High Mountain Palaeoecology and Palaeolimnology Fo Central Pyrenees, Based on Pollen and Diatom Analyses

Résumé du livre

High-mountain ranges are suitable ecosystems for studying local environmental shifts driven by large-scale climate changes. Sedimentary records obtained from those regions contain information that allows to understand past environmental changes that would help to predict the influence of the ongoing climate warming. This thesis is focused in the palaeoecology and palaeolimnology of Central Pyrenees. In order to understand the ecosystem responses to past environmental changes and climate, we analysed several proxies from sedimentary sequences extracted from Bassa Nera pond. We combined biological indicators (pollen, diatoms, chrysophytes, non-pollen palynomorphs, microscopic charcoal particles, macroremains, tree-rings and DNA metabarcoding) and inorganic proxies (Loss-on-ignition and chemical elements) to assess the diverse questions proposed in this work. To infer vegetation shifts and aquatic changes during the past millennium, we analysed pollen and diatom at multidecadal resolution. A montane pollen ratio was introduced as a new palaeoecological indicator of altitudinal shifts in vegetation. Results emphasize the sensitivity of the montane ratio to detect upward migrations of deciduous forest and the presence of the montane belt close to Bassa Nera during the Medieval Climate Anomaly. Changes in aquatic taxa allowed to date the development of the peat bog in the coring site around AD 1565. Overall, the studied proxies suggest that Bassa Nera had a low-intensity human pressure and that people changed from farming in the Medieval Climate Anomaly to livestock in the Little Ice Age. To reconstruct the vegetation and lacustrine dynamics during the last 10,000 years we analyzed pollen, plant macroremains, charcoal, chemical elements and loss-on-ignition. The montane ratio was also applied to track altitudinal shifts and it was compared to the ice-rafted debris index. Results revealed upward shifts of deciduous forest and its presence in Bassa Nera from the onset of the Holocene until 4200 cal yr BP. The montane ratio showed a link between vegetation and North Atlantic influence, while changes in macroremains and aquatic taxa allowed the description of the transition from the initial pond to the present peatland. First anthropic pressures were grazing activities by 7300 cal yr BP, while cereal agriculture appeared around 5190 cal yr BP. The late Bronze Age, Roman Period and Middle Ages periods presented the highest human pressure. To assess the dynamics of subalpine forests of Central Pyrenees for the last 700 years, we compared the sedimentary pollen and montane ratio from Bassa Nera with nearby Pinus uncinata tree-rings. To study the climate-growth associations, we related the chronologies with instrumental meteorological records of the 1901-2010 period and with temperature reconstructions for the Pyrenees and Northern Hemisphere for the last 700 years. Few robust associations were found between any specific arboreal pollen taxa and tree-rings. However, a significant correlation was found between the montane ratio and the pine growth of nearby subalpine forests. Results suggest that tree-growth variability at high elevations is more constrained by low than by high temperatures, although a relaxation of this constrain in recent decades was also noted. To explore the eukaryotic communities of Bassa Nera, we performed a metabarcoding study of four different micro-habitats and five sedimentary depths using 18S and COI genetic markers. The sedimentary DNA from palaeoecological communities were compared to the modern communities and also to the environmental reconstruction from pollen and macroremains from the same record. Results show that even though 18S could amplify a broader group of organisms, the taxonomic resolution was lower than that obtained from COI and that the taxonomic assignment of the COI sequences yielded mostly metazoans. This first molecular approach has allowed to prove that the diversity of modern and past eukaryotic peat bog communities can be assessed using universal metabarcoding markers.