Do we need to care about water loss through bark and bark photosynthesis in trees under changing climate?

In non-stressed conditions, majority of water loss from plants occurs through stomata in leaves. During drought, plants close their stomata or even drop leaves to prevent massive embolism formation that disconnects leaves and above-ground parts hydraulically from roots and can ultimately lead to hydraulic failure. However, plants loose water also through leaf cuticle and bark. While water loss from leaves after stomatal closure has received increasing attention in recent years, water loss through bark has been largely ignored although bark covers 30 to 50% of whole tree surface area. The outer bark layers are practically impermeable to gases and water, but they are pierced by lenticels that provide channel for exchange of water and gas with ambient air to allow oxygen intake for the metabolic processes of the stem. In contrast to active stomatal control in leaves, gas exchange through bark cannot be actively regulated by plants and therefore water loss through bark continues after stomatal closure (together with water loss through leaf cuticle).

Stomatal closure in leaves also reduces photosynthesis; thus, drought can cause both hydraulic failure and carbon starvation and these processes are strongly linked. When leaf photosynthesis is minimized, bark photosynthesis can locally compensate the decreasing leaf photosynthesis. This helps to avoid carbon starvation, because bark photosynthesis utilizes recycled CO₂ released from internal respiration resulting in more efficient carbon fixation in terms of water use. Bark thus plays a role in tree water and carbon balance, and it is crucial to understand the bark water and carbon dynamics in trees under changing climate.

We will show results and discuss three different aspects of bark gas exchange: 1) Drivers and seasonality of water loss and CO₂ exchange through bark. These results are based on continuous stem chamber measurements of Pinus sylvestris in boreal environment. We successfully partitioned stem CO₂ exchange into bark photosynthesis driven by light, respiration driven by temperature, and transport of CO₂ dissolved in xylem sap; 2) Species-specific differences in water loss rate through bark and bark photosynthesis. These results are based on sampling branches from 11 coniferous and 4 broadleaved species grown in a boreal arboretum and comparing their bark characteristics regarding water loss and photosynthesis; 3) The role of water loss through bark in the whole tree water loss in dry conditions. These published results show that water loss rate per bark area was typically ~76% of the shoot transpiration rate (on projected needle area basis) in Pinus halepensis growing in semi-arid conditions but could even surpass the shoot transpiration rate during the highest evaporative demand. Irrigation of trees did not affect bark water loss rate, whereas shoot transpiration was greatly increased due to stomatal control.

The role of bark in tree water and carbon balance is often neglected in research, because its share of the whole tree water and carbon balance is negligible in good growing conditions, but when trees get stressed and stomata in leaves are closed, the role of bark may become dominant.