Reconstruction of a fossil forest reveals details of the palaeoecology, palaeoenvironments and climatic conditions in the late Oligocene of South America

Abstract

This research focuses on the three-dimensional reconstruction of an in situ forest based on fossil wood assemblages recovered in the Rancahué Formation (Upper Oligocene), Neuquén, Argentina. Atherospermataceae, Lauraceae, Nothofagaceae, Eucryphiaceae, Cunoniaceae and Myrtaceae specimens are described. The mapping of a forest floor section and in situ tree diameters enabled the estimation of the following palaeoecological quantitative data: tree density, dominance, basal area, biomass, diametric classes, canopy height, and age classes. Palaeoclimatical data was determined on the basis of physiognomic anatomical features using multivariate anatomical analyses. These results were compared with other proxies including Carlquist's index, Coexistence Approach (CA), Nearest Living Relatives (NLRs), and growth-rings analyses. The structural data from the Aluminé forest inferred from these analyses includes: tree density of 463-701trees/ha, mean height of 15.22m, dominance of the genus Nothofagoxylon (89.66m2/ha), total basal area of 158.20m2/ha, biomass between 43 and 712tn/ha and mean age of 223years (specimens between 31 and 700years old). These results are comparable to those of mature low-to-middle altitude extant forests dominated by Nothofagus and developed under humid-temperate conditions. Based on the NLRs method, the Aluminé forest has a floristic composition similar to the present-day Valdivian forest. The persistence of Nothofagus as the dominant element in temperate rainforests correlates with regimes where large-scale disturbances, such as volcanism and earthquakes are prevalent. The fossil taxa are closely related to the extant Laurelia, Persea, Eucryphia, Nothofagus, Weinmannia, Myrceugenia and Luma. The forest shows intermixed deciduous and evergreen elements, and taxa with shade-intolerance and intermediate shade tolerance. Also, the majority of these taxa need soils with available water. The integrated analysis of multiple sets of proxy data suggests that the late Oligocene forest grew under temperate and humid climate, while the eco-anatomical features and sedimentary data provide information about the environmental stress conditions of its development and the violent causes of burial.