The seasonality of non-structural carbohydrates in mature boreal Betula papyrifera and potential constraints in their remobilization

Abstract

Assimilated non-structural carbohydrates (NSC) can be stored as reserves in plants and remobilized during periods of asynchrony between carbon acquisition and carbon demand to fuel essential metabolic functions and growth. However, the framework of NSC allocation to reserves and their remobilization remains unclear, especially for mature trees which potentially can store large quantities of reserves. Here, I characterize the seasonal dynamics of NSC reserves in mature boreal Betula papyrifera and relate them to seasonal growth processes (first study), as well as determine the patterns and potential constraints of between-organ reserve remobilization. I used different patterns of stem phloem girdling to separate crown, stem and root NSC storage and their remobilization in response to induced C stress (second study). In the first study, I found that whole-tree NSC pools increased seasonally by 72% from a spring minimum to a maximum during late summer bud-set — greatly exceeding the relative change in reserves reported for more temperate conspecifics. At the organ level, the branches were the largest and most dynamic storage pool, suggesting that storage changes at the branch level largely drive whole-tree storage dynamics in these trees. In the second study, I found evidence that the crown of birch trees could store nearly double the reserves of what was observed under normal conditions, and that the stored NSC in the stem may not be universally available for remobilization to other organs (here roots) under C stress. Together, these results suggest that seasonal NSC allocation patterns appear to be highly regulated to ensure adequate distribution (allocation) of NSC to reserves throughout organs over time to support organ-level processes — however, if organs are constrained by carbon limitations, it appears that the remobilization of the organ reserve storage pools are regulated somewhat autonomously which could potentially limit the sharing of reserves within the large organism.