MODISTE · MOdelling and Data assimilation of the Isotopic Signature of carbon pools and fluxes in Terrestrial Ecosystems

Final Activity Report Summary - MODISTE (MOdelling and Data assimilation of the Isotopic Signature of carbon pools and fluxes in Terrestrial Ecosystems)

Identifying the mechanisms that control the magnitude as well as the spatial and temporal distribution of the terrestrial carbon sink remains one of the key challenges in climate change research. Stable isotopes of carbon and oxygen are a powerful tool to trace the flow of carbon and water through terrestrial ecosystems and hence enable us to gain a mechanistic understanding of the underlying processes. The project MODISTE combines ecosystem scale modelling of isotope processes at selected sites in Europe and the United States with datasets of stable carbon and oxygen isotopes from a new established isotope network and continuous trace gas flux measurement from the FLUXNET network. During the project we built a water isotope sampling network at 12 selected FLUXNET sites across the United States. This network is part of the global Moisture Isotopes in Biosphere and Atmosphere network (MIBA) initiated by the International Atomic Energy Agency aiming to provide a global dataset of water isotopes in order to constrain global climate models. The network in the United States covers forests, woodlands and arable land ranging from Alaska to Florida and from 400 mm to 1400 mm precipitation. We further advanced the multi-layer ecosystem model CANVEG and implemented it a three sites: a dense old-growth deciduous forest in Germany, a semi-arid woodland in the United States and a set of Eucalyptus stands in Hawaii. Our studies in the dense deciduous forest revealed how vertical gradients in microclimate affect carbon isotope discrimination within dense forest canopies and enabled us to disentangle the relative contribution of various fractionation steps from boundary layer, to stomata and mesophyll to the site of carboxylation inside the leaf. Furthermore, we improved our understanding how and why diffuse light increases carbon uptake of dense forest canopies and how this impacts isotope discrimination. Additionally, we applied a simplified carbon isotope model to constrain stable carbon isotopes in tree rings. Our results indicate that a major part of wood growth at the beginning of the growing season derives from stored carbohydrates assimilated during the previous growing season. Tree rings and their isotopic composition are often used as recorders of historic climate changes. With our results we challenge a simply interpretation of the isotopic composition of tree rings, since they contain a mix of recent and old carbohydrates. Finally, we used the model to understand the biosphere-atmosphere exchange of other trace gases, i.e. isoprene. Isoprene is an important precursor of ozone in the surface boundary layer and changes the oxidative capacity of the atmosphere. We adapted and validated our model for three different Eucalyptus stands in order to study the effect of nitrogen fertilization on isoprene emissions. Our results indicate that canopy scale isoprene emissions were similar among different stands, even though leaf level emissions varied depending on leaf nitrogen composition (nitrogen fertilisation). Overall, the project led to the creation of a new continental scale isotope network in the US and to advancements in ecosystem scale modelling of stable isotope processes contributing to a better mechanistic understanding of the carbon and water cycle in terrestrial ecosystems.

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