Nitrogen allocation regulates the relationship between maximum carboxylation rate and chlorophyll content along the vertical gradient of subtropical forest canopy

Abstract Leaf chlorophyll content can be retrieved from satellite observed radiance providing a new perspective for accurately mapping the global pattern of maximum carboxylation rate (Vcmax). However, it is unclear whether species and canopy positions affect the relationship between Vcmax and leaf chlorophyll content. Here, vertical and seasonal variations in leaf Vcmax, chlorophyll and nitrogen (N) content were measured synchronously along canopy profiles of coniferous and broad-leaved trees on a scaffold tower over one year in a subtropical forest ecosystem of south China. We found that the relationship between Vcmax and area-based chlorophyll content (Chlarea) was positive and significant in coniferous but not in broad-leaved trees. Vcmax and Chlarea decreased while the proportion of nitrogen partitioned to light-harvesting chlorophyll components increased from top to bottom layer along the vertical profile of coniferous trees. The proportion of nitrogen partitioned to light-harvesting chlorophyll components mediated the relationship between Vcmax and Chlarea. These findings highlight the crucial role of nitrogen allocation and potentially help to optimize global terrestrial productivity models.