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

Canopy stomatal ozone (O 3 ) flux ( F st,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 F st,O3 and accumulated F st,O3 ( AF st 0) were significantly higher in wet seasons from April to September (4.62 nmol m − 2 s − 1 and 35.37 mmol m − 2 , respectively) than in dry seasons from October to March (3.90 nmol m − 2 s − 1 and 24.15 mmol m − 1 , respectively), yet comparable between the 2 years of the experiment, being 4.23 nmol m − 2 s − 1 and 58.23 mmol m − 2 in April 2013–March 2014 and 4.29 nmol m − 2 s − 1 and 60.80 mmol m − 2 in April 2014–March 2015, respectively. At the diurnal scale, F st,O3 generally peaked in the early to middle afternoon hours (13:00–15:00), while the maximum stomatal conductance ( G st,O3 ) typically occurred in the middle to late morning hours (09:00–11:00). Monthly integrated AF st 0 reached the maximum in July, although accumulated O 3 exposure ( SUM 0) was highest in October. Seasonally or yearly, the accumulated O 3 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 O 3 and its environmental exposure level; on the other hand, indicated the potential O 3 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 F st,O3 , and the transition from the exposure-based toward flux-based metrics for assessing O 3 risk for forest trees. Further studies are urgently needed to relate stomatal O 3 uptake doses with tree growth reductions for an improved understanding of O 3 effects on trees under natural conditions. [ABSTRACT FROM AUTHOR]