Bamboo Expansion Affects the Vertical Distribution and Phenotypic Plasticity of Root Systems of Native Cunninghamia lanceolata and Expanding Phyllostachys edulisin Middle-Southern China

Bamboo expansion is widely recognized as a major driver of declining native biodiversity; however, the belowground interactions between expanding bamboo and native tree species remain poorly understood. In this study, we investigated the differences between the expanding species Phyllostachys edulis ( Pe ) and the native species Cunninghamia lanceolata ( Cl ) in terms of vertical root biomass distribution and the phenotypic plasticity of different root orders across various stages of bamboo expansion. Our results showed that as bamboo expansion intensified, Cl exhibited a notable reduction in root biomass density, particularly in the upper soil layers, coupled with increased root allocation in deeper layers. Concurrently, fine roots of Cl displayed significant increases in specific root length (SRL), root length density (RLD), and root specific surface area (RSA). In contrast, the rhizome-based root system of Pe remained relatively stable in biomass but exhibited significantly higher phenotypic plasticity indices for fine roots compared to Cl . Vertically, the two species showed clear root stratification, with Pe dominating the nutrient-rich upper soil layers, while Cl increasingly allocated fine roots to deeper layers. In mixed stands, Cl was consistently outcompeted by Pe in both spatial distribution and morphological plasticity, resulting in compromised nutrient acquisition. These findings provide mechanistic insight into how bamboo expansion alters belowground dynamics and contributes to the decline in native species, thereby reducing community biodiversity from a root system perspective.