 The soil is for the most part a sterile quartz-sand, 2 which has been 

 thoroughly elutriated and deprived of nutrient substances since the Glacial 

 Epoch ; some of the Spermophyta present occur especially on sandy soil. 

 This soil has its characters greatly changed by a more or less thick layer of 

 raw humus deposited over it by the heath vegetation. 3 Calluna and Vacci- 

 nium Myrtillus are among the plants flourishing luxuriantly on raw humus 

 in which heather-peat is formed by their matted roots, as well as by the 

 rhizoids of mosses and hyphae of Cladosporium and the like. The layer of 

 raw humus greedily takes up moisture, retains it firmly for a long time, 

 prevents evaporation from the soil, and obstructs the entrance of air, 

 thus occasioning the manufacture of humous acids that retard the absorp- 

 tion of water by roots. In times of drought this layer, owing to its dark 

 colour, may easily be heated and robbed of moisture. If the process 

 of drying is carried far the ling disappears and lichen-heath usually 

 arises, for example in North Germany. 



The production of raw humus must be regarded as the most character- 

 istic peculiarity of heath. The acid soil excludes plants belonging to 

 forest or to other formations in the temperate zone, and thus gives absolute 

 dominance to heather. The conditions necessary for the production of 

 raw humus are therefore those essential to the occurrence of heath. 

 Ramann 4 sets these conditions forth as follows : 



1. Want of nutrient substances (these are required by organisms 

 responsible for decomposition). 



2. Exclusion of air (in most cases only when the soil has been 

 covered with water for a long time). 



1 See Focke, 1893 ; E. H. L. Krause, 18920; Grabner, 1895, 1901, 1909. 

 1 See p. 59. ' See p. 62. * Ramann, 1905, p. 



159. 



