ALNION FORESTS IN LOWER SAXONY (FRG), THEIR ECOLOGICAL REQUIREMENTS, CLASSIFICATION AND POSITION WITHIN CARICI-ELONGATAE-ALNETUM OF NORTHERN CENTRAL-EUROPE

In the diluvial plain of Lower Saxony ( = Niedersachsen) two types of alder fen woods are found: alder swamp woods ('alder carrs') with a more changing water table and inundations in winter, and alder spring woods without hibernal inundation and a more or less constant water table. While all alder swamp woods are Alnion forests, the alder spring woods can be devided into Alnion and Alno-Ulmion forests. All Alnion forests in the diluvial plain of Lower Saxony belong to the association Carici elongatae-Alnetum Schwick. 1933. This association can be devided into three subassociations, C. el.-Al. betuletosum Bodeux 1955, C. el.-Al. typicum Meijer Drees 1936 and C. el.-Al. cardaminetosum Meijer Drees 1936. Low pH-values and high C/N-ratios of the soil are good indicators to distinguish the Carici elongatae-Alnetum betuletosum from the other two subassociations. Humus contents has a tendency to decrease in the sequence C. el.-Al. betuletosum > C. el.-Al. typicum > C. el.-Al cardaminetosum, but is no reliable factor for classification of a single plot. Differences in annual groundwater level variations cause a floristical differentiation only within alderwoods of meso- to eutrophic sites. Here a nearly constant water table or at least no hibernal inundation leads to a Carici elongatae-Alnetum cardaminetosum, while sites which are inundated in winter carry the Carici elongatae-Alnetum typicum. On oligotrophic sites both, swamp and spring woods belong to the Carici elongatae-Alnetum betuletosum. The subdivision of the Carici elongatae-Alnetum for the whole lowlands of northern Central Europe is generally the same as for Lower Saxony. © 1990 Kluwer Academic Publishers.