Leaf morphological classification reveals structure–function continuum in Neosinocalamus affinis: integrating non-destructive area estimation with functional trait variation across geographic gradients

Background Leaf area is a critical functional trait governing photosynthetic capacity in bamboo ecosystems. However, the relationship between leaf morphological structure and multiple functional traits remains poorly understood in clump-forming bamboo species. This study investigated how leaf shape variation, quantified by the length-to-width (L/W) ratio, influences both allometric area estimation and a suite of leaf functional traits in Neosinocalamus affinis (Rendle) Keng f., an ecologically and economically important bamboo species in subtropical China. Results A dataset of 4,497 leaves from 38 sites across seven provinces, and 162 site-level observations of 13 leaf functional traits (leaf area, SLA, LDMC, leaf thickness, SPAD, Fv/Fm, and C:N:P stoichiometry) were analyzed. Leaves were classified into broader (L/W ≤ 5.5), elliptic (5.5 < L/W ≤ 7.5), and slender (L/W > 7.5) categories. The classified fitting model significantly outperformed Montgomery equation and multiple linear regression (R² = 0.886, RMSE = 3.72 cm², lowest AIC = 3560.3). The Montgomery parameter (k) increased systematically from broader (0.450) to slender leaves (0.659). This classification also captured significant functional trait variation: broader leaves exhibited higher SLA (408.8 vs. 335.1 cm²/g), higher nitrogen (30.93 vs. 26.88 g/kg), and lower LDMC (29.9% vs. 35.6%) compared to slender leaves. A strong SLA–LDMC trade-off (r = − 0.850) and N–P coordination (r = 0.590) were consistent with the leaf economics spectrum. Conclusions Morphological classification based on L/W ratio provides a unified framework that simultaneously improves leaf area estimation and reveals functional trait coordination. Leaf shape reflects fundamental resource investment trade-offs. The validated model combined with functional trait characterization provides an integrated tool for bamboo ecosystem assessment.