Tree ring width, density, and ratio of stable isotopes (13C/12C and 18O/16O) in wood and cellulose were determined for larch (Larix sibirica Ledeb.) growing under water deficit conditions in the forest steppe zone in central Siberia (54°24′N, 89°57′E) for the period 1850–2005. Dendroclimatic analysis of the chronologies indicated precipitation to be the most important factor determining indicated parameters. Precipitation of June is significantly correlated with tree ring width and maximum density (r = 0.36 and 0.43, p < 0.05, respectively). Relations of δ13C and δ18O to precipitation are similar, but the most important month is July (r = −0.47, p < 0.05 for δ13C; r = −0.29, p < 0.05 for δ18O). Further, δ18O is positively related to the mean temperatures of July (r = 0.30, p < 0.05). Carbon and oxygen isotope ratios in wood and cellulose showed strong negative trends for the last 100 years. The reason for this decrease could be the observed shift of the beginning of the vegetation period to earlier dates (up to 1 week) in spring and the increased use of precipitation water stored in the soil from October of the previous year, although other explanations cannot be excluded (in particular the physiological effect of increasing atmospheric CO2 also responsible for lower δ13C values). Thus, an earlier start of the vegetation period could lead to tree ring formation during a period with higher water availability, resulting in stronger isotopic fractionation and 13C depletion, also reflected in a higher earlywood to latewood ratio. At the same time, highly 18O depleted water from October precipitation of the previous year is absorbed. The incorporation of this isotopically lighter water during photosynthesis is reflected in the wood and cellulose of tree rings.