Pedunculate (Quercus robur) and sessile (Quercus patraea) oak, dominant species in European hardwood forests, are declining in many regions throughout Europe and extreme climatic events (summer drought, winter frost) are considered to be key factors contributing to this decline via a negative effect on wood formation. An extensive sampling of scattered oak trees within a landscape of small groves and flower meadows in the White Carpathians, a hilly chain in the warm south-eastern part of the Czech Republic, was undertaken in order to determine the association between growth in diameter and climate over the last 100 years. The association with climate was evaluated by comparing latewood, earlywood and total ring widths with monthly climatic data over the period 1900–2006, using a combination of response function and pointer year analyses. The two approaches clearly showed that late wood growth of oak trees, growing on deep calcium-rich soils, which dry out in summer, is mainly associated with rainfall in May–June, while early wood growth is associated with previous autumn and winter temperatures. Extreme growth years coincided with an abnormally wet or dry May–June periods, which are often associated with cool or hot Junes. Deficient water balances resulting from low rainfall and high temperatures during the summer period are negatively associated with late wood formation and hence total annual growth increment. The results provide support for a crucial role of climate change (decline in rainfall and increase in summer temperatures over the last three decades) among other external factors in the high number of oaks dying prematurely in the White Carpathian wooded grasslands. Prolonged periods of unfavourable climatic conditions cause attenuated trees to become prone to fungal attack and mistletoe hemiparasites, which predispose the oaks to damage or death, especially solitary pedunculate oaks.
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.