Investigating tree-ring isotopes and trace elements in larch to determine links with climate in Southern Siberia

Relationships of various tree–ring parameters with climate have been investigated in many studies, however their full capacity for reconstructing past climate has yet to be fully realized. Here we expand on this earlier work and add trace-element measurements in tree rings to gain better understanding of the physiological and environmental controls on the isotopic and elemental composition of tree rings. Multivariable mathematical analysis is needed to extract information of interest from the multiple parameters that are measured. We employ conventional tree-ring characteristics (width and density) plus isotopes and ~30 trace elements with temperature and moisture parameters. Tree-ring width (TRW), maximum latewood density (MXD) and stable isotope composition (δ¹³C and δ¹⁸O) in wood and cellulose were determined for larch (Larix sibirica Ledeb.) growing under water deficit conditions in the forest-steppe zone in Central Siberia. Dendroclimatic analysis of the chronologies indicated precipitation to be the most important factor determining tree-ring composition and structure. Precipitation of June is significantly correlated with TRW and MXD (r=0.36 and 0.43, p<0.05, respectively). δ¹³C and δ¹⁸O are similarly related to precipitation, most strongly for 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). δ¹³C and δ¹⁸O in wood and cellulose strongly decline over the last 100 years. The main reason for this decrease is most likely a shift of the beginning of the vegetation period to earlier dates (up to 1 week) in spring and the use of water stored in soil from previous October precipitation, which has been increasing over the last three decades. 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, and the use of water from previous October precipitation results in the uptake of depleted δ¹⁸O. The incorporation of this isotopically lighter water during photosynthesis is reflected in the wood and cellulose of tree rings. While growing season precipitation is decreasing at this location, our results show that changes in the seasonality (more winter precipitation, earlier start of growing season) may more than compensate the reduction in summer precipitation and therefore lower the drought stress for trees in the forest steppe.

We are in the process of obtaining elemental composition of tree rings with respect to ~30 elements using ICP-MS, the first study of this type for this region. Our goal is to compare the data for elements measured with 5-year resolution with instrumental climate data for the last 70 years. A particular element (e.g., chlorine) or combination of elements may be developed as new proxies of climate to supplement and help explain previous results obtained for carbon and oxygen isotope trends in tree rings (Goldberg et al., 2007).