Although stable carbon isotopic composition (δ¹³C) of plants has been widely used to indicate different water regimes in terrestrial ecosystems over the past four decades, the changes in the plant δ¹³C value under waterlogging have not been sufficiently clarified. With the enhanced global warming in recent years, the increasing frequency and severity of river floods in Arctic regions lead to more waterlogging on willows that are widely distributed in river lowland. To investigate the δ¹³C changes in plants under different water conditions (including waterlogging), we measured the δ¹³C values in the leaves of willows with three species,Salix boganidensis,S. glauca, andS. pulchra, and also monitored changes in plant physiology, under several major flooding conditions in Northeastern Siberia. The foliar δ¹³C values of willows varied, ranging from −31.6 to −25.7‰ under the different hydrological status, which can be explained by: (i) under normal conditions, the foliar δ¹³C values decrease from dry (far from a river) to wet (along a river bank) areas; (ii) the δ¹³C values increase in frequently waterlogged areas owing to stomatal closure; and (iii) after prolonged flooding periods, the δ¹³C values again decrease, probably owing to the effects of not only the closure of stomata but also the reduction of foliar photosynthetic ability under long period of waterlogging. Based on these results, we predict that plant δ¹³C values are strongly influenced by plant physiological responses to diverse hydrological conditions, particularly the long periods of flooding, as occurs in Arctic regions.

The Qinling Mountains, serving as a natural geographical and climatic boundary in China, require comprehensive climatic records to elucidate the trends in climate changes across the country. While stable isotopes in tree rings are widely employed to indicate historical environmental changes, investigations into tree ring isotopes in the Qinling Mountains, particularly within the widespread broad-leaf oaks, remain limited. In this study, we investigated both intra- and inter-annual variations in the δ¹³C values of tree rings and their correlations with climatic signals over the past two decades for Quercus aliena var. acuteserrata, a dominant species among oak trees on the main peak of the Qinling Mountains. Our results reveal that responses to climate differ among altitudes and individual trees, with trees at higher altitudes exhibiting higher sensitivity to extreme climate, which is low temperatures and rainfall fluctuations during the growth period in intra-annual δ¹³C variations. Furthermore, the positive correlations are observed between temperature during growing season and both tree growth and the inter-annual δ¹³C variations. However, the climate signal appears to be hampered by oak-specific factors, such as intense competition among individuals and the age of trees. Therefore, we suggest a more rigorous selection of sampling and propose further investigations into isotopic fractionation processes in oaks for future studies.

The oxygen isotopic composition of tree‐ring cellulose (δ¹⁸O_cell) has been widely used to reconstruct historical environmental changes; however, the control factors on δ¹⁸O_cell have not been fully constrained—especially in high latitudes. To evaluate the influence of metabolic processes and related environmental factors on δ¹⁸O_cell, we analyzed the δ¹⁸O values of soil water (1998–2015), stem water (1997–2016), leaf water (3 days in each 2014 and 2015), and tree‐ring cellulose (1981–2016), on a dominant larch species (Larix cajanderi) in an eastern Siberian boreal forest. We determined that the δ¹⁸O variability of water sources is dependent on the precipitation, and the ¹⁸O enrichment in leaf water is determined by relative humidity. These findings suggest that both water source uptake and leaf water enrichment processes can affect the δ¹⁸O values of oxygen‐containing compounds in larch trees in the study site. However, the δ¹⁸O_cell (one of the end oxygen‐containing products) was found dependent on water sources on which was related to the amount of summer rainfall in the previous year. This finding significantly differs from that of studies in other areas, which infer the positive correlation between δ¹⁸O_cell and leaf water enrichment rather than precipitation or water sources. These differences are predominantly due to the specific conditions of high‐latitude areas, such as the large seasonal δ¹⁸O difference in precipitation, the existence of permafrost, and the low growth rate. Our findings contribute toward the development of tree‐ring paleoclimate reconstructions—especially in eastern Siberia.