Unlocking the physiological and climatic hydrogen isotope signal in tree rings and leaf n-alkanes in a boreal forest

It is essential to comprehensively understand past climate and tree response to climate change because trees are directly exposed to rapid, intensifying, and widespread climate change. Both tree rings and leaf n-alkanes are long-term biomarkers which can help to better understand past climate and/or tree response to climate change. For example, interpreting carbon and oxygen stable isotopes in tree rings contribute to understanding past climate and tree response to climate change. However, greater insight could be achieved if hydrogen stable isotopes can also be clearly interpreted. To clearly interpret hydrogen isotopes in tree rings it is necessary to discover which aspects of climatic variability and tree physiology are most clearly expressed by hydrogen isotopes in tree rings. Additionally, the climatic signal of hydrogen stable isotopes in leaf n-alkanes can be deposited to soil and sediments from plants, and their climatic soil record can date back to thousands to millions of years ago. However, there are still aspects of plant physiology that have not been accounted for in leaf n-alkane hydrogen isotope interpretation, which limits the reliability of their interpretation. While data from tree rings and leaf n-alkanes are rarely combined, both tree rings and leaf n-alkanes contain a hydrogen isotope signal that originates from a hydrogen isotope signal in source water that is changed by physiological processes and interacting climatic factors during their biosynthesis. This project aims to improve our understanding of the physiological and climatic signals contained in the hydrogen isotope signal in both tree rings and leaf n-alkanes. It helps increase the useability of the hydrogen isotope signal in tree rings and helps improve the reliability of interpreting climatic signals from leaf n-alkanes. It uses a unique dataset from a field survey with samples collected multiple times during a growing season, with high temporal resolution, at one to two boreal forests in southern Finland (e.g., Leppä et al. 2022; Tang et al., 2022). This dataset is rich with information from multiple sources, such as multiple element isotope concentrations in various plant tissues and water isotope pools, as well as leaf gas exchange data, meteorological data, and eddy covariance data. The hydrogen isotope signal is traced from the climatic signal in source water, to the physiological and climatic signal in leaf water, sugars and starches and leaf n-alkanes, then from the same leaf water, sugars, and starches to bark and stem phloem sugars and starches, to high-resolution intra-annual tree ring α-cellulose. This presentation gives the project outline and some main findings.

Leppä et al. (2022) doi:10.1111/nph.18227

Tang et al. (2022) doi: 10.1093/treephys/tpac079