SCANDIVAVIA. 239 



The fonnerly extensive forests are now represented only by oak scrub, and 

 by subordinate herbaceous species. The heath formation is typically devel- 

 oped in west Jutland, with Calluna as dominant, and Empetrum nigrum, 

 Ardostaphylus uva-ursi, and Cladonia rangiferina next in importance succes- 

 sively. Heath reaches to the sea, where it is covered gradually by the dunes. 

 In the dune vegetation a distinction is made between (1) lower dune levels 

 and valleys, (2) dunes proper and dry valleys, (3) strand. In the first, the 

 sequence from the margin to the wet center is Calluna, Erica tetralix, Myrica 

 gale, and Aira uliginosa. Dune-crests are dominated by Psamma arenaria, 

 and the dry levels by Calluna and Empetrum. In the dunes of Holland, 

 sedges and grasses dominate in the valleys, and scrub frequently appears 

 likewise. 



Kihlmann (1890 : 116) has given a detailed account of the successional 

 relations of bog and tundra in Russian Lapland, and has dealt especially with 

 causes of the dying out of Sphagnum: 



"In accordance with the foregoing viewpoints it appears to us that the 

 entrance into the peat of such species as were excluded by the dryness, is 

 easily xmderstood. It is not necessary to presuppose that the rainfall or 

 humidity has decreased. It indicates that the balance between absorption 

 and transpiration can only be maintained by those leaf-organs which can 

 reduce them to a relatively smaller proportion. This reduction, however, 

 can arise from other causes than an increasing dearth of water. In the occur- 

 rence of pine and Calluna upon the dying Sphagnum moors, I see no more 

 certain proof for the well-known theory of Blytt concerning the alternating 

 wet and dry periods, and according to which we now Uve in a relatively dry 

 period. The visible recession and gradual dying out of the Sphagna in the 

 northern peat moors and their occupation by lichens and mosses which demand 

 less moisture, is very conmion, especially in Russian Lapland. As will be 

 shown below, it is not based upon too slight a rainfall, but upon the physical 

 constitution of the peat and the annual temperature movements. That is 

 to say, the mass of peat is a very poor conductor of heat and the smaller the 

 annual amount of heat in a region, the later will the groimd-ice covered by 

 the peat thaw, or the melting be stopped because of a higher snow-level. By 

 means of continued growth the moss itself becomes a hindrance which isolates 

 the transpiring living surface from the humid subsoil. Effective in the begin- 

 ning only at certain seasons-, the separation extends gradually over the entire 

 vegetative period, and by the continued increase in height of the Sphagnum 

 hummock moves the middle level of the ground-ice more and more upwards, 

 thus making more and more difficult an abundant, not to say a sufficient supply 

 of water from below. The melting of the ground-ice taies place too slowly 

 to compensate for the lack of water. 



"In many cases the dying of the Sphagnum hummock is retarded in local 

 depressions where the water runs off. But independently of this, almost 

 every large peat moor of the northern half of the peninsula shows a similar 

 dying-off in a most extensive degree. Just as little as in the previously 

 described peat hiunmocks do we need to refer to great climatic changes for 

 the explanation of this process. Common experience teaches us that it is 

 connected with the imequal growth in the surface of the Sphagnum moor and 

 coincides with the raising of a Sphagnum hmrmaock by means of growth. 



" Even when we are forced, on the basis of other appearances, to the accept- 

 ance of a complete climatic change in postglacial times, still we can not admit 

 that the Sphagnum hummocks die off as a consequence of diminished precipi- 



