Tropical Climate Reconstruction with Tree-Ring Chronologies from the Central Amazon and Eastern Highlands of Brazil

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This collaborative project will involve the University of Arkansas (David Stahle), the Brazilian National Institute for Amazonian Research/Max Planck Institute (Jochen Schongart), and the Federal University at Lavras, Brazil (Ana Carolina Barbosa). Rigorous dendrochronological methods will be used to train students and faculty at both institutions and to develop several exactly dated, well replicated ring-width chronologies covering the last 300-years in the seasonally flooded bottomland forests of the central Amazon basin and in seasonally dry upland forests of eastern Brazil. Exactly dated tree-ring chronologies provide a vital preindustrial perspective on global warming and have produced the only spatially distributed estimates of past megadroughts and pluvials needed for testing climate model simulations of the dynamics responsible for decadal moisture regimes over North America. The vast majority of tree-ring chronologies available worldwide have been developed from temperate and boreal forests. However, the most biodiverse forest ecosystems in the world, the forests of the Amazon Basin in Brazil, have not produced a single freely available multi-century tree-ring chronology useful for exact dating or climate reconstruction. In the absence of a cold winter dormant season, most tropical tree species do not produce anatomically distinctive annual growth rings and therefore cannot be used for routine dendrochronology. However, strong environmental rhythms do exist in some tropical forests and can induce annual ring formation in a few native species. This includes the profound precipitation seasonality and the prolonged flood pulse of lowland forests in the central Amazon. Preliminary research has proven that annual rings exist in Macrolobium acacifolium in these inundation forests and their potential value for hydroclimatic applications has been demonstrated. Annual rings have also been proven in Cedrella fissilis and C. odorata from seasonally dry upland forests of Brazil and eastern Bolivia, and ring width and oxygen isotope chronologies from both are related to large-scale climate variability. A major impediment to the development of long climate sensitive tree ring chronologies in Amazonia has been the lack of expertise with the rigorous application of dendrochronology, the most accurate and precise dating method in geochronology. The 300-year long master chronologies of tree growth and climate history that will be developed through this project are expected to leverage a wide range of interdisciplinary research, including climate dynamics, isotope paleoclimatology, and forest ecology. Once this research concept is proven, further collaborative investigations are envisioned that involve US and Brazilian scientists, in order to model the climate and stream flow response of the derived chronologies, develop calibrated reconstructions of annual hydroclimatic conditions, and investigate the large scale ocean-atmospheric forcing of moisture regimes over tropical South America.