Contrasting roles of biogeochemical niche conservatism and convergence in shaping growth form similarity during subtropical forest plant species diversification

Understanding how different plant species develop similar growth forms remains a challenge. We analyzed leaf and soil nutrients from climbers, herbs, shrubs, and trees in subtropical karst, valley, and montane forests to evaluate the relative roles of biogeochemical niche conservatism and convergence in shaping growth form similarity. Leaf nutrient concentrations varied significantly among growth forms, suggesting their distinct biogeochemical niche space. Across all growth forms, leaf nutrient concentrations showed a significant phylogenetic signal. However, growth form did not exhibit a significant phylogenetic signal, leading to species with similar growth forms showing dissimilar biogeochemical niches. But certain lineages with shared growth forms displayed a significant phylogenetic signal for specific leaf nutrients, indicating that biogeochemical niche conservatism can drive growth form similarity at the lineage level. The evolution of leaf C and N concentration was most accurately described by stabilizing selection towards multiple opima for different growth forms across forest types, promoting plant stratum specialization. In contrast, the evolution of leaf P, K, Ca, and Mg concentrations was best characterized by stabilizing selection favoring distinct opima for each growth form in montane, valley or karst forest, allowing each growth form to occupy different forest types through its adjustments of optimal leaf nutrient profiles to local soil nutrient availabilities. Our findings thus highlight biogeochemical niche convergence as the primary driver of growth form similarity, underscoring the need for future management to conserve various growth forms and their associated biogeochemical niches to maintain the functional stability in subtropical forests. • Plant species with similar growth forms had their own biogeochemical niche space. • Biogeochemical niche conservatism shaped growth form similarity in some lineages. • Biogeochemical niche convergence was the main driver of growth form similarity.