Effects of Microplastics on Growth Pattern of <i>Pinus massoniana</i> and <i>Schima uperba</i>

As ubiquitous environmental contaminants, microplastics (MPs) have garnered global concern due to their persistence, bioaccumulation potential, and multifaceted threats to ecosystem health. These particles threaten terrestrial ecosystems via soil contamination; however, research on their phytotoxicity remains predominantly focused on herbaceous plants. The responses of woody plants to MPs and their interspecific differences are severely unexplored. Here, two important ecological and economical tree species in southern China, Pinus massoniana (P. massoniana) and Schima superba (S. superba), were selected to explore the ecotoxicity effects of polyethylene (PE) and polypropylene (PP) MPs (the two most abundant species in the soil) on seedling growth characteristics, specific leaf area (SLA) and biomass allocation at 0%, 1%, 5% and 10% concentration gradients in the 120-day potted experiment. The results showed that the inhibition effect of MPs was concentration and tree species-dependent. Seedling height, basal diameter, and total biomass of P. massoniana decreased significantly with increased concentration, while S. superba showed a non-significant growth effect at 1% concentration. The SLA was generally increased, revealing that plants enhanced their light capture ability through leaf morphological plasticity to compensate for the loss of carbon assimilation. There were interspecific differences in root investment strategies: the root-shoot ratio of P. massoniana was significantly reduced by 48.43% under 10% PP treatment. In comparison, the root-shoot ratio of S. superba was significantly reduced by maintaining a higher root-shoot ratio (65.26% higher than that of P. massoniana on average) and phased resource allocation (5% concentration biomass is higher than 10%) partially alleviated the toxic pressure. Collectively, our results indicate that the ecotoxicity of MPs was mainly driven by concentration and was not correlated with type (PE/PP), while the differences in tree species response were closely related to their resource allocation strategies and morphological plasticity. These findings imply that MPs contamination can differently impact the growth and development of dominant tree species, potentially altering the structure, diversity, and function of forest ecosystems. This study systematically revealed the growth response mechanism of native common tree species to MPs pollution and provided a theoretical basis for sustainable management of plantations and toxicological risk assessment of forest ecosystems.