Stomatal uptake of O3 in a Schima superba plantation in subtropical China derived from sap flow measurements

Canopy stomatal ozone (O3) flux (Fst,O3) in a plantation of Schima superba, an ecologically and economically important evergreen pioneer tree species in subtropical China, was quantified based on sap flow measurements during a 2-year period. Mean Fst,O3 and accumulated Fst,O3 (AFst0) were significantly higher in wet seasons from April to September (4.62nmolm−2s−1 and 35.37mmolm−2, respectively) than in dry seasons from October to March (3.90nmolm−2s−1 and 24.15mmolm−1, respectively), yet comparable between the 2years of the experiment, being 4.23nmolm−2s−1 and 58.23mmolm−2 in April 2013–March 2014 and 4.29nmolm−2s−1 and 60.80mmolm−2 in April 2014–March 2015, respectively. At the diurnal scale, Fst,O3 generally peaked in the early to middle afternoon hours (13:00–15:00), while the maximum stomatal conductance (Gst,O3) typically occurred in the middle to late morning hours (09:00–11:00). Monthly integrated AFst0 reached the maximum in July, although accumulated O3 exposure (SUM0) was highest in October. Seasonally or yearly, the accumulated O3 doses, either exposure-based or flux-based, notably exceeded the currently adopted critical thresholds for the protection of forest trees. These results, on the one hand, demonstrated the decoupling between the stomatal uptake of O3 and its environmental exposure level; on the other hand, indicated the potential O3 risk for S. superba in the experimental site. Therefore, the present study endorses the use of sap flow measurements as a feasible tool for estimating Fst,O3, and the transition from the exposure-based toward flux-based metrics for assessing O3 risk for forest trees. Further studies are urgently needed to relate stomatal O3 uptake doses with tree growth reductions for an improved understanding of O3 effects on trees under natural conditions.