Ecological and environmental controls on plant wax production and stable isotope fractionation in modern terrestrial Arctic vegetation

Abstract. Terrestrially-derived plant waxes and their compound-specific stable carbon (δ¹³C) and hydrogen (δ²H) isotope ratios are valuable tools for inferring past changes in vegetation and hydrology in sedimentary archives. Such inferences require knowing the ecological (i.e. plant growth form) and environmental (i.e. temperature, precipitation, relative humidity, elevation) mechanisms that govern the production of different plant wax carbon chain-lengths and the fractionation of stable isotopes. These mechanisms, however, are uncertain in the Arctic, limiting our ability to infer past vegetation and hydrology changes. To address this, we produced terrestrial plant n-alkanoic acid and n-alkane abundance and δ¹³C and δ²H data from a latitudinal environmental gradient along the Eastern Canadian Arctic (105 individuals), which we compiled with published data across the Arctic (additional 281 individuals). We compared this dataset with environmental parameters to assess the mechanisms that govern plant-wax production and isotope fractionation. We found that total plant wax concentrations and Average ChainLength (ACL) were statistically different between vascular (trees, shrubs, forbs, ferns, graminoids) and non-vascular plants (mosses, liverworts) and lichens, whereas δ¹³C values and δ²H apparent fractionation relative to growing season precipitation δ²H often did not differ significantly between plant growth forms. Correlations between plant wax indices and mean of the months above freezing (MAF) environmental parameters were generally weak (r ≤ 0.4), and/or not significant (p > 0.05). These results suggest that a fundamental assumption to paleoclimate research holds in the Arctic: for individual plant taxa and plant communities, the abundance, ACL, and δ¹³C/δ²H isotopic fractionation of both n-alkanoic acids and n-alkanes is independent of temperature, precipitation, humidity, and elevation. Instead, changes in sedimentary plant wax distributions reflect changes in plant taxa present through time, and changes in plant wax δ²H reflect changes in source water δ²H. Therefore, plant waxes can be used to infer past changes in climate and ecology.