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Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ13 C) and oxygen isotope ratios (δ18 O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ18 O and δ13 C of Picea abies and Fagus sylvatica in relation to tree age, size, and distance to the upper canopy at seven temperate sites across Europe. We observed strong positive trends in δ13 C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ13 C in shade-tolerant species. We also detected positive trends in δ18 O with increasing tree size. However, the observed slopes are less steep and consistent between trees of different ages and thus can be more easily addressed. We recommend restricting the use of δ13 C to years when trees are in a dominant canopy position to infer long-term climate signals in δ13 C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ18 O should be in principle the superior proxy for climate reconstructions.

Summary Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ18OV) influence water and assimilates in plants remains limited, restricting our ability to use δ18O as a tracer of ecophysiological processes. We exposed oak (Quercus robur) saplings under wet and dry soil moisture conditions to 18O-depleted water vapour (c. − 200‰) at high relative humidity (c. 93%) for 5 h, simulating a fog event. We then traced the step change in δ18OV into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h. The immediate δ18OV effect was highest for δ18O of leaf lamina water, but 40% lower on δ18O of main vein water. To a smaller extent, we also observed changes in δ18O of twig xylem water. Depending on the individual assimilation rate of each plant, the 18O-label was partitioned among different assimilates, with highest changes in δ18O of starch/sucrose and lowest in δ18O of quercitol. Additionally, 18O-label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status. Our results have important implications for water isotope heterogeneity in plants and for our understanding of how the δ18O signal is incorporated into biomarkers.

Elevated CO2 along with rising temperature and water deficits can lead to changes in tree physiology and leaf biochemistry. These changes can increase heat- and drought-induced tree mortality. We aim to reveal the impacts of climatic drivers on individual compounds at the leaf level among European larch (Larix decidua) and mountain pine (Pinus mugo) trees, which are widely distributed at high elevations. We investigated seasonal carbon isotope composition (δ13C) and concentration patterns of carbohydrates and organic acids in needles of these two different species from a case study in the Swiss National Park (SNP). We found that average and minimum air temperatures were the main climatic drivers of seasonal variation of δ13C in sucrose and glucose as well as in concentrations of carbohydrates and citric acid/citrate in needles of both tree species. The impact of seasonal climatic drivers on larch and mountain pine trees at the needle level is in line with our earlier study in this region for long-term changes at the tree-ring level. We conclude that the species-specific changes in δ13C and concentrations of carbohydrates and organic acids are sensitive indicators of changes in the metabolic pathways occurring as a result of climatic changes.

Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ

Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ