Physiological carry-over responses across inter-heatwave intervals in two subtropical tree species

Recurrent heatwaves can induce physiological carry-over effects, but how inter-heatwave intervals influence photosynthesis, oxidative stress, and photochemical performance in trees remains poorly understood. We investigated two subtropical tree species, Schima superba and Phoebe bournei , exposed to a single heatwave or two heatwaves separated by intervals of 7, 15, or 23 days. Leaf gas exchange, chlorophyll fluorescence, antioxidant enzyme activities, and malondialdehyde (MDA) content were measured at a common final sampling date. Under the shortest inter-heatwave interval, net photosynthetic rate declined by 50.8% in S. superba and 32.4% in P. bournei relative to the control, with higher photosynthetic rates observed at longer intervals in both species. However, their carbon–water responses contrasted: water-use efficiency was higher at 23 than at 7 days in S. superba , whereas the opposite pattern occurred in P. bournei ( P < 0.001). Oxidative responses also differed between species. MDA content peaked under the 7-day interval in S. superba but under the 15-day interval in P. bournei , while catalase and peroxidase activities exhibited contrasting interval-associated patterns. Chlorophyll fluorescence parameters varied among treatments, although significant species × interval interactions were not detected. Exploratory association analyses further revealed contrasting relationships between antioxidant activity and photosynthetic performance, including a negative catalase–photosynthesis association in S. superba and a positive peroxidase–photosynthesis association in P. bournei . The findings suggest that inter-heatwave interval length is associated with contrasting physiological responses in subtropical trees, particularly in carbon–water relations, oxidative stress, and antioxidant–photosynthesis associations. These results highlight the value of integrating multiple physiological traits when evaluating potential heatwave carry-over effects. However, the lack of independent greenhouse replication and repeated physiological measurements limits treatment-level inference and prevents direct characterization of recovery kinetics or causal mechanisms. Further replicated experiments are needed to test the proposed physiological response patterns.