Potentiality of past vegetation landcover reconstruction in forest- high andean steppe ecotone of southern Patagonia, Argentina. First results

SOTTILE, G.D.; TONELLO, M. S; MANCINI, M. V.

San Salvador de Bahía

Congreso; XIV International Palynological Congress y X International Organisation of Paleobotany Conference; 2016

High- altitude treelines are sensitive biomonitors of past and recent climate change and variability. Many authors have demonstrated that high elevation treeline responses can act as an "early- warning" of climate change impacts through structural, positional and compositional changes. Southern hemisphere temperature reconstructions show extreme warm period between 850-650 cal yrs BP (and since 1950), with an extreme cold period between 550-250 cal yrs BP. These oscillations are expected to have influenced the upper treeline of the forest and the forest- high andean steppe in southern Patagonia. these variations last for more than a hundred years, long enough to have a sustained effect on several tree cohort life cycle attributes that could be registered in the palaeoecological high altitude records. Past vegetation dynamics have been qualitatively or semiqualitatively reconstructed largely by pollen and charcoal records from peatbogs and lakes since middle twentieth century. There are not yet reported Holocene reconstructions of past vegetation land cover for South Americaa ecosystems. The main limiting factor to apply this technique is the lack of information about two main parameters, the sedimentation velocity and the relative pollen productivity estimates from the main pollen types. Southern Patagonia high Andean steppe communities adjacent to treeline present a high plant diversity and include peatbog, praire and dwarf shrub patches. In contrast, the upper Nothofagus forest are composed by monospecific closed canopy stands with a low understory vegetation diversity. We presenta first approach to understand pollen- vegetation relatonship by comparing modern pollen samples coupled with vegetation surveys around each sampling point. We highlight the main pollen types associated to modern communities by ordination analysis and correlate modern forest land cover at different concentric radii with Nothofagus pollen percentages from modern pollen samples. The ordination analysis suggests that pollen assemblages from the high Andean steppes differ from treeline forest assemblages although pollen samples were arranged close to one another. Thus, past change onn Southern Patagonia treeline position could be detected palynologically from peat-bog sediments. The correlation analysis show an r= 0.8 between forest land cover at 1500 meters concentric radii (oriented according to the prevailing wind direction) and Nothofagus pollen percentages. These first results will help us to focus future research to apply Multiple Scenarios Approach to reconstruct past treeline communities land cover at different periods during the last 1500 years ad compare the vegetation changes to those expected in the context global warming.