Effects of Understory Interplanting of Broad-Leaved Trees on a Pinus massoniana Plantation’s Biomass and Soil Stoichiometric Characteristics in Southern China

In the context of global warming and declining forest ecosystem services, this study explores underplanting effects on Pinus massoniana stand biomass allocation and soil C–N–P stoichiometry to support sustainable plantation management in southern China. Two underplanting treatments were established: P. massoniana underplanted with Schima superba (denoted by PS), and P. massoniana underplanted with S. superba + Michelia macclurei + Castanopsis hystrix (denoted by PSMC). A pure P. massoniana stand was set as the control (CK). The tree height, DBH and soil C–N–P stoichiometric characteristics were measured, and we compared biomass distribution among the tree layer, understory vegetation layer and litter layer. The results showed that compared with CK and PS, the PSMC significantly promoted the height and DBH growth of P. massoniana and substantially increased the biomass of all tree (leaves, branches, stems, and roots), sustaining higher stand productivity. In contrast, underplanted S. superba inhibited the growth of P. massoniana. Both underplanting treatments significantly reduced understory vegetation biomass, with herbs dominating the understory layer in both treatments. Litter biomass was the lowest under the PS treatment and was significantly lower than others. Compared with CK, both the PS and PSMC treatments significantly increased soil C, N, and P contents as well as soil C:P and N:P ratios in the 0–20 cm and 20–40 cm soil layers. The DBH of P. massoniana was positively correlated with soil C, N, and P concentrations in the 0–20 cm and 20–40 cm soil layers, and it exhibited an extremely significant positive correlation with soil N concentration and soil N:P ratio in the 40–60 cm soil layer. Tree-layer biomass was significantly increased with soil P concentration and the N:P ratio in the 0–20 cm soil layer, and it showed an extremely significant positive correlation with soil C and N concentrations in the 0–20 cm and 20–40 cm soil layers, as well as with soil N concentration and the N:P ratio in the 40–60 cm soil layer. In addition, litter-layer biomass was significantly and positively correlated with soil P concentration in the 20–40 cm soil layer. This association occurs because mixed underplanted broadleaves capture phosphorus released by litter decomposition via their well-developed roots and redistribute P to subsoil, and subsoil P availability controls litter production by regulating tree nutrient status. In conclusion, compared with CK, PSMC significantly improved P. massoniana individual growth and tree-layer biomass, while decreasing understory vegetation biomass and raising the 40–60 cm soil N:P ratio. Deep-soil N:P ratio was positively correlated with tree-layer biomass, implying that PSMC supports high stand productivity.