Minimum levels and dynamics of carbon reserves in temperate trees at severe carbon limitation

Non-structural carbohydrates (NSC, i.e. free sugars and starch) are regarded as freely available carbon (C) reserves in plants. They are often quantified to estimate the C supply status of plants, especially of tall forest trees. However, it is still unclear, whether high NSC concentrations indicate a sufficient C supply and, respectively, if low NSC concentrations indicate an insufficient C supply. Furthermore, the temporal dynamics of NSC in trees under long-term (year-long) C-limitation are still unknown, despite the possibility that NSC concentrations are subject to acclimation. Additionally, it is still unclear, to what extent a sufficient C supply can improve the fitness of trees.
I conducted multiple experiments on young trees to (i) identify the minimum tissue concentrations of NSC in tree organs at lethal C-starvation following complete darkness, (ii) investigate the long-term effects of C-limitation (under 6% light) on growth, gas exchange and C storage of different tree species to identify possible trade¬-offs between biomass production and storage, and to (iii) assess the significance of the presence of NSC pools for the survival of trees under environmental stress, like drought.
At lethal C-starvation, I observed minimum NSC tissue concentrations lower than 20% compared to control concentrations, which I never observed at non-lethal C-limitation. In contrast, I observed higher NSC concentrations (at least 30% compared to controls) during three years of non-lethal C-limitation. No photosynthetic acclimation was observed after years of shading, while NSC generally increased under shade. Manipulation of NSC during drought was not efficient enough to show the effect of stored NSC on drought survival.
This thesis showed, that measurements of substantial NSC concentrations in potentially C-limited tree seedlings or saplings do not exclude the presence of non-lethal C-limitation, while they clearly exclude lethal C-starvation. Thus, lethal C-starvation can be predicted more easily from NSC measurements, than non-lethal C-limitation. These findings improve the predictive value of NSC concentrations, and hopefully help to prevent erroneous assessments about the C-relations of trees.