Abstract: To better understand the photosynthetic ecophysiological characteristics of Larix principis-rupprechtii and explore the mechanism of photoinhibition, we measured canopy photosynthetic indicators of Larix principis-rupprechtii plantation under intense irradiation using Li-6400 portable photosynthetic instrument, in Saihanba Mechanical Forest Farm of Hebei Province. We analyzed the main drivers of canopy photoinhibition and the relationships between net photosynthetic rate (P_n) and environmental factors. The results showed that under intense irradiation condition, photoinhibition occurred in the upper and middle layers of canopy. The change trend of stomatal limit values (L_s) was opposite to that of the intercellular CO₂ concentration (C_i). The P_n in the upper and middle canopy was positively correlated with relative humidity (RH) (with the correlation coefficients of 0.720 and 0.934, respectively), but negatively correlated with vapor pressure deficit (VPD) and temperature (T) (with the correlation coefficients of -0.850 and -0.820, respectively). The P_n in the lower canopy was significantly correlated with RH, VPD, and T, but their trends were opposite to upper and middle canopy. Our results indicated that the photoinhibition mainly occurred in the middle and upper canopy, with RH, VPD, and T as the main factors driving P_n. The response patterns of P_n to environmental factors were significantly different among upper, middle and lower canopy. The results lay a foundation for regulating canopy structure, improving the microenvironment, and mitigating canopy photoinhibition.

Key words: strong light, stomatal limitation, non-stomatal limitation, environmental factor