Comparative Functional Traits of Bamboo Monospecific Stands and Bamboo–Casuarina Mixed Stands in Coastal Sandy Land Under Different Silvicultural Regimes

Coastal sandy ecosystems face compounded abiotic stresses, including nutrient-poor soils, strong winds, and salt spray, making vegetation restoration highly challenging. Plant functional traits are key indicators of adaptive strategies under environmental stress, yet systematic studies on leaf, twig, and root functional traits of bamboo stands under different silvicultural regimes in coastal sandy land remain scarce. This study examined six stand types on Dongshan Island, Fujian Province: monospecific stands of Bambusa oldhamii Munro, Phyllostachys nidularia f. farcata Wen, and Bambusa tuldoides ‘Swolleninternode’, and their corresponding mixed stands with Casuarina equisetifolia L. (bamboo-to-C. equisetifolia ratio 7:3). Eighteen morphological and structural indices spanning three organ categories, leaf morphology (leaf area, specific leaf area [SLA], leaf tissue density [LTD], etc.), twig structure (wood density, dry matter content, etc.), and root morphology (specific root length [SRL], specific root surface area [SRA], root tissue density [RTD], etc.), were measured and analysed using Pearson correlation and principal component analysis (PCA). Results: (1) Coefficients of variation (CV) for leaf and twig traits ranged from 11.29% to 74.61%; leaf area (CV = 74.61%) and twig wood density (CV = 71.91%) were most variable, indicating high phenotypic plasticity of bamboo in coastal sandy environments. (2) Twig wood density in monospecific stands of B. oldhamii (0.346 g cm−3) was significantly higher than in all other stands (p < 0.05), reflecting a conservative water-transport strategy; mixed stands of B. oldhamii had significantly higher SRL and SRA than other stands (p < 0.05), indicating stronger root resource-acquisition capacity. (3) PCA revealed that leaf area, leaf volume, SLA, LTD, SRL, average root diameter, total root volume, total root surface area, and twig wood density (TWD) were the key traits distinguishing stands under different silvicultural regimes; monospecific stands scored higher overall than mixed stands, reflecting superior leaf, twig, and root functional coordination. Different silvicultural regimes significantly shape the functional adaptive strategies of bamboo in coastal sandy land. Bamboo plants integrate leaf, twig, and root traits in a coordinated, resource-conservative manner to withstand coastal stresses. These findings provide a theoretical basis for bamboo species selection and mixed-stand configuration in coastal shelterbelt management.