Abstract: Understory plants are an important component of biodiversity in forest plantations and play a key role in maintaining ecosystem functions. To understand the impacts of planting age on understory plants, we conducted a comparative analysis of the composition and diversity of understory plants in the first and second generation Eucalyptus plantations. The interspecific associations of understory plants were evaluated using variance ratio (VR), χ² test and Spearman rank correlation test. The results showed that: (1) The Sorenson similarity index of species in shrub layer and herb layer was high between the first and second generations. The shared species were mainly pioneer species and those with strong disturbance resistance. The dominance of species changed obviously. The ranking of importance value of species changed significantly. The dominant species in the shrub and herb layers of the first generation plantation, Litsea glutinosa and Dicranopteris pedata were replaced by Eucalyptus urophylla and Microlepia speluncae in the second generation plantation. (2) Species richness significantly decreased from the first generation to the second generation plantation, with a significant increase in the species diversity of the herb layer and no significant difference in the shrub layer. (3) The overall species association in the shrub layer shifted from a significant positive association to a non-significant negative association with increasing planting age, while that in the herb layer remained significantly positive but weakened. The proportion of positively associated species pairs decreased, and there was a tendency towards negative associations. Successively planting Eucalyptus plantation exerted a controlling impact on the composition and diversity of understory species, leading to weakened interspecific associations. However, the importance value of certain species, such as Litsea glutinosa, Pteris semipinnata, Microlepia speluncae, and the number of the positively associated species pairs either remained stable or showed a rapid increase in the second generation plantation, indicating that they are of great significance for maintaining understory biodiversity. Our results reveal understory species composition and interspecific associations in plantations, which provides a new perspective for understanding the maintenance of understory biodiversity and the enhancement of ecosystem functions of plantations.

Key words: interspecific association, species composition, plantation age, Eucalyptus plantation, understory plant