Role of Stand Density in Shaping Interactions and Growth Strategies of Dioecious Tree Species: A Case Study of Fraxinus mandshurica

Stand density is a primary limiting factor affecting the accumulation of timber volume, growth, and development of trees in plantations. However, the impact of stand density on the spatial structure and developmental strategies of male and female plants in dioecious tree species remains unclear. In this study, we focused on female, male, and unknown-sex plants of Fraxinus mandshurica across four initial densities (1 m × 1 m, 1.5 m × 1.5 m, 2 m × 2 m, 3 m × 1.5 m). From 2018 to 2022, continuous observations were conducted to determine sex and growth traits (tree height, diameter at breast height, and crown width) with measurements taken annually during the peak growing season. In 2022, in the same season, we measured the morphology and nutrient contents of vegetative organs (shoots, leaves, and absorptive roots) in plants of different genders and assessed the soil properties of their rhizosphere soil. The competition intensity among female plants at high density (D4) increased significantly by 46.32% compared to low density. The gender mingling between female and male plants remained relatively stable across all densities and was greater than 0.7, and the plants occupied a sub-dominant position within their spatial structure. As density increases, the annual growth in height and crown width of female, male, and unknown-sex plants significantly decreases (p ≤ 0.05), while the annual timber volume growth of males and unknown-sex plants also experiences a significant reduction (p ≤ 0.05). Density was a primary factor affecting the ratio of the leaf area, branch thickness, diameter of the absorbing roots, and root tissue density in female and male plants. It also significantly influenced the changes in nitrogen (negatively) and phosphorus (positively) levels within the vegetative organs (p ≤ 0.05). Collectively, these changes were related to the moisture content, ammonium nitrogen, and total phosphorus levels in the rhizosphere soil. These findings emphasize the important of density and spatial structure in shaping the interactions between male and female plants, with the density influencing their growth and reproductive strategies. Research findings provide important insights into the cultivation strategies for dioecious tree species in plantations.