Response of plant architecture and biomass allocation of under-canopy Phoebe bournei to forest gap sizes

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.