Declines in oak-forest ecosystems have been reported frequently throughout the world since the beginning of this century. They have been assoeiated with the death of individual trees, of small groups of trees or of entire stands. In this paper, the phenomenon of oak decline is presented in the context of the local site eonditions in seleeted eountries. Oak deeline is regarded as a result of the synergistic aetion between damaging agents, both biotie and abiotie. Periodic changes of the climate in selected experimental plots of Quercus robur stands in Poland were analysed by comparing Walter's climatic diagrams and annual growth rings. Climatic conditions have a major intluenee on the vigour of oaks. An increase in the frequency ot dry years (espeeially when drought occurs at the beginning of the glowing season, e.g. in May-June) has eontributed to the eomplex phenomenon of oak deeline.

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The goal of this study was to investigate the postglacial recolonization pathways of the white oaks Quercus robur and Quercus petrea in Poland, and especially to evaluate the impact of Iberian refugium in this part of Europe. Chloroplast DNA polymorphism of 310 individuals older than 200 years was analyzed. Six haplotypes in total were found to differentiate three maternal lineages: the Balkan (haplotypes 4, 5, and 7), the Apennine (haplotypes 1 and 2), and the Iberian (haplotype 12). The most abundant were members of the Balkan (71.5% of all samples) and the Apennine lineage (23.1%), and only 5.4% of individuals were of Iberian origin. The geographic distribution of the three lineages is clearly structured. The northernmost territories of Poland are occupied by Apennine (haplotype 1) and Iberian (haplotype 12) lineages, whereas samples in central and southern Poland represents the Balkan lineage. The population structure might be the result of competitive colonization among lineages after migration from different refugia. It is likely that colonization of northernmost parts of Poland by the Balkan lineage was halted or at least hampered due to the arrival of the Apennine populations. The most significant result of this study concerns the presence and status of the Iberian lineage in Poland, which is most likely of natural origin.