| Summary: | To better understand the phenotypic plasticity of the highly invasive native weed, <i>Pedicularis kansuensis</i>, we investigated and compared phenotypes (morphology, biomass, and nutrient composition) at different levels of invasion (low: 0 < cover ≤ 30%; medium: 30% < cover ≤ 70%; and high: cover > 70%). With the increase in invasion level, the plasticity of inflorescence length, single-leaf thickness, and specific leaf area increased, while the plasticity of single-leaf area and crown width decreased. During the invasion process, we observed significant density-dependent effects, including changed morphological characteristics, increased total aboveground biomass, and decreased plant height, inflorescence length, root length, crown width, single-leaf area, structure biomass of structures (root, stem, inflorescence), and individual biomass (<i>p</i> < 0.05). During the reproductive period of <i>P. kansuensis</i>, the resource allocation (C, N, and P content, total biomass, biomass allocation) to inflorescence was significantly higher than to root and stem, while the elemental ratios (C:N, C:P, N:P) of inflorescences were significantly lower than those of roots and stems (<i>p</i> < 0.05). When the invasion level increased, the ratio of inflorescence C:N and biomass allocation to roots increased significantly; conversely, inflorescence N and biomass allocation to inflorescences and stems decreased significantly (<i>p</i> < 0.05). This led to a decrease in resource allocation to aboveground parts and more resources allocated to the roots, significantly increasing the root-to-shoot ratio (<i>p</i> < 0.05). Based on the phenotypic differences among different invasion levels, we suggest that <i>P. kansuensis</i> adapted to a competitive environment by regulating morphology, biomass, and nutrient allocation, thereby enhancing the potential of invasion and spread.
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