Bamboo Expansion Affected the Vertical Distribution and Phenotypic Plasticity of Root Systems of Native Cunninghamia Lanceolata and Expanding Phyllostachys Edulis in Middle-Southern China

Bamboo expansion is widely recognized as a major driver of declining native biodiversity; however, the belowground interaction between expanding bamboo and native species remain poorly understood. In this study, four expanding stages (PES, PCM, CPM, and CLS) were established in the native Cunninghamia lanceolata ( Cl)-expanding Phyllostachys edulis ( Pe) interface, and compared with their differences in terms of root vertical distribution and phenotypic plasticity. The results showed that a vertical stratification of root biomass of two species as the expansion intensified, Cl exhibited a reduction in root biomass density, coupled with increased root allocation in deeper layers. Concurrently, fine roots of Cl displayed significant increases in specific root length, root length density, and root specific surface area by expanding effect. In contrast, the root biomass of Pe remained relatively stable but exhibited significantly higher phenotypic plasticity, with dominating the nutrient-rich upper soil layers. These contrasting responses highlight the competitive advantages of P. edulis in belowground niche occupation and reveal the competitive response patterns of expanding moso bamboo and native Chinese fir from the perspective of root system. Active restoration measures should be considered to promote the growth of native trees in bamboo–Chinese fir mixed forest management.