Stable hydrogen isotope values of lignin methoxyl groups of four tree species across Germany and their implication for temperature reconstruction.

Stable hydrogen isotope ratios of lignin methoxyl groups (δ2HLM values) in wood have been shown to mirror the δ2H signatures of precipitation (δ2Hprecip values). Thus, δ2HLM values were suggested to serve as a potential paleotemperature proxy since δ2Hprecip values are dominantly controlled by air temperature in the mid-latitudes. A recent study where a significant δ2HLM-temperature relationship was found for a European transect with mean annual temperatures ranging from -4 to 17°C strengthened this assumption. However, using δ2HLM values as a paleotemperature proxy requires quantification of noise from site-, species- and biosynthetic-specific influences to determine the significance of recording smaller temperature changes. Here, we measured δ2HLM values of tree-ring sections covering 1981-1990 and 1991-2011 of four different tree species (European beech, English oak, Scots pine, Norway spruce) at 15 sampling sites across Germany. The maximum difference in mean annual temperature between sample sites was 5°C and all sites showed small temperature increases from 1981 to 1990 to 1991-2011 (mean Δ=0.7°C). For all species investigated, the maximum difference of δ2HLM within the tree was <10mUr or ‰ (median values) and between trees at a single site was ≤28mUr (median values). The general pattern of the spatial δ2HLM-temperature relationship found for the European transect was confirmed here although a significant correlation was lacking. This can be explained by the lower spatial δ2Hprecip-temperature correlation (R2=0.39) found for sampling sites in this study and the δ2HLM differences between trees. Nevertheless, the temporal changes in δ2HLM values of European beech trees correctly reflected within ±2°C the temperature change at every sampling site. Therefore, we suggest that δ2HLM values of European beech trees have considerable potential for reconstructing temperature changes when applied on tree-ring chronologies and consider this approach particularly suited for Late Holocene climate studies.