The influence of high evaporative conditions on peat-forming mosses in the Antarctic Peninsula

Low-elevation coastal ecosystems of the northern Antarctic Peninsula are responding to rapid climate change by expansion of ice-free areas and increased vegetation cover. The record-setting air temperatures during the austral summer of 2020 provided an opportunity to evaluate the sensitivity of peat-forming and carbon accumulating mosses to conditions with high evaporative demand, a departure from the typical wet, cold, and windy conditions of the northern Antarctic Peninsula. Mosses are sensitive to environmental change and have been used as archives of paleoclimate information where few terrestrial records exist. However, it is still unclear how much of an influence increased evaporative demand, source water variation, or microclimate can have on moss leaf cellulose. We collected environmental waters and moss surface samples to explore the influence of microclimate, persistent snowmelt, and evaporation on moss tissue waters and cellulose for two moss species, Chorisodontium aciphyllum and Polytrichum strictum. Using the δ¹⁸O isotope values of moss tissue waters and leaf alpha-cellulose from surface samples, we compared the enrichment found in mosses to the enrichment in local water sources.  

δ²H and δ¹⁸O isotopes of moss tissue waters ranged from -87 to -23‰ and -10.6 to 0.6‰, respectively, indicating significant enrichment relative to our environmental water samples (δ²H values of -98.3 to -48.7‰ and δ¹⁸O values of -12.1 to -5.21‰) and long-term summer precipitation (February, δ²H values of -66.9 ± 15.9‰ and δ¹⁸O values of -8.11 ± 2.20‰, Vernadsky Station). Negative correlations between δ¹⁸O of moss water and the daily average and maximum relative humidity (on the day of collection) (r = -0.44; p < 0.001 and r = -0.62; p < 0.001, respectively) suggest that conditions resulting in high evaporative demand may have a dominant effect and imprint on the δ¹⁸O of moss cellulose. We found no relationship between δ¹³C values of alpha-cellulose, which reflect changes in CO₂ diffusivity with moisture conditions, and microclimate variables or average temperature and relative humidity on the day of sample collection. This is possibly the result of discrepancies between peak growth and seasonal peaks in cumulative evaporation leading the tissues to incorporate multiple seasons. The isotope values of moss waters likely reflect the anomalously warm summer on the Antarctic Peninsula, as indicated by the divergence from moss water lines published in non-record-setting summers. Although the δ¹⁸O values of moss tissue waters are a proxy for the composition of δ¹⁸O values of summer precipitation, it is critical to consider the imprint of high evaporative conditions, in addition to summer precipitation composition, when reconstructing past environmental conditions using cellulose from peat archives.