Reconstruction of soil moisture for the past 100 years in eastern Siberia by using δ13C of larch tree rings

A stable carbon isotope ratio (C-13) chronology for the past 100 years was developed from larch tree rings in eastern Siberia (near Yakutsk, 62 degrees 14N, 129 degrees 37E), to reconstruct past soil moisture water equivalent (SWE). Based on the correlation analyses between SWE and tree ring C-13, we developed a linear regression model for SWE in the late growing period (LGP: 15 July to 31 August) using annual tree ring C-13, which was calculated from the combination of latewood in a current year and earlywood in the following year, and then reconstructed SWE (LGP) for 1908-2007. Reconstructed SWE was compared with factors such as the output of the land surface model, annual precipitation, and Palmer Drought Severity Index for July. From the results, the reconstructed SWE appears reasonable and shows a large variation, including repeated occurrences of severe drought and an unprecedented high soil moisture event in 2006-2007 during the past 100 years. The reconstruction also captured a past documented record of severe drought in the 1940s. Despite the generally good performance of the reconstruction, by the 1930s the estimated SWE was higher than that expected from the annual precipitation. Tree ring width and C-13 were negatively correlated in most periods. However, the negative correlation was weaker for the period from 1919 to 1925, when relatively low air temperature was observed. This result suggests that the rate of photosynthesis, together with the degree of stomata opening, also affected the tree ring C-13 during cool periods.