Plant water use strategies drive the fate of newly fixed carbon in an experimental rainforest under drought

Nonstructural carbohydrates (NSCs) play a crucial role in plant functioning and survival. Nonetheless, substantial knowledge gaps remain regarding NSC mobilization and transport in forests experiencing more frequent extreme droughts. We combined drought manipulation and 13CO2 pulse-labeling in an enclosed rainforest, and assessed changes in tissue NSC content and allocation of recent photosynthates in eight species that represent ecosystem biomass and cover different positions and hydraulic strategies. Drought reduced starch in leaves but not in stem phloem and roots across species. However, soluble sugars remained constant or increased in understory plants and anisohydric trees, and decreased only in leaves of isohydric trees with relatively constant leaf water potential and sap flow. Drought slowed export and transport of recent photosynthates, not only for anisohydric species with a strong decrease in leaf water potential and sap flow but also for isohydric species with a strong decrease in photosynthetic supply and carbohydrate levels.  We provide evidence that tropical plants under drought mobilize starch to buffer carbon deficiency, while regulating local utilization, export and transport of soluble sugars depending on position and isohydricity. We highlight the importance of plant functional types for understanding NSC dynamics and their role in determining forest carbon balance under drought.