Differences in nonstructural carbohydrates use and radiocarbon ages between fine roots, coarse roots, and stems

Radiocarbon measurements indicate that nonstructural carbohydrates (NSCs) in trees can be decades old. The NSCs can be critical resource during ecological disturbances that impede carbon assimilation, however, it is unclear to what degree old NSCs are accessible to tree metabolism and growth, or how their allocation and use differ between above- and belowground. To address these knowledge gaps, we girdled the stems of 60–70-year-old aspen trees, a species which can grow in clones and establish root connections with neighboring trees. During the first years after girdling (2018–2021), the girdled tree stems contained six times less NSC than the controls, respired at a rate five times slower, and increasingly used older carbon for respiration, reaching 10 years old by 2021 (Helm et al., 2024). Although most root sampling took place later, during 2021–2023, the roots’ response to the girdling was much milder. In cross sections of roots > 10 cm in diameter in 2022, the girdled trees had two to three times less NSC and NSC that was 8–14 years older than the controls. Individual NSCs showed inverted trends along the annual rings: the oldest sugars (as estimated by water-soluble carbon) were found in the bark and outer rings, whereas the oldest starch was found in the interior rings. Some rings contained NSCs aged 20–30 years. In fine roots (less than 2 mm in diameter), compared to the controls, the girdled trees contained half of the NSC, respired 30% slower while emitting 1–5 years older CO₂, and contained 1–6 years older NSC. In contrast to these relatively young ages, fine roots collected from screens buried in the soil for up to one year – probably representing growth from spring and early summer – had radiocarbon ages of 16–33 years with no clear effect of girdling. Overall, the NSC pools in the tree stems depleted faster than those in the large coarse and fine roots, suggesting either that the root NSCs were replenished by carbon from neighboring trees or that the tree stems play a more significant role in storage at the tree level. The extremely old radiocarbon ages of new fine roots suggest that old reserves in large roots are accessible and have a physiological function, even in undisturbed trees.

 

Helm, J., Muhr, J., Hilman, B., Kahmen, A., Schulze, E. D., Trumbore, S., Herrera-Ramirez, D., & Hartmann, H. (2024). Carbon dynamics in long-term starving poplar trees-the importance of older carbohydrates and a shift to lipids during survival. Tree Physiol, 44(13), 173-185. https://doi.org/10.1093/treephys/tpad135