| Popis: |
A deep understanding of the responses of photosynthesis to air temperature (Ta) is essential for accurately assessing carbon budgets. Photosynthetic light response (PLR) parameters, including apparent quantum yield (α), maximum photosynthesis rate (Pmax) and daytime ecosystem respiration (Rd), are widely used to describe and quantify photosynthesis at the ecosystem scale. However, an understanding of the specific responses of PLR parameters to temperature in different seasons is still lacking. Therefore, this study aimed to address this issue based on 17-year continuous eddy covariance measurements (2003–2019) in a subtropical coniferous plantation. The responses of PLR parameters to air temperature (Ta) were obtained by classifying the observed net ecosystem exchange (NEE) and the corresponding photosynthetically active radiation (PAR) according to Ta with a class width of 2 °C. The general regulations showed that α, Rd and Pmax first increased with Ta until the optimal Ta and then decreased. The optimal Ta for α, Rd and Pmax were 28, 30 and 32 °C, and the inflection points were consistent with the soil water content (SWC) at depths of 5, 20, and 50 cm, respectively. For the specific seasonal responses, the optimal Ta for α and Rd was highest in summer, with a value as high as 30 °C, and the optimal Ta was highest in autumn, with a value of 32 °C. Interestingly, although the temperature environments were similar in spring and autumn, the optimal Ta for α was 2 °C higher in spring than in autumn, but the optimal Ta for Pmax was 6 °C higher in autumn than in spring. Some special response patterns were also found according to delicate studies on a seasonal scale, which were found to be related to SWCs at different depths. These findings reflected the inner function changes of the ecosystem and the potential effects of SWC, which should be given special attention when the PLR parameters are used in models to predict the carbon budgets in the future. |
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