Grade-specific biomass allocation and allometric growth strategies in Pinus yunnanensis seedlings

Pinus yunnanensis, an evergreen coniferous species native to southwestern China, possesses considerable ecological and economic importance. In this study, two-year-old P. yunnanensis seedlings were categorized into three distinct grades according to seedling height to investigate variations in biomass accumulation, allocation patterns, and allometric growth relationships. Total biomass gradually decreases with decreasing seedling grades. Aboveground biomass consistently exceeded belowground biomass across all grades. The biomass levels of various organs followed the sequence: leaf biomass > root biomass > stem biomass > sprout biomass, with relatively stable allocation ratios maintained among them. Allometric and isometric relationships between organs, as well as between organs and whole-plant size, varied with seedling grade, reflecting differences in relative growth rates among plant components. Roots and aboveground parts share a common slope demonstrating a general consistency in resource allocation strategy during growth. In contrast, sprouts exhibit isokinetic growth relationships with other organs, showing poorer linear fitting. Correlation network heatmaps further illustrated divergent growth patterns and resource allocation strategies among grades. Most indicators within each grade showed significant positive correlations. Weaker correlations were observed between Grade I and Grade II seedlings, between seedling height and various indicators, and between sprouts and indicators. Conversely, Grade III seedlings exhibited significant negative correlations. These findings reflect resource trade-off mechanisms governing biomass allocation among organs in seedlings of varying grades. Through stable biomass allocation ratios and plastic allometric growth relationships, P. yunnanensis seedlings adapt to varying growth conditions.