Description:
<jats:title>Abstract</jats:title><jats:p>
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<jats:list-item><jats:p>Tree lines are supposed to react sensitively to the current global change. However, the lack of a long‐term (millennial) perspective on tree line shifts in the Pyrenees prevents understanding the underlying ecosystem dynamics and processes.</jats:p></jats:list-item>
<jats:list-item><jats:p>We combine multiproxy palaeoecological analyses (fossil pollen, spores, conifer stomata, plant macrofossils, and ordination) from an outstanding ice cave deposit located in the alpine belt <jats:italic>c</jats:italic>. 200 m above current tree line (Armeña‐A294 Ice Cave, 2,238 m a.s.l.), to assess for the first time in the Pyrenees, tree line dynamics, and ecosystem resilience to climate changes 5,700–2,200 (cal.) years ago.</jats:p></jats:list-item>
<jats:list-item><jats:p>The tree line ecotone was located at the cave altitude from 5,700 to 4,650 cal year <jats:sc>bp</jats:sc>, when vegetation consisted of open <jats:italic>Pinus uncinata</jats:italic> Ramond ex DC and <jats:italic>Betula</jats:italic> spp. Woodlands and timberline were very close to the site<jats:italic>.</jats:italic> Subsequently, tree line slightly raised and timberline reached the ice cave altitude, exceeding its today's uppermost limit by <jats:italic>c</jats:italic>. 300–400 m during more than four centuries (4,650 and 4,200 cal year <jats:sc>bp</jats:sc>) at the end of the Holocene Thermal Maximum. After 4,200 cal year <jats:sc>bp</jats:sc>, alpine tundra communities dominated by <jats:italic>Dryas octopetala</jats:italic> L. expanded while tree line descended, most likely as a consequence of the Neoglacial cooling. Prehistoric livestock raising likely reinforced climate cooling impacts at 3,450–3,250 cal year <jats:sc>bp</jats:sc>. Finally, a tree line ecotone developed around the cave that was on its turn replaced by alpine communities during the past 2,000 years.</jats:p></jats:list-item>
<jats:list-item><jats:p><jats:italic>Synthesis</jats:italic>. The long‐term Pyrenean tree line ecotone sensitivity suggests that rising temperatures will trigger future <jats:italic>P. uncinata</jats:italic> and <jats:italic>Betula</jats:italic> expansions to higher elevations, replacing arctic–alpine plant species. Climate change is causing the rapid melting of the cave ice; rescue investigations would be urgently needed to exploit its unique ecological information.</jats:p></jats:list-item>
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