Tracing carbon and nitrogen reserve remobilization during spring leaf flush and growth following defoliation.

This article investigates the remobilization and allocation of carbon (C) and nitrogen (N) reserves in trembling aspen (Populus tremuloides) saplings during spring leaf flush and following defoliation. Using stable isotope labeling (^13C and ^15N) combined with grafting techniques, the study demonstrates that both stem and root reserves contribute to leaf growth, but remobilized reserves are preferentially allocated to sinks closer to their source, with this distance effect being more pronounced for carbon after defoliation. Following defoliation, re-flush leaves rely more on current photosynthate than on stored root carbon despite substantial remaining root starch, while nitrogen remobilization patterns differ, showing less distance-related allocation constraints. The findings suggest that aspen prioritize maintaining root carbon reserves over rapid canopy recovery, highlighting spatial and temporal constraints on reserve use that may influence growth and stress responses in woody plants.