Depletion of non-structural carbohydrate reserves in temperate tree seedlings under stress

Background/Question/Methods

The carbon starvation hypothesis is one explanation of the mechanisms involved in tree death due to limited resources. The few studies that have tested this directly give mixed support and do not take into account species-level differences in allocation of carbon to storage versus growth. Species that store larger amounts of non-structural carbohydrates (NSC) could have higher survival under low resources because they can use carbon reserves to respire during non-photosynthetic periods. However, this explanation assumes that carbon reserves can be remobilized. The aim of this research was to establish whether stressed seedlings deplete their carbon reserves over time while under different stresses and to assess the assumption that differences in allocation of NSC to storage mediate species-level differences in mortality. Seedlings from four temperate tree species (Betula papyrifera, Quercus rubra, Quercus velutina, and Fraxinus americana) were subjected to combinations of three different types of stress: complete shade, drought, and herbivory. At several points after imposed stresses, we measured seedling mass accumulation and NSC pools and concentrations.

Results/Conclusions

In the control treatment, biomass growth rates were highest for B. papyrifera, followed by Q. rubra, Q. velutina, and F. americana respectively. As expected, the fastest growing species, B. papyrifera, had the smallest root mass fraction. Q. rubra and Q. velutina started with larger NSC pools than B. papyrifera and F. americana before stress treatments were applied. Most NSC was starch, located primarily in the roots. In the controls, NSC accumulated in all species, with the fastest rate in F. americana, followed by Q. rubra, Q. velutina, and B. papyrifera. In all stress treatments, except for defoliation alone, biomass growth was minimal. The defoliation-only treatment was similar to the control for all species. In all other treatments, NSC accumulation ceased, and in many cases , pools decreased over time, especially for the drought treatments. There was an increase in the concentration of soluble sugars for drought treatment combinations in high light. These results support the hypothesis that NSC reserves are used for seedling respiration during periods of negative carbon balance, and the extent to which this happens depends on species and type of stress.