Keep your feet warm? A cryptic refugium of trees linked to a geothermal spring in an ocean of glaciers.

Up to now, the most widely accepted idea of the periglacial environment is that of treeless ecosystems such as the arctic or the alpine tundra, also called the tabula rasa paradigm. However, several palaeoecological studies have recently challenged this idea, that is, treeless environments in periglacial areas where all organisms would have been exterminated near the glacier formed during the Last Glacial Maximum, notably in the Scandinavian mountains. In the Alps, the issue of glacial refugia of trees remains unanswered. Advances in glacier reconstructions show that ice domes did not cover all upper massifs, but glaciers filled valleys. Here, we used fossils of plant and malacofauna from a travertine formation located in a high mountain region to demonstrate that trees (Pinus, Betula) grew with grasses during the Lateglacial-Holocene transition, while the glacier fronts were 200-300 m lower. The geothermal travertine started to accumulate more than 14,500 years ago, but became progressively more meteogene about 11,500 years ago due to a change in groundwater circulation. With trees, land snails (gastropods) associated to woody or open habitats and aquatic mollusc were also present at the onset of the current interglacial, namely the Holocene. The geothermal spring, due to warm water and soil, probably favoured woody glacial ecosystems. This new finding of early tree growth, combined with other scattered proofs of the tree presence before 11,000 years ago in the western Alps, changes our view of the tree distribution in periglacial environments, supporting the notion of tree refugia on nunataks in an ocean of glaciers. Therefore, the tabula rasa paradigm must be revisited because it has important consequences on the global changes, including postglacial plant migrations and biogeochemical cycles.

[Fire and Pinus mugo Turra communities in the western Alps (Susa Valley, Italy) during the Lateglacial-Holocene transition: an evidence of refugia area]. / Incendies et peuplements à Pinus mugo Turra dans les Alpes occidentales (Val de Suse, Italie) durant la transition Tardiglaciaire-Holocène: une zone refuge évidente.

In western Italian Alps, small distinct populations of Pinus mugo Turra raise some questions concerning its ecological status and dynamics in the occidental Alps. This note present new palaeobotanical data based on cone imprints of Pinus mugo, identified in travertine systems located in the Val di Susa and dated back to the Late Dryas (11506+/-66 BP) and the Early Holocene (10145+/-225 et 9475+/-670 BP). Heliophilous species and charcoal fragments were also identified, testifying to the oldness of wildfires in this region. The data support the hypothesis that this zone was a refuge area of this pine during the last glaciation. Here we discuss about the postglacial dynamics of the Pinus mugo in the occidental Alps.

[Proof of the indigenous nature of Populus alba L. in the western Mediterranean Basin]. / Preuve de l'indigénat de Populus alba L. dans le Bassin méditerranéen occidental.

Around the western Mediterranean Basin, the ecological status of Populus alba, whether indigenous or introduced, is controversial. This note presents new palaeobotanical data based on analyses of leaf imprints from a travertine formation located in southern France. This travertine presents two levels with Populus alba imprints. The oldest level is dated back by 14C to the Early Holocene, i.e., 8890 +/- 70 BP. This demonstrates that Populus alba is an autochthonous species of the southern-France vegetation, removing speculations reporting that its distribution area was greatly influenced by Roman civilization. Finally, we discuss this new data in regard to other Pleistocene and Holocene deposits circum the Mediterranean Basin and in Europe, where this species was identified.

A new, isolated and endangered relict population of dwarf pine (Pinus mugo Turra) in the northwestern Alps.

Dwarf pines were discovered in 2004 during a paleoecological survey in the Mont Cenis massif (Savoy, France). These dwarf pines are the sole natural and spontaneous population in the NW French Alps of Pinus mugo Turra, ssp. mughus (Scop.) O. Schwarz. The population, fragile in light of the individual numbers, is currently isolated, but likely results from populations that would have covered larger areas during the Lateglacial or the early-Holocene, from the SW Alps (France) toward the Mont Cenis, throughout the Susa valley (Italy). With a fragmented distribution area of dwarf pine, the future of the Mont Cenis population seems altered due to important necroses observed on the topmost part of pine crowns, except for individuals that do not grow in the local ski station area. We stress the need for a conservation program to preserve this natural population, the sole known in the NW French Alps.