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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. 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 c. 200 m above current tree line (Armena-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. The tree line ecotone was located at the cave altitude from 5,700 to 4,650 cal year bp, when vegetation consisted of open Pinus uncinata Ramond ex DC and Betula spp. Woodlands and timberline were very close to the site. Subsequently, tree line slightly raised and timberline reached the ice cave altitude, exceeding its today's uppermost limit by c. 300-400 m during more than four centuries (4,650 and 4,200 cal year bp) at the end of the Holocene Thermal Maximum. After 4,200 cal year bp, alpine tundra communities dominated by Dryas octopetala 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 bp. Finally, a tree line ecotone developed around the cave that was on its turn replaced by alpine communities during the past 2,000 years. Synthesis. The long-term Pyrenean tree line ecotone sensitivity suggests that rising temperatures will trigger future P. uncinata and Betula 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.