Calcicolous rock-outcrop lime forests of east-central Europe

We studied the diversity of calcicolous rock-outcrop forest vegetation dominated by lime (Tilia cordata, T. platyphyllos and T. tomentosa) in northern Austria, the Czech Republic, southern Poland, Slovakia, northern Hungary and north-western Romania. This vegetation includes species- rich forests with a mixture of mesophilous and thermophilous forest species, dry grassland species and species of rock outcrops. It is classified in the alliance Melico-Tilion platyphylli of the order Aceretalia pseudoplatani (class Carpino-Fagetea). It is rare in the study area, usually occurring on the upper parts of steep rocky slopes with shallow soil on limestone or other types of base-rich bedrock. Since such conditions are unfavourable for the development of closed-canopy zonal forests, numerous light-demanding relict species occur there (e.g. Dianthus praecox, D. spiculifolius, Primula auricula, Sesleria spp., Tephroseris integrifolia and Viola jooi). Based on the results of unsupervised classification using original (n = 118), previously published (n = 87) and unpublished relevés stored in the EVA database (n = 6; thus 211 relevés in total), we distinguished three phytosociological associations: (i) Tilio platyphylli-Fraxinetum excelsioris occurring in central Slovakia and northern Hungary, (ii) Spiraeo chamaedryfoliae-Tilietum cordatae, a new association recorded in northern and western Romania, and (iii) Seslerio caeruleae-Tilietum cordatae occurring in the Czech Republic, northern Austria, northern Hungary, western Slovakia and also in southern Poland, where we recorded this community for the first time in this country. We created an expert system for automatic classification of these forests, which includes formal definitions of the three associations and seven subassociations.