# Thinning altered the optimum photosynthetic environment in a subtropical coniferous plantation

## TL;DR

Thinning in a subtropical forest increased the optimal conditions for photosynthesis, boosting carbon uptake and forest productivity.

## Contribution

Quantified how thinning alters the optimal photosynthetic environment and maximum productivity in a subtropical plantation.

## Key findings

- Thinning increased the optimal values of net radiation, air temperature, and vapor pressure deficit for gross primary productivity.
- The maximum GPP increased by 13% after thinning when net radiation reached its optimal value.
- Optimal photosynthetic environment configurations changed post-thinning, with higher GPPmax observed.

## Abstract

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 (GPPmax) corresponding to each single optimum environmental factor. Among them, the highest GPPmax, 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 GPPmax of 0.98 mg CO2 m− 2 s− 1. After thinning, the GPPmax 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.