Which water sources do trees on floodplains in southeastern Brazil use for transpiration?

Seasonal floodplain forests are important ecosystems that attenuate floods and have high biodiversity. However, floodplains are threatened by human activities, such as dam building, agricultural water use, and climate change. Improving our understanding of the functioning of floodplain forests can aid in their protection. Trees in the floodplain forests in southeastern Brazil experience flooding for more than a month per year but also must endure very dry periods where the groundwater level is several meters below the surface. Species composition depends on the flooding regime, but which water the trees use for transpiration is largely unknown. As a result, their vulnerability to changes in climate or flooding regime remains poorly understood.

We sampled the different water sources (precipitation, streamflow, groundwater, and soil water at different depths) and vegetation (covering more than 60 tree species) across six floodplain forests in the Rio Grande and São Francisco basins in southeastern Brazil during four campaigns (two dry and two wet seasons). At each floodplain, we took samples from three different “eco-units”: levees (close to the river), terraces (wettest parts of the floodplain), and plains (regions that do not get flooded). The samples were analyzed for the abundance of hydrogen and oxygen stable isotopes. These data were used together with the MixSIAR model to investigate the contribution of soil water (down to 1 m) for tree water uptake.

The variability in xylem water was large and there was no consistent variation in the isotopic composition of the soil water between the dry and wet seasons. Instead, soil water reflected the isotopic signature of wet season precipitation and overbank flow. We hypothesize that the soil isotopic signature is reset by precipitation and overbank flow every wet season. There was also no consistent pattern in the isotopic composition of the xylem water across the three “eco-units”. The mixing model analyses suggest that for the floodplains in the Rio Grande basin, soil water was the main source during the wet season (64% ± 17) but not during the dry season (43% ± 17), when groundwater or stream water were the predominant sources. For the floodplains in the drier São Francisco basin, soil water was the main source of tree water uptake (60% ± 17 and 72% ± 15 for wet and dry seasons, respectively). However, uncertainties are very high due to the similar isotopic composition of the potential source waters.