Description: The tree-ring stable C, O and H isotope compositions have proven valuable for examining past changes in the environment and predicting forest responses to environmental change. However, we have not yet recovered the full potential of this archive, partly due to a lack understanding of fractionation processes resulting from methodological constraints. With better understanding of the biochemical and tree physiological processes that lead to differences between the isotopic compositions of primary photosynthates and the isotopic compositions of substrates deposited in stem xylem, more reliable and accurate reconstructions could be obtained. Furthermore, by extending isotopic analysis of tree-ring cellulose to intra-molecular level, more information could be obtained on changing climate, tree metabolism or ecophysiology. This chapter presents newer methods in isotope research that have become available or show high future potential for fully utilising the wealth of information available in tree-rings. These include compound-specific analysis of sugars and cyclitols, high spatial resolution analysis of tree rings with UV-laser, and position-specific isotope analysis of cellulose. The aim is to provide the reader with understanding of the advantages and of the current challenges connected with the use of these methods for stable isotope tree-ring research.

Description: This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ2Hc) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency climate signals in the δ2Hc chronologies were weaker than those recorded in carbon (δ13Cc) and oxygen isotope signals (δ18Oc) but similar to the tree-ring width ones (TRW). The δ2Hc climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant 2H-enrichment in tree-ring cellulose. The δ2Hc inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, but also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in δ2Hc, we investigated its relationships with δ18Oc and TRW. We found significant negative relationships between δ2Hc and TRW (7 sites), and positive ones between δ2Hc and δ18Oc (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ2Hc models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ2Hc variations. Our results suggest that the information recorded by δ2Hc is significantly different from that of δ18Oc, and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of 2H-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of δ2Hc. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.

Description: 24 European annually resolved stable isotope chronologies have been constructed from tree ring cellulose for the last 400 years (1600CE – 2003CE) for carbon and oxygen and for the last 100 years for hydrogen. Data was produced within the ISONET project (400 Years of Annual Reconstructions of European Climate Variability Using a Highly Resolved Isotopic Network,) to initiate an extensive spatiotemporal tree-ring stable isotope network across Europe funded as part of the fifth EC Framework Programme “Energy, Environment and Sustainable Development”. This data set comprises the ISONET δ13C records.

Description: 24 European annually resolved stable isotope chronologies have been constructed from tree ring cellulose for the last 400 years (1600CE – 2003CE) for carbon and oxygen and for the last 100 years for hydrogen. Data was produced within the ISONET project (400 Years of Annual Reconstructions of European Climate Variability Using a Highly Resolved Isotopic Network,) to initiate an extensive spatiotemporal tree-ring stable isotope network across Europe funded as part of the fifth EC Framework Programme “Energy, Environment and Sustainable Development”. This data set comprises the ISONET δ18O records.

Description: The vapor pressure deficit reflects the difference between how much moisture the atmosphere could and actually does hold, a factor that fundamentally affects evapotranspiration, ecosystem functioning, and vegetation carbon uptake. Its spatial variability and long-term trends under natural versus human-influenced climate are poorly known despite being essential for predicting future effects on natural ecosystems and human societies such as crop yield, wildfires, and health. Here we combine regionally distinct reconstructions of pre-industrial summer vapor pressure deficit variability from Europe’s largest oxygen-isotope network of tree-ring cellulose with observational records and Earth system model simulations with and without human forcing included. We demonstrate that an intensification of atmospheric drying during the recent decades across different European target regions is unprecedented in a pre-industrial context and that it is attributed to human influence with more than 98% probability. The magnitude of this trend is largest in Western and Central Europe, the Alps and Pyrenees region, and the smallest in southern Fennoscandia. In view of the extreme drought and compound events of the recent years, further atmospheric drying poses an enhanced risk to vegetation, specifically in the densely populated areas of the European temperate lowlands.