Diversity distribution characteristics and influencing factors of understory herbaceous in Poplar plantations of different ages in the LuXi Yellow floodplain.

To explore the distribution characteristics of plant diversity and its relationships with environmental factors under poplar plantations of different ages in the Luxi Yellow River Floodplain, and to reveal the maintenance mechanisms of plant diversity in planted forests, four poplar plantations with different stand ages (10 y, 30 y, 40 y, 50 y) and a grassland used as the control were selected in Jiucheng Forest Farm, Gaotang County, Shandong Province. Field investigations combined with laboratory analyses were adopted to systematically examine the plant community characteristics and environmental factors under poplar stands of different ages. A total of 21 plant species were recorded in all quadrats, belonging to 9 families and 21 genera. Setaria viridis, Phragmites australis, and Erigeron canadensis were recognized as the dominant species. The highest species rich (12 species) was observed in the control grassland, which was significantly higher than those recorded in all stand ages. Compared with the control grassland, the Shannon-Wiener index and Simpson index of herbaceous plants under poplar plantations of different ages were found to increase initially and then decrease with increasing stand age. The Pielou index reached its maximum value at the stand age of 10 years and decreased gradually thereafter. The Margalef index exhibited slight overall fluctuations and declined slowly after reaching its peak at the standing age of 30 years. No significant differences were detected in soil pH, electrical conductivity (EC), and total nitrogen (TN) among different stand ages. Soil bulk density (SBD) and total phosphorus content (TP) were observed to decrease with increasing stand age, and the minimum values (1.28 g·cm-³ and 0.45 g·kg-¹) were obtained at the stand age of 50 years. Soil water content (SWC), total porosity (STP), soil organic carbon (SOC), available potassium (AK), and TN were found to increase with stand age, and the maximum values (6.36%, 54.68%, 9.44 g·kg-¹, 93.67 mg·kg-¹, and 0.54 g·kg-¹, respectively) were reached at the stand age of 50 years. Results obtained from correlation analysis, multiple regression analysis, demonstrated that canopy closure (CD), stand density (SD), SWC, and SOC were identified as the key environmental factors regulating variations in plant diversity of poplar plantations in the Luxi Yellow River Floodplain.