Understory plant species diversity and allelochemicals in rhizosphere soils of Eucalyptus grandis plantations with different densities

Abstract

To explore the influencing factors of understory plant species diversity of Eucalyptus grandis, we examined understory plant species diversity and phenolic allelochemicals in the rhizosphere soils of four-year-old and eight-year-old E. grandis plantations with different densities (1200, 1600, 2000 ind·hm-2) in Danling County of Sichuan Province. The results showed that a total of 45 plant species were recorded, belonging to 33 families and 44 genera. With the increases of stand age and decreases of stand density, plant species richness increased and their importance values were evenly distributed. Phanerophytes was the dominant life form across all stands. Shannon index and Margalef index of shrub in the four-year-old E. grandis plantations increased significantly at the density of 1600 ind·hm-2. All herbage plant diversity index except for Margalef index in four-year-old ones and Pielou index in eight-year-old plantations increased significantly with the decreases of density. Shannon index and Margalef index of shrub in 1200 ind·hm-2 were significantly higher in eight-year-old E. grandis stands than those in four-year-old ones. Five phenolic allelochemicals in the rhizosphere soils of E. grandis plantations were identified. In four-year-old stands, salicylic acid concentrations decreased significantly at the density of 1600 ind·hm-2. The concentrations of chrysin in four-year-old stands reduced significantly with the decreases of density. The concentrations of salicylic acid in eight-year-old stands increased significantly with the decreases of tree density. Salicylic acid concentrations in 2000 ind·hm-2 were significantly higher in four-year-old stands than that in eight-year-old ones, but the result was opposite in stands with density of 1600 ind·hm-2. The concentrations of chrysin in stand with density of 1200 ind·hm-2 were significantly higher in eight-year-old stands than that in four-year-old ones. Rundancy analysis (RDA) result showed that soil pH, bulk density, and the concentrations of soil organic matter, total nitrogen, total phospho-rus, and allelochemicals were the main environmental factors influencing understory plant species diversity. Therefore, prolonging rotation period and reducing stand density would improve micro-environmental condition of forests and buffer the allelopathy of E. grandis, which could facilitate the development of understory vegetation.