Water taken up through the bark is detected in the transpiration stream in intact upper-canopy branches

Alternative water uptake pathways through leaves and bark complement water supply with interception, fog or dew. Bark water-uptake contributes to embolism-repair, as demonstrated in cut branches. We tested whether bark water-uptake could also contribute to supplement xylem-water for transpiration. We applied bandages injected with 2H-enriched water on intact upper-canopy branches of Pinus sylvestris and Fagus sylvatica in a boreal and in a temperate forest, in summer and winter, and monitored transpiration and online isotopic composition (δ2H and δ18O) of water vapour, before sampling for analyses of δ2H and δ18O in tissue waters. Xylem, bark and leaf waters from segments downstream from the bandages were 2H-enriched whereas δ18O was similar to controls. Transpiration was positively correlated with 2H-enrichment. Isotopic compositions of transpiration and xylem water allowed us to calculate isotopic exchange through the bark via vapour exchange, which was negligible in comparison to estimated bark water-uptake, suggesting that water-uptake occurred via liquid phase. Results were consistent across species, forests and seasons, indicating that bark water-uptake may be more ubiquitous than previously considered. We suggest that water taken up through the bark could be incorporated into the transpiration stream, which could imply that sap-flow measurements underestimate transpiration when bark is wet. © 2022 John Wiley & Sons Ltd.

Special thanks to Beatriz Fernández-Marín and Jose I. García-Plazaola for their help during site selection in Spain, for the fruitful scientific discussions, the logistical support and their contribution to field wok. Thanks also to David Moreno-Mateos, Javier Porras, Asun Rodríguez-Uña and Steven Wohl for their assistance in the field at the Spanish site and to Bastien Frejaville for his help in the laboratory. We also thank Göran Wallin for setting up the GUS system in Sweden and Dr. Rita B. Stangl for the suggestion to use medical bandages for the labelling. Funding was provided by the Spanish Ecological Terrestrial Society (“Ayudas a proyectos liderados por jóvenes investigadores” 2018), the Spanish Ministry of Science (Grant PHLISCO, PID2019-107817RB-I00) and by the Knut and Alice Wallenberg Foundation (Grant #2015.0047). TEG received funding from the Severo Ochoa (CEX-2018-000828) and María de Maeztu (MDM-2017-0714) excellence accreditations awarded to CREAF and BC3, respectively, by the Spanish Ministry of Science.