Bark wounding triggers gradual embolism spreading in two diffuse-porous tree species.

Xylem transport is essential to the growth, development, and survival of vascular plants. Bark wounding may increase the risk of xylem transport failure by tension-driven embolism. However, the consequences of bark wounding for xylem transport are poorly understood. Here, we examined the impacts of the bark wounding on embolism formation, leaf water potential, and gas exchange in terminal branches of two diffuse-porous tree species (Acer platanoides L., Prunus avium L.). The effects of bark removal were examined on field-grown mature trees exposed to increased evaporative demands on a short-term and longer-term basis (6 h vs. 6 d after bark wounding). Bark removal of 30% of branch circumference had a limited effect on xylem hydraulic conductivity when embolized vessels were typically restricted to the last annual ring near the bark wound. Over the six-day exposure, the non-conductive xylem area had significantly increased in the xylem tissue underneath the bark wound (from 22-29% to 51-52% of the last annual ring area in the bark wound zone), pointing to gradual yet relatively limited embolism spreading to deeper xylem layers over time. In both species, the bark removal tended to result in a small but non-significant increase in percent loss of hydraulic conductivity compared to control intact branches six days after bark wounding (from 6% to 8-10% in both species). The bark wounding had no significant effects on midday leaf water potential, CO2 assimilation rates, stomatal conductance, and water use efficiency in leaves of the current-year shoot, possibly due to limited impacts on xylem transport. The results of this study demonstrate that bark wounding induces limited but gradual embolism spreading. However, the impacts of bark wounding may not significantly limit water delivery to distal organs and leaf gas exchange at the scale of several days.