Carbon Allocation to Wood Formation in European Beech Forests – From Cell to Ecosystem

Abstract

Understanding the magnitude and drivers of tree biomass increment is essential for improving model-based projections of the land carbon sink, which in turn is key to adequately supporting climate change mitigation efforts and nature-based climate solutions. Traditionally, carbon uptake through photosynthesis and the allocation of that carbon to woody tissues have been assumed to be closely linked. On one hand, the expansion of developing wood cells relies on turgor pressure, which is generated by water absorbed through the roots and transpired through the leaves. On the other hand, building secondary cell walls depends on the transport of photosynthetic products, such as sugars, from the leaves to the growing tissues where biomass is formed. Importantly, only a small fraction (1.68–23.08%) of the total sequestered carbon is allocated to xylem tissues where it will be stored for a long time and can help offset anthropogenic carbon emissions on climate-relevant timescales. However, we still do not fully understand the dynamics of this carbon allocation or its response to changes in a tree’s abiotic and biotic environment. In particular, the magnitude and temporal variability of carbon investment in wood formation remain poorly quantified. The objectives of this dissertation are to (1) reduce uncertainties in estimates of carbon allocation to tree woody biomass, (2) explore carbon sink–source relationships across spatial scales from the cell to the forest-stand level, and (3) address some of the most important and persistent uncertainties in our understanding of climate change impacts on carbon uptake and allocation processes. In Appendix A, I integrate climatic, eddy-covariance, tree-ring, and forest inventory data in a multidisciplinary approach to demonstrate the drought responses of carbon uptake and allocation to long-lasting woody biomass pools at two European beech (Fagus sylvatica) forests of Hainich (DE-Hai, Germany) and Sorø (DK-Sor, Denmark). In Appendix B, I continue to investigate the relationship between photosynthesis and woody biomass growth at both Hainich and Sorø at much finer (i.e., intra-annual) temporal resolution. Lastly, in Appendix C, I examine the degree to which wood core sampling strategies can bias estimates of forest productivity in two contrasting plots at the Vielsalm flux-tower site in Belgium (BE-Vie). As a whole, this dissertation captures both interannual and intra-annual dynamics of carbon uptake and allocation to wood formation in European beech forests under changing climate conditions. The three main studies are tightly connected and offer novel insights into carbon dynamics from wood tissues and individual trees to the ecosystem level. Our discussions and recommendations also aim to help improve estimates of tree biomass growth at flux-tower sites elsewhere, particularly those characterized by heterogeneous forest structure and composition. This combined empirical insight will support the development of improved mechanistic models capable of more accurately representing forest growth dynamics and contributing to more reliable projections of future land carbon trajectories.