Element-specific scaling between green and senesced leaf element concentrations in mangrove forests

Nutrient resorption is a key mechanism of nutrient conservation in nutrient-limited mangrove forests. However, conventional resorption efficiency indices are mathematically dependent on both green- and senesced-leaf nutrient concentrations and may obscure how nutrient withdrawal varies with foliar nutrient status. We used an allometric framework to examine green-to-senesced leaf scaling for 12 elements across ten mangrove species in Dongzhaigang National Nature Reserve, China, and tested whether scaling relationships varied with plant functional attributes, tidal position, season, and species identity. Senesced-leaf concentrations increased with increasing green-leaf concentration for all the elements examined. Scaling was predominantly sublinear for nitrogen (N; 0.72), phosphorus (P; 0.56), potassium (K; 0.68), magnesium (Mg; 0.80), iron (Fe; 0.32), manganese (Mn; 0.67), copper (Cu; 0.80), sodium (Na; 0.54), and aluminum (Al; 0.64), indicating that elemental retention in senesced leaves decreased proportionally with increasing green-leaf concentration. In contrast, calcium (Ca; 0.91), zinc (Zn; 0.91), and boron (B; 1.01) showed approximately isometric scaling. Functional and environmental effects were selective: P differed between growth forms; Ca differed by leaf structure and salt-tolerance strategy; Na differed between tidal positions; and Fe, Cu, and B differed between seasons. Species-specific variation occurred only for Mg. These results show that green-to-senesced elemental coupling in mangroves is primarily element specific, whereas functional and environmental factors modify scaling for a limited subset of elements. Combining allometric scaling with conventional resorption metrics can improve understanding of internal nutrient conservation and senesced-leaf elemental composition, with potential implications for litter-mediated nutrient cycling in nutrient-limited mangrove forests.