Holocene Vegetation and Disturbance Dynamics in the Araucaria araucana Forest: a paleoecological contribution for conservation

Abstract

Climatic changes have changed the vegetation distribution in northern Patagonia over millennial time-scale; while, fire and volcanoes disturbance might play an important role in the vegetation dynamics, especially in the Araucaria araucana forest. The Holocene history of the Araucaria araucana is poorly studied despite of its evolutionary, ecological, and social importance. Araucaria araucana is well adapted to resist moderate disturbances, and it is though that human activity has threatened the persistence of the Araucaria araucana. Little is known about the history of Araucaria araucana forest, its natural variability and past disturbances regime. An insight into the long-term patterns and process could provide information for conservation management. Therefore, this thesis aims to reconstruct the Holocene vegetation and disturbance history throughout the analysis of pollen, charcoal, and tephra deposited in lake sediments. In addition, I assessed the impacts of recent land-use change, fire and volcanic events in the Araucaria araucana forest in three separated chapters. The results show that land-use change since early colonization of the northern Patagonia have had a strong impact on the vegetation only for the last 50 years, in the form of Pinus plantation. In the long term, interpreted from a 9000-years-old sediment record from Lake Relem, an overall change in pollen dominance from Poaceae toward Nothofagus dombeyi-type about 4.5 ka was observed, suggesting that the landscape shift from a grassland to a forest and scrubland. The forest-steppe ecotone was probably fragmented, but the degree was likely less than today. This general trend has been disrupted by 20 fire and 39 volcanic events. Volcanic eruptions buried the vegetation around the studied lake by tephrafall. Tephrafall were relatively frequent, but with low impacts on the vegetation. Only one large eruption, the Sollipulli-Alpehue, triggered a dramatic change about 3 ka. Vegetation recovered short after with dominant Ephedra, yet return to pre-eruption condition after c.500 years. Fire regime, was variable between 6-3 ka BP when fire frequency and magnitude increased. Pollen abundance related to macro-charcoal denotes small severity of disturbances though. Fire during Euro-American colonization was comparatively small, and with the data I cannot confirm the change in fire regime. Although the early colonization does not change vegetation until more recent time, the magnitude and rate of changes produced by humans can be compared with the largest eruption in the region. The vegetation shows some sensitivity to past fire and volcanic disturbances; however, could recover fast to small-scale disturbances. In synthesizing records from the area, the comparison suggests that Araucaria araucana might reach the Andes before 10 ka, and that during the Holocene history until present there is no unique trend of decreasing populations or distribution changes of Araucaria araucana. During the Holocene, Araucaria araucana pollen abundance in Lake Relem suggests a low variability, disregard of disturbance. Future trend are unpredictable and more studies are required, but surely the spread of invasive Pinus toward natural areas is already affecting regeneration process and increasing the fuel biomass. Despite of some limitations, paleoecological studies are powerful tools to understand present vegetation patterns, thus could contribute to develop conservation strategies to Araucaria araucana forest.