Environmental and taxonomic controls of carbon and oxygen stable isotope composition in Sphagnum across broad climatic and geographic ranges

Abstract. Rain-fed peatlands are dominated by peat mosses ( Sphagnum sp.), which for their growth depend on nutrients, water and CO 2 uptake from the atmosphere. As the isotopic composition of carbon ( 12,13 C) and oxygen ( 16,18 O) of these Sphagnum mosses are affected by environmental conditions, Sphagnum tissue accumulated in peat constitutes a potential long-term archive that can be used for climate reconstruction. However, there is inadequate understanding of how isotope values are influenced by environmental conditions, which restricts their current use as environmental and palaeoenvironmental indicators. Here we tested (i) to what extent C and O isotopic variation in living tissue of Sphagnum is species-specific and associated with local hydrological gradients, climatic gradients (evapotranspiration, temperature, precipitation) and elevation; (ii) whether the C isotopic signature can be a proxy for net primary productivity (NPP) of Sphagnum ; and (iii) to what extent Sphagnum tissue δ 18 O tracks the δ 18 O isotope signature of precipitation. In total, we analysed 337 samples from 93 sites across North America and Eurasia using two important peat-forming Sphagnum species ( S. magellanicum , S. fuscum ) common to the Holarctic realm. There were differences in δ 13 C values between species. For S. magellanicum δ 13 C decreased with increasing height above the water table (HWT, R 2  = 17 %) and was positively correlated to productivity ( R 2  = 7 %). Together these two variables explained 46 % of the between-site variation in δ 13 C values. For S. fuscum , productivity was the only significant predictor of δ 13 C but had low explanatory power (total R 2  = 6 %). For δ 18 O values, approximately 90 % of the variation was found between sites. Globally modelled annual δ 18 O values in precipitation explained 69 % of the between-site variation in tissue δ 18 O. S. magellanicum showed lower δ 18 O enrichment than S. fuscum (−0.83 ‰ lower). Elevation and climatic variables were weak predictors of tissue δ 18 O values after controlling for δ 18 O values of the precipitation. To summarize, our study provides evidence for (a) good predictability of tissue δ 18 O values from modelled annual δ 18 O values in precipitation, and (b) the possibility of relating tissue δ 13 C values to HWT and NPP, but this appears to be species-dependent. These results suggest that isotope composition can be used on a large scale for climatic reconstructions but that such models should be species-specific.