Light mediates within shoot trait variations and coordinations and biomass allocation across plant sizes

Investigating differences in trait expression and biomass allocation within shoots of different plant sizes contributes to predicting long-term dynamics in forest ecosystems. Size effects may be partly explained by differences in light to which adults and saplings are exposed. Here, we incorporated saplings growing in canopy gaps to partially disentangle the effects of light from those of plant sizes. We sampled current-year shoots from adults, saplings in gaps, and saplings in understory and measured twig, petiole, and lamina traits as well as biomass of each structure. We found that acquisitive traits, such as specific twig length (STL) and lamina phosphorus content (LP) of saplings in gaps were comparable with other two groups, whereas conservative traits, including lamina carbon content (LC), lamina non-structural carbohydrates (LNSC), and petiole dry matter content (PDMC) were mainly influenced by plant size. Light drove lamina traits toward a more conservative strategy, but shifted twig traits toward a more acquisitive strategy. Traits within shoots were defined by two axes, comprising a structural investment axis driven by plant size and a metabolism axis of leaves driven by both plant size and light. Biomass allocation between twig mass (TM) and total leaf mass (TLM) varied with light, but the scaling relationship was determined by plant size, with saplings exhibiting an allometric scaling while adults showing an isometric scaling. Our results demonstrate that plant size effects on trait expression and biomass allocation within shoots is related to light conditions of saplings. Light should be explicitly considered when comparing individuals of different plant sizes.