P2C2: Amazonian Tree-Ring Chronologies for Climate and Streamflow Reconstruction

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This project aims to develop 200-400 year-long tree-ring chronologies from the Amazon Basin to extend the short instrumental record of precipitation and streamflow.Amazon River discharge, the largest on Earth, is modulated in part by ocean-atmospheric forcing from the tropical Atlantic and Pacific. Amazon discharge has been monitored since 1902 and appears to have entered a new regime of amplified hydrological variability. The recent divergence in minimum and maximum flows has no precedent in the short instrumental record. The cause of this intensified hydrological cycle over Amazonia is not known, but may arise from deforestation or moisture advection changes associated with the observed warming of the tropical North Atlantic.A severe decline in annual precipitation is predicted for the mid-21st century over most of Amazonia based on CMIP5 climate model ensemble simulations under business-as-usual greenhouse gas emissions and land cover changes. How unprecedented the recent and predicted hydroclimatic changes might be in the context of natural climate variability during the late Holocene cannot be determined empirically from the few short and spatially discontinuous instrumental observations from the Amazon Basin.Tree-ring reconstructions have provided well-dated spatially distributed estimates of past megadroughts and pluvials needed for verifying model simulations of the dynamics responsible for decadal moisture regimes over North America and Asia. However, the most biodiverse forest ecosystems in the world, the forests of Amazonia, have not produced freely available multi-century tree-ring chronologies 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 tree-ring formation in a few native species. These annual changes include the profound precipitation seasonality and prolonged flood pulse of lowland forests in the central, eastern, and southern Amazon. Colleagues on the research team have proven that annual rings exist in Macrolobium acaciifolium in these inundation forests and have demonstrated their potential value for hydroclimatic applications. Annual rings have also been proven in Bertholletia excelsa, Cedrella fissilis, C. odorata, and Centrolobium microchaete from seasonally dry upland forests of Brazil and eastern Bolivia, and ring width and oxygen isotope chronologies derived from them are related to large-scale climate variability. This project will use new tree-ring chronologies to develop gridded regional moisture reconstructions, to frame the most extreme pre-instrumental and modern droughts in the context of climate change using CMIP5 ensemble simulations of the past millennium and the 21st century, and to explore the stability of ocean-atmospheric forcing of climate over Amazonia.The project will also leverage research resources and unite students and investigators from the United States, Brazil, Bolivia, and Argentina in the application of dendroclimatology to select tropical hardwoods in Amazonia. An international collaboration of climatologists, dendrochronologists, and forest scientists has been organized with the expertise required to solve the complexities involved in the exact tree-ring dating of tropical hardwoods.