Abstract:

Objective: Intensive human activities have resulted in significant changes in aerosols and particulate matter content in the atmosphere, thereby posing an important impact on the total amount and proportion of global diffuse radiation. In this study, based on the clarification of the canopy photosynthetic parameters corresponding to the radiation components, the seasonal dynamics and the main biophysical regulators were explored to gain insight into the effects of diffuse radiation on the gross primary productivity (GPP) of a poplar plantation, providing a reference for the accurate assessment of ecosystem functions and carbon sink of plantations in the future. Method: The carbon cycling processes and biophysical factors of a poplar (Populus × euramericanacv. '74/76') plantation in Gongqing Farmland, Shunyi District, Beijing, were observed for the consecutive four years (2015—2018). A nonlinear fitting method was used to split the ecosystem maximum photosynthetic rate (A_max) and apparent quantum efficiency (α) into two components influenced by direct radiation (A_max,dir, α_dir) and diffuse radiation (A_max,dif, α_dif), respectively. Since the rectangle hyperbola light response model (Michaelis-Menten model) would appear unsaturated phenomenon when simulating GPP, GPP at PAR of 2 000 μmol·m^–2s^–1 was used as the maximum photosynthetic rate (A₂₀₀₀) in this study. Result: In this study, simulated diffuse radiation based on clearness index and solar altitude angle was found to be consistent with the measured values (slope 0.82, R² = 0.87, RMSE = 51.67 W·m^–2, P < 0.01). The α _dif, A_2000,dir and A_2000,dif of this poplar plantation during the growing season showed an increasing trend and then a decreasing trend. The α_dif and A_2000,dif were significantly higher than α_dir and A_2000,dir (P_α = 0.02; P_A2000 = 0.03). The α_dif/α_dir and A_2000,dif/A_2000,dir were 5.6 and 6.1, respectively, and the two ratios showed significant differences in summer. The growing season GPP showed a single-peaked variation pattern during the study period, and GPP was significantly correlated with A_2000,dif only (P < 0.01), and GPP showed a significant increasing trend with increasing PAR _dif/PAR_dir (P < 0.01). In summer, A _2000,dif was mainly influenced by T_a and increased with the increase of T_a (partial correlation coefficient = 0.74, R² = 0.49, P < 0.05); in spring and autumn, LAI dominated the change in A_2000,dif (partial correlation coefficient = 0.69, R² = 0.53, P < 0.01). In contrast, the Prestley-Taylor coefficient ( η) and clearness index (CI) had no significant effect on A_2000,dif in either spring and autumn or summer. Conclusion: α and A₂₀₀₀ show significant seasonal trends. The effects of diffuse radiation on A₂₀₀₀ and α are significantly higher than those of direct radiation, and are more pronounced in the middle of the growing season. The changes of A_2000,dif in spring and autumn, and summer are mainly dominated by biological factors (LAI) and environmental factors (T_a), respectively. A_2000,dif is the most critical factor regulating GPP changes, thus diffuse radiation is the key radiation component affecting photosynthetic productivity in this poplar plantation.

Key words: diffuse radiation, apparent quantum yield, maximum photosynthetic rates, gross primary productivity, poplar plantation