Isotopomers as tools to unravel forest carbon balance over decades

Carbon dioxide [CO₂] has reached almost 420 ppm in 2022 (Friedlingstein et al. 2022) and may increase to 600 ppm by the year 2100. Understanding plant responses to increasing CO₂ is essential for predictions of plant productivity and of future climate (Ehlers et al. 2015). The hydrogen isotopes protium (¹H) and deuterium (²H or D) exhibit the largest isotope effects, and D is fractionated by both physical and biochemical processes. Thus, hydrogen isotope compositions of plant compounds have a remarkable potential to further our knowledge about plant physiological and environmental processes. However, whole-molecule δD depends on the δD of the plant’s water source, fractionation by transpiration, and enzyme isotope effects. To disentangle these influences, isotopomer analysis is required since enzyme isotope effects influence stable isotope abundance in specific intramolecular positions (Ehlers et al. 2015), called isotopomers. As CO₂ increases over decades, plant responses to T and CO₂ over decades are important. For forests, opposing effects of CO₂ and T determine if forests will in the future be a sink or source of CO₂ (Van der Sleen et al. 2015; Sperry et al. 2019). Furthermore, a mechanistic understanding of physiological responses is essential to be able to estimate future C assimilation using ecosystem models. Photorespiration is a side reaction of photosynthesis that reduces C assimilation in most vegetation, and photorespiration is reduced by increasing CO₂ yet exacerbated by rising T (Van der Sleen et al. 2015; Sperry et al. 2019). Therefore, we aim to unravel how photorespiration will develop under scenarios of rising CO₂ and climate change.

Tree rings help us understand interactions of plants and environmental drivers over decades-millennia. Variables that can be measured on tree rings fall into two groups. Variables like ring width are valuable for integrating effects of several environmental drivers on tree growth. In contrast, isotopomers depend on individual biochemical events and are therefore better for mechanistic studies.

We use an NMR (nuclear magnetic resonance) method to analyze isotopomers of the glucose units of tree-ring cellulose, to elucidate physiological changes in trees during past decades of increasing CO₂. In this contribution, we will report results of two kinds of experiments to investigate long-term tree responses.

First, in manipulation experiments we calibrate isotopomer responses to environmental drivers, in particular CO₂ and T. Second, we analyse tree-ring series over previous decades of rising CO₂, and use the calibrations from the manipulation experiments to deduce shifts in photosynthetic metabolism over decades. For selected tree species, we will present combined results from both kind of experiments, conclusions on physiological changes of these trees over past decades, and implications for future C assimilation by broadleaved trees.   

 

References

Ehlers et al., 2015. https://doi.org/10.1073/pnas.1504493112.

Friedlingstein et al., 2022. https://doi.org/10.5194/essd-14-1917-2022.

Sleen et al., 2015.  https://doi.org/10.1038/ngeo2313.

Sperry et al., 2019. https://doi.org/10.1073/pnas.1913072116.