Divergent Altitudinal Responses and Contrasting Environmental Drivers of Rhizome Chemistry in Two Co-Occurring Polygonatum Species

Forest understorey herbs are an under-studied component of subtropical mountain forest biodiversity, yet they include several genera of high medicinal and economic value. The rhizomes of Polygonatum (Liliaceae) are a prominent example, but the forest-ecological controls on their bioactive composition in wild populations—particularly for co-occurring congeners—remain poorly resolved. We sampled 92 wild plants of Polygonatum cyrtonema and P. filipes along four altitudinal transects (330–1730 m) in a subtropical mountain forest reserve in southeastern China, quantifying total polysaccharide, three flavonoid monomers (rutin, quercetin, and methylophiopogonanone B), and two LC–MS class signals (ΣFlavonoid, ΣSaponin), together with 13 topographic, edaphic, and biotic predictors. The two species displayed the following distinct rhizome chemical phenotypes: P. cyrtonema tended toward higher ΣSaponin; P. filipes toward higher ΣFlavonoid. The clearest pattern was a robust species × altitude interaction for total polysaccharide (p = 0.002), with the two species following opposite altitudinal trajectories. In multivariate forward-selected redundancy analysis, canopy closure and species identity emerged as the only retained environmental predictors, identifying forest light environment as the strongest single environmental correlate of rhizome chemical variation. Species-specific bivariate analyses further revealed contrasting driver hierarchies as follows: P. cyrtonema chemistry tracked topography, whereas P. filipes chemistry tracked rhizosphere soil enzymes and chemistry; only soil temperature and urease activity were shared across species. These results argue that altitude is not a uniform predictor of rhizome chemistry in wild Polygonatum, and support species-specific, canopy-aware management of medicinal forest understorey herbs in subtropical mountain forests.