Active layer thermal regime under different vegetation conditions in permafrost areas. A case study at Signy Island (Maritime Antarctica)
Climate change is now evident also in Antarctica, with impacts both on the abiotic and the biotic components of ecosystems, particularly on permafrost, active layer thickness, vegetation, and soil properties. Permafrost ecosystems are recognized to be sensitive to the influences of the changing climate, which may activate, through complex mechanisms, both positive and negative feedbacks relating to CO2 fluxes. For this reason we analysed, through a data set collected over a complete year, the thermal regime of the active layer at four sites with different vegetation (bare ground, lichen vegetation with Usnea aurantiaco-atra, moss vegetation with Sanionia uncinata, grass vegetation with Deschampsia antarctica) but with similar topographic and geomorphological conditions at Signy Island (Maritime Antarctica). Except for the Deschampsia site, the other three sites are the same formerly studied by Chambers in the 1960s. The three sites show significant differences of the mean annual ground surface temperature (MAGST), ranging from -1.9 (Usnea) to -2.6 degrees C (Sanionia). Despite the clear differences in MAGST at the investigated sites, the mean annual ground temperature at 30 cm is virtually identical. Our results confirm that mosses play an important role in cooling the ground. The results of our study allow us to suggest also that the thawing degree days should be used instead of the growing degree days as a more suitable measure of the favourable conditions for the growth of the Antarctic cryptogam vegetation. Comparing our data with those of Chambers [Chambers, M.J.G., 1966b. Investigations on patterned ground at Signy Island, South Orkney Islands: II. Temperature regimes in the active layer. British Antarctic Survey Bulletin, 10: 71-83.], we can stress that the thermal conditions favourable to the frost heave is actually even more limited in depth (30 cm vs 40 cm in Chambers). Moreover, the freeze-thaw days near the surface appeared to be more frequent in the vegetated sites than in the bare ground.