The weed communities have changed dramatically in the 20th century. Because no weed vegetation relevés were recorded for South Moravia early in this century, changes in potential distribution of the most endangered vegetation type, the association Caucalido daucoidis-Conringietum orientalis, are evaluated using records of its diagnostic species. The diagnostic species group of this association was specified by the statistical calculation of fidelity. It includes Scandix pecten-veneris, Caucalis platycarpos, Bupleurum rotundifolium, Thymelaea passerina, Nigella arvensis, and Ajuga chamaepitys. The coincidence distribution maps of these diagnostic species were prepared. The resulting maps show that the potential distribution of Caucalido-Conringietum is the South Moravian region of thermophilous flora and possibly the adjacent regions of mesophilous flora. The incidence of this plant association in the area declined remarkably in the second half of the 20th century, but in the 1990s its diagnostic species appeared again at several sites.
Vegetation classification should reflect the major environmental and phytogeographical gradients that influence species composition. However, the importance of different gradients depends on the geographical scale of particular studies. Locally defined vegetation units usually reflect local gradients, such as disturbance and soil properties, while regionally defined units reflect macroclimatic patterns and different evolutionary and migration histories of large regions. The classification of Central European oak-hornbeam forests (Carpinion alliance) is an example of a widely accepted, broad-scale classification with geographically delimited associations. However, in some cases it fails to describe adequately local vegetation patterns. In the Czech Republic, six associations, based on a broad-scale regional classification, were traditionally distinguished: (1) Melampyro nemorosiCarpinetum – Hercynian association; (2) Primulo veris-Carpinetum – Pannonian association; (3) Carici pilosae-Carpinetum – Carpathian association; (4) Tilio-Carpinetum – Polonian association; (5) Stellario-Tilietum – local association of southern Bohemia; (6) Tilio-Betuletum – ecologically delimited and local association. The goal of this study was to evaluate, using a cluster analysis of a set of 601 geographically stratified relevés, whether the variation in species composition of oakhornbeam forests in the Czech Republic reflects the traditional geographically based classification. Hercynian, Pannonian and Carpathian types of oak-hornbeam forests were reproduced by cluster analysis. No support was found for the Polonian type and the two local associations, StellarioTilietum and Tilio-Betuletum. Instead, a distinct group confined to wet soils emerged. On the basis of this analysis, we suggest a classification that combines ecological and geographical principles and distinguishes four associations: (1) Hercynian (Melampyro nemorosi-Carpinetum or Galio sylvatici-Carpinetum); (2) Pannonian (Primulo veris-Carpinetum); (3) Carpathian (Tilio cordataeCarpinetum or Carici pilosae-Carpinetum); (4) Stellario holosteae-Carpinetum, which is a geographically delimited association of atlantic northwestern central Europe, defined ecologically by its occurrence on wet soils within the Czech Republic.