Oxygen Isotope Compositions of Cellulose in Earlywood of Larix cajanderi Determined by Water Source Rather Than Leaf Water Enrichment in a Permafrost Ecosystem, Eastern Siberia

The oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) has been widely used to reconstruct historical environmental changes; however, the control factors on δ18Ocell have not been fully constrained—especially in high latitudes. To evaluate the influence of metabolic processes and related environmental factors on δ18Ocell, 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 δ18Ocell (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 δ18Ocell 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. Plain Language Summary Scientists often find correlations between the oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) and environmental parameters, such as temperature, precipitation, and relative humidity (RH), etc, and δ18Ocell become a good indicator for paleoclimate reconstruction. However, the trend of these correlations is not always the same among different sites. What and why has not been clearly clarified, especially in high latitudes. In this study, we focused on the δ18Ocell of larch trees and most related environmental factors in eastern Siberia. We also attempted to explain why the correlations occurred through the aspects of isotopic fractionation processes and compounds related to oxygen utilization. For the most important result, we found positive correlations between pre‐rain (previous summer rainfall) and δ18Ocell. We also found so