Objective To discuss the mixed forest model of interplanting precious tree species under Eucalyptus urophylla × Eucalyptus grandis clone plantation with different cutting intensity and analyze the change trend of stand growth and understory vegetation diversity of mixed forest after interplanting, in order to provide a theoretical basis for the transformation of E. urophylla × E. grandis clone plantation into mixed forest of Eucalyptus and precious tree species.

Method According to the split plot experimental design, the transformation experiment of 9-year-old E. urophylla × E. grandis clone plantation in State-owned Dongmen Forest Farm was carried out with precious tree species. Four cutting treatments were set in the main area: A: cutting 1 row every other row (1 E. urophylla × E. grandis + 1 precious species trees; B: cutting 2 rows every other 2 rows (2 E. urophylla × E. grandis + 2 precious species trees); C: cutting 2 rows every other 3 rows (3 E. urophylla × E. grandis + 2 precious species trees); D: cutting 3 rows every other 2 rows (2 E. urophylla × E. grandis + 3 precious species trees); six tree species were used for interplanting, i.e. Castanopsis hystrix Hook. f. & Thomson ex A. DC., Erythrophleum fordii Oliv., Manglietia conifera Dandy, Acacia melanoxylon R. Br., Dalbergia cochinchinensis Pierre ex Laness, and Aquilaria sinensis (Lour.) Spreng., and the control plot without transformation was set up. Each tree in the test area was measured, and the understory vegetation was investigated according to the conventional method at the age of 3.5 years after interplanting.

Result The average tree height, DBH and individual volume of E. urophylla × E. grandis increased significantly under different mixed forest models, with the highest values of 27.63 m, 21.89 cm and 0.38 m³ respectively, which increased by 16.98%, 19.75% and 80.96% compared with the CK. The growth indexes of C. hystrix were the best among the interplanting species, followed by Manglietia conifera and Acacia melanoxylon. The volume of E. urophylla × E. grandis plantation interplanting Acacia melanoxylon in 3 Eucalyptus + 2 precious species trees was the best, reaching 244.87 m³·hm⁻², which was 3.65% higher than CK. In addition, the Simpson indexes of herb layer of 2 Eucalyptus and 2 precious trees and the Shannon-Winner index of herb layer of 3 Eucalyptus and 2 precious species trees interplanting C. hystrix were 0.84 and 1.71 respectively, which were 25.40% and 30.50% higher than that of the CK; while the Simpson index and Shannon-Winner index of shrub layer in the stand with interplanting of D. cochinchinensis were 0.80 and 1.61, respectively, which were 42.9% and 71.27% higher than that of the CK.

Conclusion In this study, the precious tree species were used to transform E. urophylla × E. grandis clone plantation to form a Eucalyptus and precious trees multi-layer mixed forest, which significantly improved the average tree height, DBH and single tree volume, optimized the stand structure, enriched the species diversity of the stand, changed the landscape of a single forest phase of E. urophylla × E. grandis and helped to improve the ecological and economic benefits of E. urophylla × E. grandis clone plantation. The results may provide scientific and technological support for the cultivation of medium- and large-diameter Eucalyptus and the management of national reserve forest.