Ice cave reveals environmental forcing of long‐term Pyrenean tree line dynamics

Abstract

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 (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. 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.

Treeline Research—From the Roots of the Past to Present Time. A Review

“…During the last three decades, especially since the 1990s, publications on this topic have rapidly increased (by about 90%), partly in a broader context with the change of vegetation and biodiversity, and the expected implications for the ecosystem functions and services of high-elevation forests (e.g., [88,134,167,) ( Figure 3). Dendrochronology, pollen analysis, sediment analysis, and radiocarbon-dating of fossil wood remains (mega fossils) have provided a profound insight into Holocene treeline fluctuations (e.g., [203,[206][207][208][209][210][211][212][213][214][215][216][217][218][219][220][221][222][223][224]). In not a few cases their after-effects have lastingly influenced the current treeline position and spatial patterns (e.g., [1,2] and references therein).…”

Neoglaciation in the Spanish Pyrenees: a multiproxy challenge

“…Studies on speleothems in Pyrenean caves have revealed various cold and wet phases that favored stalagmite growth, particularly during the YD, the very early Holocene (9.1-8.3 ka), mid-Holocene (6.3-6.2 ka), which could be related with the onset of the Neoglacial period, and the late Holocene, at 3.0-2.5 ka (the Iron Age), 1.8-1.4 ka (the end of the Roman Period and the early Middle Ages) and 0.6-0.2 ka (the LIA) (Bartolomé et al 2012). The information from the ice caves is still embryonic, although encouraging, showing significant oscillations in the elevation of the treeline ecotone in the Cotiella Massif (Leunda et al 2019), and different phases of rapid snow/ice accumulation (Sancho et al 2018b) that do not exactly coincide with those obtained from speleothems: 6.1-5.5 ka, 4.9-4.2 ka, 3.8-3.1 ka, and 2.4-1.9 ka, the LIA ice accumulation having melted because of post-LIA warming.…”

“…The 10 Be CRE ages (Palacios et al 2017;Crest et al 2017) were recalculated with the online exposure age calculator (Martin et al 2017; https ://crep.crpg.cnrs-nancy .fr), using the parameters proposed by Lifton et al (2014). (iii) The analysis of organic matter content within the ice accumulated in high-altitude caves, developed during the Mid-and Late-Holocene (Sancho et al 2018b;Leunda et al 2019).…”

“…The Armeña-A294 ice cave (2238 m a.s.l.) in the Cotiella Massif preserves a declining ice deposit ranging between 5680 and 2230 cal year BP, and providing information on the occurrence of wetter winters during 4900-4200 cal year BP, suggesting "increasing snowfall at the onset of the Neoglacial" (Leunda et al 2019). In the same ice cave, Sancho et al (2018a, b) identified four phases of ice accumulation, two of which corresponded approximately to the beginning of the Neoglacial period: 6100-5515, and 4945-4250 cal year BP, probably associated with wetter and colder winters, and separated by intervals of drier and warmer winters.…”

Stable isotopes in cave ice suggest summer temperatures in east-central Europe are linked to Atlantic Multidecadal Oscillation variability

“…While several global (Jones and Mann, 2004;Mann et al, 2009) and hemispheric C.-A. Bȃdȃluţȃ et al: Summer temperatures in east-central Europe are linked to AMO variability (Moberg et al, 2005;Neukom et al, 2019;PAGES 2k Consortium, 2019;Ljungqvist et al, 2019) climatic reconstructions have been published, these made no seasonal differentiation -a task that became recently increasingly necessary to constrain seasonally distinctive climatic changes (e.g., Ljungqvist et al, 2019), as these respond to different forcing mechanisms (e.g., Pers , oiu et al, 2019). On multidecadal timescales, the summer climate over Europe is mainly influenced by the Atlantic Multidecadal Oscillation, or AMO (Schlesinger et al, 1994;Kaplan et al, 1998;Kerr, 2000;Knudsen et al, 2011Knudsen et al, , 2014.…”