Probability distributions of non-structural carbon ages and transit times provide insights in carbon allocation dynamics of mature trees

In trees, the use of non-structural carbon (NSC) under limiting conditions impacts the age structure of the NSC pools. We compared model predictions of NSC ages and transit times for Pinus halepensis Mill., Acer rubrum L. and Pinus taeda L., to understand differences in carbon storage dynamics in species with different leaf phenology and growth environments. We used two carbon allocation models from the literature to estimate the NSC age and transit time distributions, to simulate carbon limitation, and to evaluate the sensitivity of the mean ages to changes in allocation fluxes. Differences in allocation resulted in different NSC age and transit time distributions. The simulated starvation flattened the NSC age distribution and increased the mean NSC transit time, which can be used to estimate the age of the NSC used, the NSC remaining in the system,  and the time it would take to consume the reserves. Mean NSC ages and transit times were sensitive to carbon fluxes in roots and allocation of carbon from wood storage. Our results demonstrate how trees with different storage traits are expected to react differently to starvation. They also provide a probabilistic explanation for the “last-in, first-out” pattern of NSC mobilization from well-mixed carbon pools. In addition, they unveil determinant sink fluxes in NSC dynamics for mature trees. These findings open the possibility to better understand NSC dynamics in mature trees based on estimated NSC ages and transit times in different tree organs of species with contrasting life strategies and growth environments.