How does thinning affect the carbon budget of a subtropical coniferous plantation?

To mitigate climate change, China has established extensive plantations since the 1970s, making a substantial contribution to the terrestrial carbon sink. However, after decades of growth, plantations require effective management due to potential ecological risks. While thinning can provide long-term economic and ecological benefits for planted forests, it also imposes significant short-term disturbances that may result in temporary carbon sink losses. This raises a critical question: How does thinning affect forest carbon budgets on Earth? This issue is very controversial, largely due to variations in climate conditions, methodological approaches, and study scales. Consequently, in this study, three methods were used to quantify the effects of thinning on carbon fluxes and provide a theoretical and practical basis for carbon budget estimation and forest management.

The 1-tower, look-up table (LUT), and 2-tower methods were used to investigate the effects of thinning on carbon fluxes. Among them, the 2-tower method is considered the most dependable which tracked the seasonal variation pattern of the carbon fluxes best. The 1-tower method is direct and easy to conduct but may involve estimation biases introduced by climatic interannual variations. While the LUT method could theoretically diminish the effects of the varying climate, it was weak in tracking relatively extreme values. The three methods yielded consistent results, indicating that 25% thinning enhanced gross primary productivity (GPP), net ecosystem productivity (NEP), and ecosystem respiration (Re). According to the 2-tower results, in the first year after thinning, the GPP increased by 9.8%, and it increased more in the second year after thinning, reaching 14.6%. However, a much greater increase in Re was found in the second year after thinning than in the first year, with values of 23.2% and 12.6%, respectively. Consequently, the increases in NEP induced by GPP were offset by increasing Re, which was 5.4% and 0.5% in the first and second years after thinning, respectively.

Thinning enhanced the GPP, Re, and NEP in the subtropical forest, indicating the positive effects of thinning in a near-mature coniferous plantation, even shortly after thinning.