Isotope variability in larch tree rings of Siberia: climate and ecology

Paleoclimate reconstructions from tree-ring widths and maximum wood density are most successful in localities with extreme climates for particular tree species that are most responsive. Climate proxy records from other, less conventional, tree-ring parameters have been rapidly increasing over the last decade. We assembled a unique dataset of carbon and oxygen isotope ratios of larch tree rings from the northern and southern tree-lines of Siberia, variously sub-sampled and analyzed (whole wood and cellulose & annual and 5-year sequences from individual trees and pooled). Larch samples from the north in Taymyr (Larix gmelinii Rupr.) published by Sidorova et al. (2010) and from the south collected in Khakasia (Larix sibirica Ledeb.) both came from highly temperate continental climates exhibiting similar amounts of precipitation and observed temperature trends. However, the sites differ because temperature is the dominant factor limiting radial tree growth in the north, whereas precipitation is the dominant limiting factor in the south. Climatic signals documented in the chronologies of tree-ring widths, wood density, and stable carbon and oxygen isotopes were compared from 1896 to 2005 and interpreted based on site ecology and larch physiology. We found a wide range of climatic responses in the variability of isotopic ratios, which suggest influence by combined interaction of precipitation and temperature changes rather than either climate factor alone. We discuss the improvement in our understanding of climatic mechanisms that control isotope compositions and tree growth in boreal forests. At certain locations where tree-ring widths are less sensitive to climate factors, isotope analysis may have greater value to successful climate modeling. It seems crucial to measure both isotopes (C and O) in tree rings and to incorporate these mechanisms properly in developing reliable climate predictors. It is noteworthy that despite the identified differences in climatic signatures of carbon and oxygen isotopes from the studied dataset, all isotope chronologies show a strong trend of progressively depleted values after the mid-20th Century. We also noticed a change in correlation sign from negative to positive between oxygen and carbon isotope records around the same time. A similar fluctuation in relationship between these two isotopes was reported previously in other periods and locations, e.g. during the onset of the Younger Dryas event documented by tree rings in the US Midwest. The phenomena of inconsistency in isotopic compositions of tree rings needs to be studied closely to help us better understand and interpret tree-ring isotope signals as possible indicators of climate change in the past. Reference: Sidorova, O. V., Siegwolf, R. T. W., Saurer, M., Naurzbaev, M. M., Shashkin, A. V., Vaganov, E. A., 2010. Spatial patterns of climatic changes in the Eurasian north reflected in Siberian larch tree-ring parameters and stable isotopes. Global Change Biology 16(3):1003-18.