Floodplain forests in Estonia: Typological diversity and growth conditions

Flooded forests in 79 sample areas were studied throughout the country. Using multivariate methods, six community types were established: (i) Tilia cordata-Mercurialis perennis (ii) Ulmus laevis-Allium ursinum, (iii) Populus tremula-Convallaria majalis, (iv) Alnus incana-Cirsium oleraceum, (v) Alnus glutinosa-Filipendula ulmaria and, (vi) Alnus glutinosa-Carex acutiformis. The species composition of these types is partly overlapping but the abundance proportions of species are clearly different and all types have several significant (P < 0.05) indicator species. Communities of the first type grow mainly on temporarily overmoistened Dystri-Gleyic Arenosols on floodplain terraces but sometimes also in lower areas close to a riverbed on Eutric Gleysols or Gleyic Fluvisols. The second and third types embrace communities on levees where mainly Eutric Gleysols or Gleyic Fluvisols are represented. Communities of the fourth type are characteristic of low levees or flat areas with Mollic Fluvisols or Eutric Gleysols. The communities of the fifth and sixth types represent floodplain backswamp black alder forests on Eutric Gleysols, Molli-Histic and Histic Fluvisols. The convergence of communities, i.e., a situation where plant communities growing in various hydrological conditions and on different soils can have similar structure and belong to one community type is rather conspicuous for floodplain forests. In the conditions of Estonia, like in the Central European and Scandinavian countries, it is reasonable to consider as floodplain forest not only those growing on Fluvisols sensu stricto but all forests that are almost annually regularly covered with flood-water at least for a couple of weeks; the amount of alluvial sediments is not a decisive criterion in this case.