Canopy-level stomatal narrowing in adult Fagus sylvatica under O-3 stress - Means of preventing enhanced O-3 uptake under high O-3 exposure?

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. (C) 2014 Elsevier Ltd. All rights reserved.