Isotopic Composition of Soil and Xylem Water Across Six Seasonal Floodplain Forests in Southeastern Brazil

Seasonal floodplain forests are unique but threatened ecosystems. Trees in these ecosystems have adapted to extreme conditions of excess water and drought. However, what water sources these trees use is not fully known and therefore the vulnerability of floodplain forests to changes in the hydrological cycle remains poorly understood. In this study, we sampled different water sources (precipitation, streamflow, groundwater, soil water at different depths) and vegetation across six floodplain forests in the Rio Grande and S & atilde;o Francisco basins in southeastern Brazil during four campaigns (two dry and two wet seasons). We used water stable isotope data to investigate the sources for root water uptake. There was no consistent dry-to-wet season variation in the isotopic composition of soil water, but soil water reflected the isotopic signature of the wet season's precipitation and overbank flow. There was a very large variability in the isotopic composition of xylem water, but no consistent pattern across the floodplains, or between tree species. Mixing model analyses suggest that for the floodplains in the Rio Grande basin, soil water was the main source of root water uptake during the wet season (64% +/- 17) but not during the dry season (43% +/- 17), when streamflow and groundwater were the main sources. In the S & atilde;o Francisco basin, soil water was the main source of root water uptake (60% +/- 17 and 72% +/- 15 for the wet and dry seasons, respectively). However, the uncertainties are large due to the similarity in the isotopic composition of potential water sources.