The role of the bole: Constraints on the remobilization of stem non‐structural carbohydrates under experimental carbon limitation

Plants store non‐structural carbohydrates (NSC) for remobilization when carbon (C) demand exceeds supply. However, large trees may not readily remobilize stored NSC across longer distances, limiting C availability for functions like growth. Furthermore, whether NSC availability limits physiological processes like defence and hydraulic function remains poorly understood, limiting our ability to predict when C limits growth and survival. To test whether organs remobilize C over long distances and to create variation in NSC availability in different organs, we applied phloem girdles to large Betula papyrifera trees, creating four treatments—no girdles, upper stem girdle only (top‐girdled), lower stem girdle only, and both upper and lower girdles (double‐girdled)—and monitored NSC dynamics in all organs for a year. Upper girdles restricted import of new photosynthate below the crown; lower girdles inhibited transport of remobilized NSC between roots and stems. If cross‐organ remobilization occurs under C limitation, we predicted that stem and root NSC concentration differences would develop between top‐girdled and double‐girdled trees where such exchange is prevented. We then used organ‐level NSC variation created across all treatments to explore the relationships between storage and survival‐linked functions. NSC concentrations declined more in the lower stem of double‐girdled trees compared with top‐girdled trees (−3.6% vs. −0.7%), suggesting some root NSC is re‐allocated above‐ground under C limitation. However, root and mid/upper bole NSC did not differ, indicating that inter‐organ and long‐distance sharing of stored NSC was minimal. Across all treatments, root NSC was positively correlated with root collar sprouting and spring stem bleeding, an indicator of root pressure and post‐winter hydraulic recovery. Stem NSC was positively correlated with branch moisture content and spring leaf flush and negatively correlated with stem canker infection. Synthesis . The lack of substantial NSC sharing between organs and across longer distances in large trees suggests that NSC storage in a single organ like the stem is a poor indicator of C status throughout the whole tree. Without a more spatially explicit representation of storage pools, we may overestimate not only how much stored C is available to support a given organ or function but also tree survival.