Bark conductance of Scots pine stem decreases with increasing VPD and decreasing soil water content

Bark has lenticels that allow exchange of gases between the stem and ambient air. In non-stressed conditions, the majority of water loss occurs through stomata in leaves. During drought, trees close their stomata or even drop leaves to minimize water loss, and thus uncontrolled water loss through bark can increase the risk of hydraulic failure. We continuously measured bark transpiration in the field from two mature Pinus sylvestris trees with two chambers in each tree over three growing seasons. We quantified bark conductance for water vapor (gb) in different seasons and what drives it. We also analysed how well do gb correlate with total tree conductance during nighttime when stomata in leaves are largely closed. gb of boreal Pinus sylvestris stems varied between 0.25 and 2 mmol m-2 s-1 depending on the environmental conditions and season. We found that gb decreased during both atmospheric and soil drying. gb was the lowest in May, intermediate in summer and highest in September, but gb was more sensitive to VPD and soil water content during the summer months than in spring and autumn. We are not aware of previous field studies on the effect of drought on gb and discuss the possible mechanisms. Changes in gb following water availability can help trees to survive drought even if they cannot actively regulate lenticels in the bark. Similarly, at night, bark water loss was significantly associated with total conductance in boreal pine trees. While previous studies on water loss during drought periods or nighttime have primarily focused on leaf stomatal and cuticular transpiration, incorporating the role of bark water loss could provide a more comprehensive understanding in future research.