The role of the bole: constraints in the remobilization of stem reserves under experimental carbon limitation

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, we explore the role of stem reserves and potential constraints in their remobilization in large Betula papyrifera trees. To explore reserve remobilizations between organs, we use different patterns of phloem girdling to induce carbon stress and to isolate crown, stem, and root NSC storage pools.  Our results suggest that NSC reserves in the stem tissues may not be easily remobilized to other, more distant, organ sinks. However, we also found that some root reserves may be allocated toward the lowermost stem/root collar position, indicating that under carbon limiting conditions the roots might not be the strongest sink for NSC reserves. We suggest this response represents an adaptive recovery strategy for a collar-sprouting species like B. papyrifera, which occupies areas prone to aboveground disturbance. We also found that storage capacity of tissues (here stem wood and crown) can far exceed the concentrations that are normally stored in these trees.  Additionally, we found stem NSC concentrations positively correlated with disease resistance and branch water content.  These relationships suggest that a maintenance of greater stem reserves may be required to support other important roles directly or indirectly, such as defense or spring leaf flush, respectively.  Overall, it appears that the remobilization of different organ reserve storage pools is regulated somewhat autonomously, which, particularly under carbon limiting conditions, could potentially limit the sharing of reserves within a large tree.