Plant communities in broad-leaved deciduous forests in Hordaland county, Western Norway

The vegetation of broad-leaved deciduous forests in Hordaland, Western Norway is described with regards to species composition and structure. The investigation is based on phytosociological analyses of native forest stands dominated by Corylus avellana, Fraxinus excelsior, Ulmus glabra , or Tilia cordata. Two-way-indicator analysis (TWINSPAN) and correspondence analysis (CA) are used to distinguish different vegetation types and assess possible gradients in the vegetation data. The vegetation types are discussed in relation to equivalent forest communities and to syntaxonomical units. Two forest types are described at the first hierarchical TWINSPAN level: 1) A hygrophilous and slightly thermophilous Fraxinus excelsior-Cirriphyllum piliferum forest characterised by an open tree canopy, a dense field layer of tall herbs and ferns, and a high cover of bryophytes. 2) A thermophilous and less hygrophilous Corylus avellana-Brachypodium sylvaticum forest characterised by a dense tree layer, a more open field layer with larger elements of small herbs, and a somewhat lower bryophyte cover. The CA analysis clearly separates the samples from the first TWINSPAN division along the first ordination axis. Five forest types have been described at the second hierarchical level, mainly associated with differences in mesotrophic and eutrophic species, but there is clearly a gradient structure in the species composition between these plant communities. In relation to syntaxonomy, the first TWINSPAN division supports the separation of the west Norwegian broad-leaved deciduous forests into a hygrophilous plant community, the Eurhynchio-Fraxinetum (Blom 1982) Øvstedal 1985 and a drier and more thermophilous community, the Primulo-Ulmetum (Blom 1982) Øvstedal 1985.