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We monitored leaf phenology and xylogenesis of 12 Pedunculate oaks in northern Poland in 2014. We hypothesized that the individual trees, which differed in size, age and habitat (tree stand or gap), also diverged in terms of the seasonal patterns of leaf phenology and xylogenesis. The samples used for wood formation observations were collected most frequently during the early leaf phenophases (from March to end of June). The transverse sections of the cambial region were cut with a sledge microtome. We counted the number of cambial cells, measured the width of xylem increment and assessed the timing of xylogenesis and earlywood-vessel formation. We found significant differences in leaf phenology and timing of xylogenesis among individual trees. The smallest differences in wood formation among the trees were observed at the beginning of the vegetation season when the first earlywood vessels were detected (9 days). The dates of completion of the first tangential row of earlywood vessels varied by up to 30 days, while for the completion of the entire earlywood dates varied by up to 32 days. The highest productivity of cambial cells (13 cell layers) was observed around the time of bud swelling at mid-April. In the last days of April, the number of cambial cell layers decreased and subsequently increased again when the leaves were nearly fully expanded at the end of May. To summarize, we observed a high seasonal variability in the number of cambial cell layers. Differences in the time of cessation of cambial activity and xylogenesis amounted to 1 month. We conclude that: (1) oak tree-ring widths and earlywood-vessel sizes and numbers may not be sensitive indicators for early spring temperature and spring defoliation; (2) the missing association between leaf phenophases and xylogenesis as well as the phenological variability may be the reasons for the lack of a clear climatic effect on the above-mentioned parameters. [ABSTRACT FROM AUTHOR]

European beech Fagus sylvatica and Sessile oak Quercus petraea are reaching the north-eastern limits of their natural ranges in northern Poland. According to the projected changes in potential ranges in this region, climatic conditions for both species until 2080 will remain stable. On the other hand, a decrease in the vitality of mature trees and a reduction in their radial growth are currently observed. To understand these contradictory findings, we monitored the cambial activity in both species during two vegetation seasons. 2015 was characterized by a negative water balance, while 2017 was wet. This provided an opportunity to compare how the xylogenesis proceeds in diffuse-porous beech and ring-porous oak during contrasting in the summer precipitation seasons. The forming annual increments were sampled with Presler borer at variable time intervals depending on the leaf phenological phases. The cores were prepared using a sledge microtome and double-stained with safranin and astra blue. Observations of the phases of wood formation and measurements of the width of the forming increments were analysed on the microslide digital images. Differences in the dates of the phenological phases were analysed using Two-Way ANOVA, while the dynamics of the formation of annual increments were fitted to the Single and Double Gompertz function for beech and oak, respectively. The beginning of vessel formation started earlier in both seasons compared to previous studies. Inter-seasonal differences in the duration of spring phenological phases were insignificant for both species, while inter-species differences were significant due to physiological diversity between diffuse-porous beech and ring-porous oak. In dry 2015, cambial activity ceased sooner in both species than in 2017. Differences between oak and beech were insignificant in both years, but statistically significant disparities existed within each species due to varying in precipitation seasons. Rainfall significantly elongated the duration of cambial activity and caused wider tree-rings. According to our study, despite predicted maintaining their climatic niche, both species, situated at the north-eastern cooler range boundary, are expected to produce wood at a lower rate due to the projected warmer central European climate with increased heatwaves and summer droughts. [ABSTRACT FROM AUTHOR]