Effects of different gap sizes on individual growth and plant configuration of Phoebe bournei under Chinese fir forest

<bold>Objective</bold> Gap will regulate the redistribution of light，temperature，water and other resources in the forest environment，thus affecting the growth of seedlings under the forest.This study aims to reveal the influence of gap area regulation on the growth and development of <italic>Phoebe bournei</italic> interplanting under Chinese fir. <bold>Method</bold> Taking 2-year-old <italic>Phoebe bournei</italic> seedlings as the research object，three clear-cut Chinese fir sample plots with spacing configurations of 4 m×4 m（T1），8 m×8 m（T2），and 12 m×12 m（T3）were established.Additionally，an untreated 8 m×8 m stand（CK）was set as the control group to conduct an understory <italic>Phoebe bournei</italic> supplementary planting experiment.After 4 years，the growth，branch and root biomass，branching configuration，root configuration of <italic>Phoebe bournei</italic> were measured and analyzed. <bold>Result</bold> （1）There were extremely significant differences in tree height and breast diameter of <italic>Phoebe bournei</italic> under different gap sizes（<italic>P</italic>&lt;0.01），and significant differences in branch height（<italic>P</italic>&lt;0.05），among which the tree height，breast diameter and crown width reached the highest under T2 treatment，which were 44.9%，51.3%，and 15.2% higher than those of the control group，respectively；（2）Under T2 treatment，the biomass of branches and leaves in the upper and middle layers of the canopy，and the biomass of roots in the depth of 0-20 cm and 20-40 cm reached the peak，which were 109.3%，67.4%，30.0%，56.7%，206.9% and 331.3% higher than those of the control，and the root biomass showed significant（<italic>P</italic>&lt;0.05）and extremely significant difference（<italic>P</italic>&lt;0.01）at the depth of 0-20 cm and 20-40 cm，respectively；（3）Under T2 treatment，the basal diameter of the upper layer，the number of secondary branches in the middle layer and the lower layer of the canopy reached the peak，which were 9.1%，30.5% and 55.3% higher than those of the control，and the basal diameter of the secondary branches in the upper layer and the number of secondary branches in the middle layer showed significant differences（<italic>P</italic>&lt;0.05）；Under T3 treatment，the number of upper branches，primary branch base diameter，middle branch number，lower branch number，primary branch base diameter and branch length of the canopy were 79.7%，10.6%，68.2%，58.3%，12.0% and 28.7% higher than those of the control group；（4）Under T2 treatment，the root surface area in 0-20 cm soil depth，root length and root tip number in 20-40 cm soil depth，root length and root tip number in 40-60 cm soil depth were 92.7%，136.0%，79.2%，61.0% and 44.6% higher than those of the control；Under T3 treatment，the root length，root tip number in 0-20 cm soil depth and root volume in 20-40 cm soil depth increased by 80.5%，127.4% and 231.8% respectively compared with the control. <bold>Conclusion</bold> Under T2 treatment，the comprehensive performance of tree height，breast diameter，crown width，height under branches，number of primary and secondary branches，branch angle，basal diameter，branch length，leaf biomass，branch biomass and root system was the optimal.Gap treatment can effectively promote the growth and development of <italic>Phoebe bournei</italic> in seedling stage，and can significantly promote the growth and configuration of <italic>Phoebe bournei</italic>.