Regional Differences in South American Monsoon Precipitation Inferred from the Growth and Isotopic Composition of Tropical Trees

The authors present results on the relationship between treering proxies and regional precipitation for several sites in tropical South America. The responsiveness of oxygen isotopes (delta O-18) and seasonal growth as precipitation proxies was first validated by high-resolution sampling of a Tachigali myrmecophila from Manaus, Brazil (3.1 degrees S, 60.0 degrees W). Monthly growth of Tachigali spp. was significantly correlated with monthly precipitation. Intra-annual measurements of cellulose delta O-18 in Tachigali spp. were also significantly correlated with monthly precipitation at a lag of approximately one month. The annual ring widths of two tropical tree taxa, Cedrela odorata growing in the Amazon (12.6 degrees S, 69.2 degrees W) and Polylepis tarapacana growing in the Altiplano (22.0 degrees S, 66.0 degrees W), were validated using bomb-derived radiocarbon C-14. Estimated dates were within two to three years of bomb-inferred C-14 dates, indicating that these species exhibit annual rings but uncertainties in our chronologies remain. A multiproxy record spanning 180 years from Cedrela spp. showed a significant negative relationship between cellulose delta O-18 and January precipitation. A 150-yr record obtained from Polylepis spp. also showed a significant negative relationship between delta O-18 and March precipitation, whereas annual ring width showed a significant positive correlation with December precipitation. These proxies were combined in a multivariate framework to reconstruct past precipitation, revealing a significant increase in monsoon precipitation at the Amazon site since 1890 and a significant decrease in monsoon precipitation at the Altiplano since 1880. Proxy time series also showed spatial and temporal coherence with precipitation variability due to El Nino forcing, suggesting that oxygen isotopes and ring widths in tropical trees may be important diagnostics for identifying regional differences in the response of the tropical hydrologic cycle to anthropogenic warming.