Effects of Different Cultivation Patterns and Bamboo Stand Densities on Growth and Physiological Traits of Paeonia lactiflora Intercropped Under Phyllostachys edulis Forests

Interplanting Paeonia lactiflora Pall. under Phyllostachys edulis (Carrière) J. Houzeau (moso bamboo) forests forms an innovative agroforestry intercropping system. This practice has great potential to improve economic benefits and maintain ecosystem stability of moso bamboo forests. Nevertheless, the physiological and growth adaptation mechanisms of P. lactiflora in the understory microenvironment of moso bamboo are still poorly understood. In this study, a two-factor field experiment was conducted to examine how moso bamboo stand density (0, 1200, 1800, 2400 plants·ha−1) and cultivation pattern (bag cultivation, field cultivation) shape the growth, root traits, and resource allocation of P. lactiflora. The results showed that under bag cultivation, the low bamboo stand density (1200 plants·ha−1) promoted biomass accumulation in P. lactiflora leaves, stems, and roots, and facilitated root elongation and radial thickening. Under this condition, P. lactiflora adopted a root-prioritized growth strategy and preferentially allocated nitrogen and phosphorus to leaves and roots. For field cultivation, P. lactiflora growth was inhibited by dual stresses of aboveground shading and underground interspecific root competition. To cope with such stress, P. lactiflora prioritized leaf growth, allocated more nitrogen and phosphorus to leaves, and developed thinner roots. This study reveals the differential growth, root plasticity, and resource allocation strategies of P. lactiflora in response to dual stresses of aboveground shading and underground root competition from moso bamboo, filling the research gaps regarding multi-factor adaptation mechanisms of understory herbaceous plants in moso bamboo forests. The optimal undercropping regime combining low moso bamboo stand density with bag cultivation pattern is identified. These findings offer practical references for the efficient and sustainable management of bamboo–peony agroforestry systems and are critical for balancing the economic benefits of moso bamboo stands and maintaining the stability of understory ecosystems.