Palaeoecological study of vegetation dynamics in the Neotropical Gran Sabana since the Late Glacial = Estudio paleoecológico de la dinámica de la vegetación en la Neotropical Gran Sabana desde el Tardiglaciar

Abstract

The Gran Sabana (GS, Northern South America) is a humid savanna region (treeless savanna intermingled with forest patches) with a long history of vegetation changes under fire disturbances. A number of hypotheses seek to explain the importance of environmental factors (e.g., climate, fire) in the origin and maintenance of the GS savannas. In order to fully comprehend the ontogeny of the GS, this thesis was aimed at conducting a detailed palaeoecological evaluation of vegetation dynamics and ecological processes (e.g. feedbacks, resilience) from the Late Glacial to the present, analysing two sedimentary archives (Lacustrine: Lake Encantada, peat bog: Fidencio). Currently, the GS savannas are being highly threatened due to excessively frequent fires. In addition, forest patches are being replaced by grassland. Therefore, in order to achieve ecosystem sustainability, this investigation was also aimed at forecasting potential vegetation responses and suggesting some guidelines for the savannas management. To accomplish the objectives, the palaeoecological assessment was based on palynological analysis and multiproxy approach (e.g., geochemical proxies, ecological indices) where possible. Lake Encantada's basin is currently dominated by grassland with morichales (Mauritia flexuosa palm stands). The results suggest that the multifactorial interplay between fire regime, diversity, grasses availability, moisture and rates of recovery (RR) could have caused the vegetation changes during the Holocene and also contributed to the degree of resilience of plant communities. Forest patches were resilient to fire during specific time intervals. The vegetation changes suggest the following general trend: expansion of forest patches between -8700 and 6400 cal yr BP, probably driven by the interplay between fires, high RRs and diversity. Despite wetter climate from 5400 cal yr BP, the savanna expanded under higher fire occurrence, which was potentially related to human land-use practices. The modern grassland with morichales established around 1700 cal yr BP, coinciding with drier conditions. The interplay between fire and erosion could have promoted forests instability, and thereby playing a part in the savanna expansion since middle Holocene. Drier climate and high availability of grasses probably reinforced this interplay. vegetation of forest-savanna mosaic was replaced suddenly by grassland around 12,100 cal yr BP. This vegetation shift seems to be the origin of grassland in Fidencio and occurred during a period characterised by marked climate changes (Late Glacial). The origin of open vegetation may have been caused by the synergistic action between rapid warming at the end of the Younger Dryas (YD), drier climate, fire (natural and/or anthropogenic), decreased diversity and grasses availability. The aforementioned synergism could have played a role in reducing resilience of the Late Glacial forests. Considering the future scenario of abrupt warming and higher occurrence of fires for South America, the significant expansion of grasses at the YD-Holocene transition suggests that savanna vegetation in the GS might expand in the future, similarly to what was observed historically.

In both localities, the vegetation changes from forest areas to savanna appear to have been irreversible on the timescales studied. The positive feedback between fire and herbs has accentuated the savannisation processes, which were associated with loss of diversity. The valuable information obtained in this thesis could help to better understand the ecology of savanna, particularly that of moist savannas.

La Gran Sabana (GS, Norte de Sur América) es una región de sabana húmeda con una larga historia de cambios de vegetación bajo perturbaciones de fuego. En esta tesis se realizó una evaluación detallada de la dinámica de la vegetación y de los procesos ecológicos de la GS desde el Tardiglaciar hasta el presente, basada en el análisis palinológico de dos archivos sedimentarios. En la cuenca de la Laguna Encantada, los resultados sugieren que la interacción multifactorial entre el régimen de fuegos, diversidad, erosión, disponibilidad de pastos, humedad y tasas de recuperación podría haber causado los cambios de vegetación durante los últimos 8700 años. La vegetación fue resiliente al fuego durante períodos específicos, en una escala centenal. La sabanización comenzó en la cuenca aproximadamente hace 5400 años, la cual se ha acentuado en los últimos dos mil años, dando lugar a la vegetación actual (pastizales con bosque de galería monoespecíficos de Mauritia flexuosa). En la localidad Fidencio, el reemplazo de densos parches de bosque por pastizales en el Tardiglaciar (sabanización), al parecer fue causado por la sinergia entre el rápido calentamiento del Dryas Reciente (Younger Dryas, YD), clima seco, fuego, disminución de diversidad y disponibilidad de pastos. Este sinergismo podría haber erosionado la resiliencia de los bosques del Tardiglaciar. Este cambio de vegetación (origen de los pastizales) en la transición YD¬Holoceno podría considerarse como un análogo del pasado del entorno ambiental actual de la GS. Esto sugiere que la vegetación de sabana podría expandirse en el futuro en la GS, de forma similar a la que ocurrió históricamente. En ambas localidades, los cambios de vegetación de áreas forestales a sabana parecen haber sido irreversible en las escalas de tiempo estudiadas. La retroalimentación positiva entre el fuego y los pastos acentuó los procesos de sabanización en ambas localidades, los cuales estuvieron asociados con pérdida de diversidad.