From theory to methods and applications – for a better understanding of stem respiration and carbon dynamics in mature trees

This doctoral thesis was motivated by the need to improve our understanding of carbon dynamics in mature tree stems. Stem respiration substantially contributes to the return of photo-assimilated carbon to the atmosphere, and thus, to the tree carbon balance. Stem CO2 efflux is often used as a proxy for stem respiration. However, estimating stem respiration by using CO2 efflux does ignore possible stem-internal transport and refixation processes that affect the fraction of CO2 emitted locally. We developped a new field-tested stem chamber device for autonomous real-time and quasi-continuous long-term measurements of O2 and CO2 simultaneously. This device allows for a more holistic approach to studying respiratory and post-respiratory processes in woody tissues. Combined with internal CO2 flux and refixation measurements, it improves stem respiration estimates and advances our knowledge on the uncertain fate of CO2 in stems. The first field experiment, looking at the fate of CO2 in mature beech trees (Fagus sylvatica L.), showed that ~30% of the respired CO2 was retained in the stem along a 3-m vertical stem gradient. However, the transport of respired CO2 away from its point of production could only partially explain the difference between CO2 efflux and O2 influx in large trees. I provided novel evidence that PEPC-mediated CO2 fixation can be seen as a relevant driver of the mismatch between CO2 efflux and O2 influx in mature trees, highlighting the potential relevance as a mechanism of local CO2 removal to close the stem C balance. In a second field experiment in the Thuringian forest, we used stem girdling in mature poplar trees (Populus tremula L. hybrids), a lipid-storing species, to permanently interrupt phloem C transport and induced C shortage in the isolated stem section below the girdle. Poplar trees can survive several years of reduced C supply from the canopy by switching in metabolism from recent carbohydrates to older storage pools with a potential mixture of respiratory substrates, including lipids. This mechanism of stress resilience can explain why tree decline may take many years until death occurs.