To investigate the influence of forest gap sizes on plant architecture and biomass allocation in young Phoebe bournei to provide a theoretical basis for under-canopy silviculture and stand structural optimization.

Four treatments were established: 4 m×15 m (F1), 8 m×15 m (F2), 12 m×15 m (F3), and an unthinned control (CK). The phenotypic responses of young P. bournei after five years of intercropping under different gap sizes were analyzed, focusing on plant growth, spatial distribution of lateral branches and roots, and biomass allocation.

1) Different gap sizes significantly enhanced tree height and ground diameter, with F2 notably increasing crown width. A significant interaction was observed between gap size and crown position or soil depth for a few phenotypic traits (e.g., level 2 lateral branch length and root tip number), while gap size, crown position, or soil depth individually influenced numerous traits (e.g., level 1 lateral branch length); 2) Compared to CK, F1 and F2 significantly improved a small number of branch configuration indicators in the upper crown, whereas F3 significantly increased length, base diameter, and number of level 1 and 2 lateral branches in the upper crown. Root tip number, length, surface area and volume generally exhibited first increasing and then decreasing to gap sizes across soil depth, peaking under F2. Branch and leaf biomass in the upper crown varied significantly among gap sizes, with F3 performing the best; 3) Within the same treatment, lower canopy branches in CK, F1, and F2 had significantly greater length and angle than the upper canopy, whereas F3 displayed higher branch numbers in the upper canopy. Lower canopy branch and leaf biomass were significantly greater in CK and F1 than in the upper canopy.

Forest gap sizes effectively enhance plant growth and upper canopy branch development in young P. bournei. Among them, F2 significantly promotes height, ground diameter, crown width, length and base diameter of level 1 and 2 lateral branches in the upper canopy, level 2 lateral branch numbers in the lower canopy, root tip number and root length in the surface soil, as well as branch length and angle in the lower canopy significantly greater than those in the upper canopy, achieving more efficient utilization of forest spatial resources and enriching the hierarchy of tree crowns. F3 significantly increases height and ground diameter, enhances branch length, base diameter and number of level 1 and 2 lateral branches in the upper canopy, as well as branches and leaves biomass of the upper canopy, with significantly more branches in the upper than lower canopy, indicating a greater utilization of light resources in the upper canopy and a more uniform canopy structure.

To provide theoretical basis for promoting individual growth and plant configuration of Phoebe bournei seedlings.

Two-year-old P. bournei was selected as the research object, with the complete interaction design of three factors, to explore the effects of soil type (S1 and S2), light intensity (25%, 50%, 75% and 100% light transmittance), naphthylacetic acid concentration (0, 25, 50 and 75 mg/L) and their interactions on its growth and configuration.

1) The height growth rate, ground diameter growth rate, the ratio of aboveground and underground biomass, stem biomass, branch biomass, leaf biomass, crown width, branch number, and leaf number of P. bournei seedlings treated with S2 were significantly better than those treated with S1. The light intensity and hormone levels in S1 significantly affected the biomass of each layer's main stem, 2/4 branch biomass, 2/4 and 3/4 leaf biomass, 2/4 and 4/4 branch number, and 2/4 leaf number. The light intensity and hormone levels in S2 only significantly affected 2/4 and 3/4 leaf biomass, 4/4 branch number, and 3/4 leaf number; 2) Among the two common soil types, P. bournei seedlings performed best under moderate light intensity of 50% or 75%. Low or high light intensity inhibited the accumulation of P. bournei stem biomass, branch biomass, leaf biomass, and leaf quantity. The ratio of aboveground and underground biomass under 75% and 100% light intensity treatments were lower than that under 25% and 50% light intensity treatments; 3) Under the same soil type conditions, there were significant differences in the effects of light intensity and NAA concentration on the growth and configuration of P. bournei, but under the same light intensity, the effect of NAA alone was not significantly different; 4) Among the three factors that had the most significant impact on the growth and configuration of P. bournei, soil type was the most significant, followed by light and its interaction with soil, hormones and their interaction with soil or light, and finally the interaction of the three factors.

The individual growth and plant configuration of P. bournei seedlings treated with soil S2 were significantly better than those treated with soil S1. The overall growth and configuration of P. bournei seedlings treated with 75% light transmittance and 0 mg/L NAA in S2 were superior to other treatments, with the best performance in ground diameter, 3/4 leaf number, and 2/4 leaf biomass. The research results are helpful in screening the key and suitable cultivation conditions for P. bournei seedlings, thereby providing scientific basis for the management of P. bournei seedlings.