394 PAST succession: the ceneosere. 



in early postdiluvial time, and reaches down to the present. The lowermost 

 layer consists of water-plants in fragments, followed by a layer of swamp- 

 plants, Phragmites, Cladium, Carex, etc., which are more or less entire. As the 

 peat thus formed reached the water-level or slightly exceeded it, a semi- 

 terrestrial association appeared, particularly alder and meadow woodland. 

 The peat thus formed raised the level so that pines and birches were able to 

 appear and control. The peat finally formed an impervious soil, which in 

 rainy climates produced swamp and pool conditions anew, favoring the devel- 

 opment of Sphagnum, Scheuchzeria, and Eriophorum. Such Sphagneta in- 

 creased in extent, and finally replaced the woodland. They are represented at 

 first by Scheuchzeria or Eriophorum peat, followed by Sphagnum peat. The 

 Sphagnetum m North Germany is the final association of moor-forming 

 vegetation. At one period it was destroyed as a consequence of a drier 

 climate, and replaced by a succession of Cladonietum, Eriophoretum, Callu- 

 netum, and poorly developed woodland. In the following moister secular 

 period Sphagnum again became dominant. Weber terms the layer between 

 the two^phagnum communities the limiting horizon. The term eutrophic is 

 used for swamp-moor formed in water rich in nutrients and oligotrophic for the 

 moors with water poor in nutrients; mesotrophic is applied to the intermediate 

 stages. Development from eutrophic to oligotrophic is typical both of moor 

 horizons and of existing moors. 



Andersson (1908) has summarized his views upon the climate of the post- 

 glacial period as follows: 



At the end of the postglacial period, the temperature increased for a long 

 time, and became considerably higher than at present in Scandinavia. It sank 

 again after this maximum. Upon the basis of the earlier distribution of 

 Corylus avellana, Naias marina, and Trapa natans, the author concludes that in 

 this warm period the growing season was considerably longer than at present, 

 and that it had a mean temperature about 2.5° C. higher. The distribution 

 of Taxus baccata and Hedera helix is thought to show that the winter was more 

 or less the same as at present. This warmer period lasted from the later part 

 of the Ancylus age into the Litorina age. Afterwards a gradual decrease of 

 temperature took place, and has continued to the present. With reference 

 to precipitation in the postglacial period, there is reason to assume that a 

 dry climate prevaOed in eastern Scandinavia during the Ancylus age, as indi- 

 cated by the presence of existing xerophilous plants which invaded Sweden 

 during this more continental period. After the climate of the latter part of the 

 Ancylus age, which was warmer and drier than that of the present, there 

 developed in the Baltic Basin during the Litorina age a similarly warm but 

 much moister period. After the maximum of the Litorina subsidence, the 

 precipitation gradually decreased to the present time. 



De Geer (1908) has based the climatic changes of the late glacial period upon 

 the layers of fluvio-glacial clay found in Sweden: 



Each annual layer of this deposit stretches somewhat further in the direction 

 of the recession of the ice than the preceding one. In different years, the 

 extent in thickness of this layer changes more or less in accordance with the 

 cUmate. The author concludes from this that the climate was still relatively 

 cold when the ice withdrew from southern Sweden, since the annual recession 

 was only about 50 meters. Somewhat farther north the melting proceeded 

 at the rate of 100 to 130 m. per year. The ice was then stationary for 100 to 

 200 years, during which a series of end moraines was formed. This interrup- 



