Carbon dynamics during long-term starving poplar trees-the importance of older carbohydrates and a shift to lipids during survival.

Carbon (C) assimilation can be severely impaired during periods of environmental stress like drought or defoliation, making trees heavily dependent on the use of C reserve pools for survival; yet, dynamics of reserve use during periods of reduced C supply are still poorly understood. We used stem girdling in mature poplar trees (Populus tremula L. hybrids), a lipid-storing species, to permanently interrupt phloem C transport and induced C shortage in the isolated stem section below the girdle and monitored metabolic activity during three campaigns in the growing seasons of 2018, 2019, and 2021. We measured respiratory fluxes (CO2 and O2), NSC concentration, the respiratory substrate (based on isotopic analysis and CO2/O2 ratio) and the age of the respiratory substrate (based on radiocarbon analysis). Our study shows that poplar trees can survive long periods of reduced C supply from the canopy by switching in metabolism from recent carbohydrates to older storage pools with a potential mixture of respiratory substrates, including lipids. This mechanism of stress resilience can explain why tree decline may take many years until death occurs.