Understory plant diversity and phenolic allelochemicals across a range of Eucalyptus grandis plantation ages

Allelopathy is an important mechanism in Eucalyptus plantations that causes detrimental impacts on understory diversity. Phenolic compounds are the main allelochemicals suppressing understory plants. However, the dynamic changes in phenolic allelochemicals and their relationship with understory diversity with increasing age of Eucalyptus plantations remain largely unclear. In this study, the understory plant diversity was assessed and phenolic compounds identified from leaf litter, roots, and rhizosphere soil samples in a Eucalyptus grandis plantation at two-year intervals for ten years using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The abundance and diversity of under story plant species were lowest in 4-year-old plantations and increased significantly with age. Seven phenolic acids and 10 flavonoids were identified from leaf litter, roots, and rhizosphere soils. Most of the potential phenolic allelochemicals, such as salicylic acid, gallic acid, 4-hydroxybenzoic acid, and epicatechin, were more abundant in younger plantations, especially at 4 years old. The concentrations of phenolic compounds in the rhizosphere zone were significantly lower than in litter and root samples and did not change significantly with an increase in age. Notably, phenolic compounds contributed more to the variation in the understory plants than soil factors. Hydroxyphenyllactic acid, ellagic acid, quercetin, salicylic acid, and 4-hydroxybenzoic acid were the main phenolic compounds explaining the variation in plant diversity with plantation age. These findings indicate that young E. grandis plantations, especially at four years of age, merit a greater focus because of their lower understory plant diversity and higher allelopathic potential.