Thinning altered the optimum photosynthetic environment in a subtropical coniferous plantation

Thinning offers long-term benefits for planted forests and is an important silvicultural practice. Meanwhile, ecosystem would respond to thinning and the optimal photosynthetic environment would change. Clarifying and quantifying the changing optimal photosynthetic environment induced by thinning is important for accurate prediction of carbon budgets and effective management of artificial forests. Based on six-year continuous in situ observations of carbon fluxes and corresponding environmental factors before and after 25% thinning in a typical subtropical plantation in China, this study revealed that thinning increased the optimum values of key environmental factors (net radiation (Rn), air temperature (Ta), and vapor pressure deficit (VPD)) for gross primary productivity (GPP). Meanwhile, thinning enhanced the ecosystem maximum GPP (GPP_max) corresponding to each single optimum environmental factor. Among them, the highest GPP_max, reaching 0.91 mg CO₂ m^{- 2} s^{- 1}, was found when Rn reached the optimum value and was 13% higher than that before thinning. In addition, the optimum photosynthetic environment configurations (the combination of multiple environmental factors) that could occur in the real world were detected and quantified. Before thinning, the optimal environment configuration was Rn = 692.85 J m^{- 2} s^{- 1}, Ta = 23.43 °C, VPD = 0.96 kPa and SWC = 0.20 m³ m^{- 3}, with a GPP_max of 0.98 mg CO₂ m^{- 2} s^{- 1}. After thinning, the GPP_max increased to 1.11 mg CO₂ m^{- 2} s^{- 1}, and the corresponding optimal photosynthetic environment configuration was Rn = 693.92 J m^{- 2} s^{- 1}, Ta = 26.70 °C, VPD = 1.10 kPa and SWC = 0.21 m³ m^{- 3}. All the values increased after thinning compared with those before thinning, and the changes in the optimal photosynthetic environment might be an important reason for the enhanced photosynthetic capacity of the thinned forests. These results indicated the positive responses of forest to proper thinning and could provide references for the development of forest management policies.