Stomatal uptake of O_3 in a Schima superba plantation in subtropical China derived from sap flow measurements

摘要

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 (SUM0) 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.