117-2 High elevation shrubification in the Subarctic during the Holocene Thermal Maximum and 21st Century

Abstract:

Future changes in high latitude shrubification are expected to amplify warming. However, the elevational range and climate sensitivity of shrubification is poorly constrained, which hinders our ability to tune predictive Earth system models. To constrain these variables, we leverage multiple lake sediment records that span low to high elevations in Iceland (151 to 422 m asl), birch (Betula) ancient DNA, and mean summer air temperature (MST) reconstructed from bacterial lipids. Our data show that Holocene Thermal Maximum MSTs were up to 2.3 ^oC warmer than present. At this time, birch woodlands quickly spread into low elevation area and were even present in highland lake catchments (>420 m asl), significantly higher than their current regional limit (250 to 350 m asl). During the Middle and Late Holocene, birch generally persisted around the lakes until its disappearance in the last millennium due to Little Ice Age summer cooling. Our findings demonstrate that climate, rather than human impact, has been the dominant Holocene control on Icelandic woodland cover and are therefore ideal constraints for Earth system models. Using Holocene Thermal Maximum warmth as a partial analog for future climate change, model ensemble climate projections suggest that the re-expansion of birch to higher elevations is likely by 2100 CE.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-16543

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