Developmental stage modulates twig trait variation across local alpine sites in Rhododendron nivale on the southeastern Qinghai-Xizang Plateau

Background and aims Alpine shrubs coordinate leaf display, stem support, and reproduction within short growing seasons, but the role of developmental stage in trait variation among sites and plant–soil associations remains poorly resolved. We examined this question in Rhododendron nivale on the southeastern Qinghai-Xizang Plateau. Methods We sampled 87–93 current-year twigs for each site and developmental stage (540 twigs in total) at three sites located at 4300, 4358, and 4414 m a.s.l. Sampling was conducted during the flowering stage in June 2024 (FG) and the late-growth stage in September 2024 (LG). We measured leaf, stem, and flower traits and seven soil properties. Site × Stage effects were tested with plant-random-intercept mixed-effects models. We also conducted an 18-trait PCA with supplementary soil projections, RDA, and stage-specific path models. Results Significant Site × Stage interactions occurred for NL, ILB, SLA, TD, TB, LI, and SLR. During FG, NF and FB were lower at the 4300-m site than at the 4358- and 4414-m sites. The first two PCA axes explained 48.9% of total trait variation. Soil temperature covaried negatively with Site at both stages (standardized β = −0.866 and −0.902), whereas the Site–SOC association was positive during FG (β = 0.235) and negative during LG (β = −0.351). Conclusion Site-associated differences did not form a single twig trait syndrome. Instead, the covariation of leaf, stem, and flower traits with the local soil environment differed between developmental stages. Developmental stage should therefore be considered when interpreting trait–soil associations in alpine shrubs.