High mountain palaeoecology and palaeolimnology fo Central Pyrenees, based on pollen and diatom analyses

Abstract

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.

Los registros sedimentarios obtenidos en ecosistemas de alta montaña contienen información que permite entender los cambios ambientales ocurridos en el pasado que ayudarían a predecir la influencia del calentamiento climático actual. Esta tesis se centra en el estudio de la paleoecología y paleolimnología de los Pirineos Centrales a lo largo del Holoceno basado en el análisis de diversos proxis de dos secuencias sedimentarias de Bassa Nera. También se introdujo un índice de polen montano como un nuevo indicador paleoecológico para resaltar cambios altitudinales en la vegetación caducifolia. Nuestros resultados mostraron cambios en el ascenso de los bosques caducifolios y su presencia en Bassa Nera desde el inicio del Holoceno hasta el año 4200 años BP, así como su ascenso durante la Anomalía Climática Medieval. El índice montano mostró una relación entre la vegetación y la influencia del Atlántico Norte, así como una correlación significativa entre el índice de polen montano y el crecimiento del pino de bosques subalpinos cercanos. Mientras que los cambios en los macrofósiles de Sphagnum y los taxones acuáticos permitieron la descripción de los cambios ontogénicos locales desde el estanque inicial hasta la turbera actual. Los cambios en los taxones acuáticos también permitieron datar cuando la turbera circundante se desarrolló y colmató el lugar donde se extrajo el sondeo, siendo esta en 1565 AD. En general, los resultados sugieren que Bassa Nera tuvo una presión humana de baja intensidad. Las primeras inferencias de presión antrópica en Bassa Nera datan de 7300 años BP y fueron actividades de pastoreo, mientras que los cereales aparecieron alrededor de 5190 años BP. Por otro lado, nuestros resultados sugieren que la sensibilidad del crecimiento del pino al clima ha variado en los últimos 700 años y que la variabilidad del crecimiento arbóreo en localidades altas está más limitada por las temperaturas bajas que por altas, aunque también observamos una relajación de esta restricción en las últimas décadas. Los resultados del estudio metabarcoding utilizando los marcadores genéticos universales 18S y COI sugieren que la diversidad de comunidades de turberas eucariotas modernas y pasadas pueden ser evaluadas usando marcadores genéticos universales.