Girdling was applied to 5-year-old potted beech individuals of early, intermediate and late phenological forms to block assimilate export from leaves. Phloem severance caused accumulation of soluble carbohydrates and starch in leaves and increased the C/N ratio. While the hexose content increased continuously until the end of the experiment, the sucrose and starch contents peaked earlier, depending on the plant's phenological features. Different rates of chlorophyll degradation and H2O2 and TBARS (thiobarbituric acid-reactive substances) production in different phenological forms implied that phloem girdling was the source of oxidative stress and, depending on the phenological form, accelerated leaf senescence to different degrees. The variable rate of the increase in soluble carbohydrate and starch content, characteristic of the different phenological forms, had different modifying effects on the antioxidant activity in leaves. Compared with the early phenological form, the late form was characterized by a smaller increase in H2O2 and TBARS content and delayed and slowed chlorophyll and carotenoid degradation. In conjunction with the larger increase in the activity of antioxidant enzymes (catalase, ascorbate peroxidase and superoxide clismutase) induced by carbohydrate accumulation and slower carotenoid degradation, these changes led to the late form having greater resistance to oxidative stress and slower senescence.

Genetic diversity is often considered a major determinant of long term population persistence and its potential to adapt to variable environmental conditions. The ability of populations to maintain their genetic diversity across generations seems to be a major prerequisite for their sustainability, which is particularly important for keystone forest tree species. However, little is known about genetic consequences of demographic alterations occurring during natural processes of ecological succession involving changes in the species composition. Using microsatellites, we investigated genetic diversity of adult and offspring generations in beech (Fagus sylvatica L.) and oak (Quercus robur L.) populations coexisting in a naturally established old-growth forest stand, showing some symptoms of ongoing ecological succession from oak- to beech- dominated forest. In general, adult generations of both species exhibited high levels of genetic diversity (0.657 for beech; 0.821 for oak), which, however, depended on the sets of selected genetic markers. Nevertheless, several symptoms such as differences in genetic diversity indices between generations, significant levels of inbreeding (up to 0.029) and low estimates of effective population size (48-80) confirmed the declining status of the oak population. On the other hand, the uniform distribution of genetic diversity indices across generations, low levels of inbreeding (0.004), low genetic differentiation among adults and offspring and, most importantly, large estimates of effective population size (119-716), all supported beech as a successive and successful tree species in the studied forest stand.