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Autumn phenology, determined mainly by temperature and photoperiod, is essential for ecosystem carbon sequestration. Usually, the variations in the maximum rate of Rubisco (<i>V</i>_cmax) and the maximum rate of ribulose-bisphosphate regeneration (<i>J</i>_max) are taken as the mechanism regulating the seasonal pattern of photosynthetic rates and autumn phenology. In this study, we used <i>Quercus mongolicus</i> seedlings as an example to examine the photosynthetically physiological mechanism of leaf coloration onset (LCO) responding to different warming and photoperiod treatments based on experimental data acquired from large artificial climate simulation chambers. The results indicated that: (1) LCO and the net CO₂ assimilation rate (<i>P</i>_n), transpiration rate (<i>T</i>_r), stomatal conductance (<i>G</i>_s), <i>V</i>_cmax, and <i>J</i>_max of <i>Quercus mongolicus</i> seedlings were significantly affected by the changes of photoperiod. (2) LCO was significantly correlated only with the <i>P</i>_n approach, supporting the view that leaf senescence is the result of a trade-off between nutrient resorption and reserves. (3) The major variation in stomatal conductance (<i>G</i>_s) is the mechanism by which photoperiod regulates the seasonal pattern of photosynthetic rates, implying that both limitations of stomatal and photosynthetical capacity (<i>V</i>_cmax and <i>J</i>_max, non-stomatal limitation) are able to modulate LCO. Our study riches the knowledge of phenology and provides a reference for phenological modelling and ecosystem carbon estimation.