Sustainable forestry is one of the most important challenges of the 21st century, which requires the cultivation of species adapted to the changing climate and, in the case of existing species, the selection of climate-resistant propagation materials. European beech (Fagus sylvatica L.) is the most important hardwood tree species in Europe. Its ecological and economic importance is outstanding. The species, however, is significantly affected by the change in climate, so it is essential to select the appropriate propagating material for future cultivation and afforestation. The aim of this work was to measure and evaluate the polyphenolic composition of the leaves of different beech provenances to find relationships between average stem diameter, climate index (EQ), ABTS (2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid) antioxidant capacity and polyphenolic composition to assess the adaptability of the tested beech provenances using factor analysis. According to the results from the altogether 44 identified polyphenols, flavonoid (quercetin, kaempferol) glycosides and flavan-3-ol compounds were the strongest antioxidants, and the two types of compounds probably participate differently in the antioxidant defence system, but both have a significant role. A significant relationship between the concentration of individual polyphenols (Procyanidin B dimer 5, Feruloylthreonic acid, Unidentified 1) and average stem diameter was found: these compounds and factors can be chemical indicators of growth and climatic adaptation and can be used in predicting the future effects of climate change and in the future selection of beech propagation material.