The stem height–diameter allometric relationship is fundamental in determining forest and ecosystem structures as well as in estimating tree volume, biomass, and carbon stocks. Understanding the effects of silvicultural practices on tree height–diameter allometry is necessary for sustainable forest management, though the impact of measures such as thinning on the allometric relationship remain understudied. In the present study, the effects of thinning on tree height–diameter allometry were evaluated using Masson pine height and diameter growth data from a plantation experiment that included unthinned and thinned treatments with different intensities. To determine whether thinning altered the height–diameter allometry rhythm, the optimal height–diameter model was identified and dummy variable methods were used to investigate the differences among model parameters for different thinning treatments. Periodic (annual) allometric coefficients were calculated based on height and diameter increment data and were modeled using the generalized additive mixed model (GAMM) to further illustrate the response of tree height–diameter allometry to different thinning treatments over time. Significant differences were detected among the parameters of the optimal height–diameter model (power function) for different thinning treatments, which indicated that the pattern of the height–diameter allometry relationship of Masson pine was indeed altered by thinning treatments. Results also indicated a nonlinear trend in the allometric relationship through time which was significantly affected by thinning. The height–diameter allometric coefficient exhibited a unimodal convex bell curve with time in unthinned plots, and thinning significantly interfered with the original trend of the height–diameter allometric coefficient. Thinning caused trees to increase diameter growth at the expense of height growth, resulting in a decrease of the ratio of tree height to diameter, and this trend was more obvious as the thinning intensity increased.

Quantifying the impact of forest management on carbon (C) stock is important for evaluating and enhancing the ability of plantations to mitigate climate change. Near natural forest management (NNFM) through species enrichment planting in single species plantations, structural adjustment, and understory protection is widely used in plantation management. However, its long-term effect on forest ecosystem C stock remains unclear. We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana (Lamb.) and Cunninghamia lanceolate (Lamb.) Hook., to explore the effects of long-term NNFM on ecosystem C storage. The C content and stock of different components in the pure plantations of P. massoniana (PCK) and C. lanceolata (CCK), and their corresponding near natural managed forests (PCN and CCN, respectively), were investigated during eight years of NNFM beginning in 2008. In 2016, there was no change in the vegetation C content, while soil C content in the 0–20 cm and 20–40 cm layers significantly increased, compared to the pure forests. In the P. massoniana and C. lanceolata plantations, NNFM increased the ecosystem C stock by 31.8% and 24.3%, respectively. Overall, the total C stock of soil and arborous layer accounted for 98.2%–99.4% of the whole ecosystem C stock. The increase in the biomass of the retained and underplanted trees led to a greater increase in the arborous C stock in the near natural forests than in the controls. The NNFM exhibited an increasingly positive correlation with the ecosystem C stock over time. Long-term NNFM enhances ecosystem C sequestration by increasing tree growth rate at individual and stand scales, as well as by likely changing the litter decomposition rate resulting from shifts in species composition and stand density. These results indicated that NNFM plays a positive role in achieving multi-objective silviculture and climate change mitigation.

Greenhouse gases are the main cause of global warming, and forest soil plays an important role in greenhouse gas flux. Near natural forest management is one of the most promising options for improving the function of forests as carbon sinks. However, its effects on greenhouse gas emissions are not yet clear. It is therefore necessary to characterise the effects of near natural forest management on greenhouse gas emissions and soil carbon management in plantation ecosystems. We analysed the influence of near natural management on the flux of three major greenhouse gases (carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)) in Pinus massoniana Lamb. and Cunninghamia lanceolata (Lamb.) Hook. plantations. The average emission rates of CO₂ and N₂O in the near natural plantations were higher than those in the corresponding unimproved pure plantations of P. massoniana and C. lanceolata, and the average absorption rate of CH₄ in the pure plantations was lower than that in the near natural plantations. The differences in the CO₂ emission rates between plantations could be explained by differences in the C:N ratio of the fine roots. The differences in the N₂O emission rates could be attributed to differences in soil available N content and the C:N ratio of leaf litter, while the differences in CH₄ uptake rate could be explained by differences in the C:N ratio of leaf litter only. Near natural forest management negatively affected the soil greenhouse gas emissions in P. massoniana and C. lanceolata plantations. The potential impact of greenhouse gas flux should be considered when selecting tree species for enrichment planting.

Understanding the responses of C:N:P stoichiometry and nutrient resorption to thinning is essential to evaluate the effects of management practices on biogeochemical cycling in plantation forest ecosystems. However, nutrient resorption and C:N:P stoichiometry do not always respond in the same way to various thinning intensities, and the underlying mechanisms are not well understood. In this study, we aimed to examine the mechanisms underlying the impacts of thinning on C:N:P stoichiometry in a Pinus massoniana plantation, focusing on interactions among soils, plant tissues (leaves and litter), and soil properties. We conducted four different thinning treatments to determine the effects of thinning on the C:N:P stoichiometric ratios in leaves, litter, and soil in a Pinus massoniana plantation ecosystem. Thinning significantly increased the C, N, and P content of leaves, litter, and soil (p < 0.05). The effects of thinning on C:N:P stoichiometry varied strongly with thinning intensity. Specifically, thinning significantly decreased all C:N:P stoichiometry except leaf N:P and litter C:N (p < 0.05). The N resorption efficiency (NRE) showed no significant change, but thinning significantly decreased the P resorption efficiency (PRE, p < 0.05). This suggests that thinning has inconsistent impacts on N and P cycling in Pinus massoniana plantations. In addition, these different responses suggest that soil physicochemical processes play a crucial role in regulating the effects of thinning. Thinning intensity regulates the biogeochemical cycles of C, N, and P in Pinus massoniana plantation ecosystems by affecting nutrient resorption and soil physicochemical processes. The inconsistent results obtained can be attributed to the complexities of stand environments and the redistribution of site resources following thinning. Therefore, incorporating the effects of thinning intensity into nutrient cycling models may improve predictions related to achieving long-term forest management strategies.

Soil enzymes are involved in the process of mineralization of soil organic matters. The close-to-nature transformation (CNT) of plantations changes the soil enzyme activities by changing the composition of stand vegetation, which in turn affects the change process of soil organic carbon. We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana and Cunninghamia lanceolate, to explore the effects of CNT on soil enzyme activities and soil organic carbon, and the relationship between them is explored through comparative study. Compared with control stands (CCK and PCK), CNT enhanced soil organic carbon; the content of water-soluble organic carbon in the 0–10 cm soil layer of the transformed C. lanceolata plantations (CCN) is 81.29% higher than those in the control stands (p < 0.05); the contents of particulate organic carbon and water-soluble organic carbon in the 10–30 cm soil layer are 95.42% and 48.68% higher than those in the control stands (p < 0.05), respectively; after the CNT, the protease, urease, and acid phosphatase in C. lanceolata plantations were higher than control stands, while protease and catalase in P. massoniana plantations were higher than control stands. Correlation analysis showed that catalase and protease were more closely related to organic carbon components than other organic enzymes; redundancy analysis (RDA) results show that pH and total nitrogen are key factors that cause changes in carbon fractions after the CNT. In general, CNT enhanced soil organic carbon in coniferous plantations, which was more conducive to soil organic carbon accumulation but had a negative effect on soil organic carbon stability to a certain extent. Therefore, the effect of tree species configuration on soil carbon stability components should be considered in the forest management practice.

The soil profile is a strong and complex physicochemical gradient that greatly affects bacterial community structure and function between soil layers. However, little is known about molecular ecological network structure and bacterial community function under differing soil profiles in planted forests. Four typical native tree species (Pinus massoniana Lamb., Castanopsis hystrix Miq., Mytilaria laosensis Lec., and Michelia macclurei Dandy) plantations were selected from subtropical China as the research object. We evaluated molecular ecological network structure as well as potential function of the soil bacterial community at different soil depths (0–20, 20–40, and 40–60 cm) within native tree plantations. Our results showed that (1) compared to the topsoil (0–20 cm), the bacterial molecular ecological network scale increased within the middle layer (20–40 cm) and the subsoil (40–60 cm), and the interaction between species was stronger; (2) module hubs and connectors were the key bacterial groups in each soil layer and increased with increasing soil depth; (3) the dominant functional groups of the bacterial communities in each soil layer were chemoheterotrophy, aerobic chemoheterotrophy, cellulolysis, ureolysis, nitrogen fixation, and nitrate reduction, and they were related to soil carbon and nitrogen cycling; and (4) the different molecular ecological network structures along with relative bacterial functional group abundances among diverse soil layers were mainly affected by soil organic carbon (SOC), NO₃⁻-N, NH₄⁺-N, available phosphorus (AP), and total phosphorus (TP). Our study provides a theoretical foundation for bacterial community structure together with function within soil profiles of native tree plantations in subtropical regions.