Biomass allocation and leaf–root trait coordination of dominant ferns in a subtropical forest

BACKGROUND AND AIMS: The coordinated variation between plant biomass allocation and organ functional traits is central to understanding resource-use strategies. However, relevant studies have focused predominantly on angiosperms, and the whole-plant trait coordination structure in ancient fern lineages remains unclear. METHODS: In this study, we simultaneously measured biomass allocation fractions, stoichiometric traits (C, N, P, C:N, C:P, N:P) and morphological traits (SLA, LDMC, etc.) in both leaves and roots of seven co-occurring subtropical forest fern species. Using correlation analyses at the individual and species levels, partial least squares path modeling (PLS-PM) and Mantel tests, we examined the allocation-stoichiometry-morphology cascade pathway and cross-organ coordination patterns. KEY RESULTS: The results showed that: (1) a significant pathway exists in leaves, with allocation influencing morphology via stoichiometry-higher leaf biomass investment was associated with lower N and P concentrations and more conservative leaf morphology, consistent with the leaf economics spectrum; (2) in roots, the positive direct effect of allocation on morphology was offset by a negative indirect effect mediated by reduced nutrient concentrations, resulting in a near-zero total effect, supporting the multidimensionality of the root economics space; (3) cross-organ coordination was relatively strong for stoichiometric traits (Mantel's r = 0.69) but weak for morphological traits (r = 0.31), indicating dimension-dependent coordination; (4) the above trait relationships were significant at the individual level but mostly disappeared at the species level, suggesting that intraspecific variation may contribute importantly to community functional patterns. CONCLUSIONS: This study reveals a cascade structure of whole-plant trait organization in ferns, provides new evidence for the applicability of the leaf economics spectrum in ancient vascular plants, and highlights the need for experimental validation with a broader sampling scope to establish causality.