Taking Eucalyptus urophylla × E. grandis plantations as the research object, two stand density treatments including TT (1650 trees/hm²) and FR (1125 trees/hm²), as well as a full sunlight control group (CK), were established. The contents of bioactive components such as total phenols, total flavonoids, bergapten and psoralen in the roots of Ficus hirta growing under the forest were determined, and their in vitro antioxidant activities were systematically evaluated. Untargeted metabolomics was further applied to analyze the composition of secondary metabolites and screen key metabolites significantly associated with antioxidant capacity. The results showed that the TT treatment significantly promoted the accumulation of bioactive components in F. hirta. Compared with CK, the contents of total phenols and total flavonoids increased by 108.3% and 30.2%, respectively. The contents of quality control markers bergapten and psoralen were markedly elevated by 25.40% and 17.6% relative to CK. Meanwhile, the TT group exhibited the highest DPPH radical scavenging activity, ABTS radical scavenging activity and total antioxidant capacity.

A total of 1852 metabolites were identified via metabolomic analysis. The metabolic profile was profoundly altered under TT treatment, with a greater number of differentially expressed metabolites (DEMs) than that under FR treatment. KEGG pathway enrichment analysis revealed that the phenylpropanoid biosynthesis and flavonoid biosynthesis pathways were significantly activated in the TT group. A total of 45 phenolic acids and 20 flavonoids were identified as DEMs, among which 28 phenolic acids were significantly upregulated. Furthermore, 11 phenolic compounds (e.g., 4-methoxycinnamaldehyde, 1,3-O-dicaffeoylquinic acid and gallic acid) and 6 flavonoids (e.g., morusflavone C and 4-hydroxychalcone) were screened out, which were closely correlated with the antioxidant capacity of F. hirta.