Canopy-level stomatal narrowing in adult Fagus sylvatica under O3 stress – Means of preventing enhanced O3 uptake under high O3 exposure?

Abstract Spatio-temporally consistent O 3 doses are demonstrated in adult Fagus sylvatica from the Kranzberg Forest free-air fumigation experiment, covering cross-canopy and whole-seasonal scopes through sap flow measurement. Given O 3 -driven closure of stomata, we hypothesized enhanced whole-tree level O 3 influx to be prevented under enhanced O 3 exposure. Although foliage transpiration rate was lowered under twice-ambient O 3 around noon by 30% along with canopy conductance, the hypothesis was falsified, as O 3 influx was raised by 25%. Nevertheless, the twice-ambient/ambient ratio of O 3 uptake was smaller by about 20% than that of O 3 exposure, suggesting stomatal limitation of uptake. The O 3 response was traceable from leaves across branches to the canopy, where peak transpiration rates resembled those of shade rather than sun branches. Rainy/overcast-day and nightly O 3 uptake is quantified and discussed. Whole-seasonal canopy-level validation of modelled with sap flow-derived O 3 flux becomes available in assessing O 3 risk for forest trees.