Positive interactions between Pinus massoniana and Castanopsis hystrix species in the uneven-aged mixed plantations can produce more ecosystem carbon in subtropical China

Silvicultural practices are an important determinant of tree growth and stand structure. However, the potential advantages of long-term forest management, i.e., mixing valuable non-N2-fixing broadleaf tree species, such as Castanopsis hystrix Miq., with Masson pine (Pinus massoniana Lamb.) to increase stand growth and ecosystem carbon storage, remain poorly understood. In this study, we assessed tree growth, aboveground carbon accumulation, and belowground carbon sequestration in 26-year-old monocultures of C. hystrix (100C26), 26-year-old monocultures of P. massoniana (100P26), 50-year-old monocultures of P. massoniana (100P50), and even- and uneven-aged (50C26 + 50P26 and 50C26 + 50P50, respectively) mixed plantations. Carbon storage within the total ecosystem and in the tree and soil layers of 50C26 + 50P50 mixed plantations was significantly greater than values predicted based on the means of the 100C26 and 100P50 monoculture plantations (p < 0.05). By contrast, differences within the shrub and litter layers were not significant, nor were the differences between observed and expected carbon storage values in the total ecosystem, except in the case of carbon stock in 50C26 + 50P26 mixed plantation soils. These results indicate that plantations of uneven-aged mixtures of P. massoniana and C. hystrix promote carbon sequestration via positive interactions.