Vessel formation in relation to leaf phenology in pedunculate oak and European ash

Abstract

The earlywood–vessel area of ring-porous species is related to environmental factors that prevailed during the time of vessel formation. However, limited knowledge is available on the time window during which environmental factors such as temperature, precipitation or flooding can be recorded in vessel area. The susceptibility of vessel area to capture these environmental signals is likely to be related to the timing and duration of vessel formation. Ring-porous species are known to have their new earlywood vessels formed before buds break to guarantee efficient water transport through the large earlywood vessels in the outermost tree ring. However, as leaf phenology differs in ring-porous species, also the timing of vessel formation may vary between species. In this study we investigated the timing and duration of earlywood–vessel formation in relation to phenological status in pedunculate oak ( Quercus robur L.) and European ash ( Fraxinus excelsior L.) growing at the same site. Microcores were taken from the stem periphery of ten oaks and five ashes in weekly intervals from March to the beginning of June in 2008. Leaf phenology was recorded in the same period. Vessel size was measured from the microcores to relate dynamics in vessel formation to the leaf phenological phases. We found that vessel formation in ash started well before bud swelling and much earlier as compared to oak. Although oak preceded ash in leaf phenology, earlywood–vessel formation in oak started on average more than 13 days later, but vessels enlarged faster than in ash. The formation of all earlywood vessels was completed in both species before leaves were fully expanded. These results indicate that timing and dynamics in vessel formation, as well as the relation between leaf phenology and vessel formation, can considerably differ between ring-porous species.