Ancient DNA and lipid biomarkers quantify the climate sensitivity of highland shrubification in Iceland

David Harning, Samuel Sacco, Jonathan Raberg, Nicolò Ardenghi, Thor Thordarson, Julio Sepúlveda, Gifford Miller, Áslaug Geirsdóttir

Future changes in high latitude shrubification are expected to lead to changes in ecosystem structure and positive climate feedbacks, but the rates and elevational range of shrubification are still poorly constrained. Using a sediment record from a small lake in Iceland’s eastern highlands (422 m asl), we merge a sedimentary ancient DNA (sedaDNA) record of Betula with mean summer lake temperature (MST) reconstructed from bacterial branched glycerol dialkyl glycerol tetraethers (GDGTs) to quantify the extent and climate sensitivity of Icelandic woodlands. Our data show that during the Early Holocene MSTs were 2.75 oC warmer than present and Betula woodlands were present in the lake‘s catchment, significantly higher than their current regional limit (250 to 350 m asl). During the Middle and Late Holocene, reconstructed MSTs are unreliable likely due to reducing conditions in the lake‘s water column inferred from archaeal isoprenoid GDGTs. However, relative temperature changes inferred from biogenic silica abundance indicate that the disappearance of Betula from the catchment was coeval with Little Ice Age cooling. Using Early Holocene warmth as a partial analog for future climate change, Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble projections suggest that the natural re-expansion of Betula woodlands to the eastern highlands is possible by 2100 CE, which would occupy over 42 % of Iceland‘s land surface area.