title: Floodplain ecohydrology: Climatic, anthropogenic, and local physical controls on partitioning of water sources to riparian trees.
abstract: Seasonal and annual partitioning of water within river floodplains has important implications for ecohydrologic links between the water cycle and tree growth. Climatic and hydrologic shifts alter water distribution between floodplain storage reservoirs (e.g., vadose, phreatic), affecting water availability to tree roots. Water partitioning is also dependent on the physical conditions that control tree rooting depth (e.g., gravel layers that impede root growth), the sources of contributing water, the rate of water drainage, and water residence times within particular storage reservoirs. We employ instrumental climate records alongside oxygen isotopes within tree rings and regional source waters, as well as topographic data and soil depth measurements, to infer the water sources used over several decades by two co-occurring tree species within a riparian floodplain along the Rhône River in France. We find that water partitioning to riparian trees is influenced by annual (wet versus dry years) and seasonal (spring snowmelt versus spring rainfall) fluctuations in climate. This influence depends strongly on local (tree level) conditions including floodplain surface elevation and subsurface gravel layer elevation. The latter represents the upper limit of the phreatic zone and therefore controls access to shallow groundwater. The difference between them, the thickness of the vadose zone, controls total soil moisture retention capacity. These factors thus modulate the climatic influence on tree ring isotopes. Additionally, we identified growth signatures and tree ring isotope changes associated with recent restoration of minimum streamflows in the Rhône, which made new phreatic water sources available to some trees in otherwise dry years.
  
  
  CONCLUSIONS
  Water shifts due to climatic fluctuations between floodplain storage reservoirsAnthropogenic changes to hydrology directly impact water available to treesEcohydrologic approaches to integration of hydrology afford new possibilities.
authors.affiliation: Department of Earth and Environmental Sciences, University of St Andrews St Andrews, UK ; Earth Research Institute, University of California Santa Barbara Santa Barbara, California, USA,.
authors.forename: Michael Bliss
authors.initials: MB
authors.lastname: Singer
authors.affiliation: Department of Earth and Environmental Sciences, University of St Andrews St Andrews, UK.
authors.forename: Christopher I
authors.initials: CI
authors.lastname: Sargeant
authors.affiliation: Université Lyon, UMR 5600-CNRS, Site ENS-Lyon, ISIG Platforme Lyon, France.
authors.forename: Hervé
authors.initials: H
authors.lastname: Piégay
authors.affiliation: Université Lyon, UMR 5600-CNRS, Site ENS-Lyon, ISIG Platforme Lyon, France.
authors.forename: Jérémie
authors.initials: J
authors.lastname: Riquier
authors.affiliation: Department of Earth and Environmental Sciences, University of St Andrews St Andrews, UK.
authors.forename: Rob J S
authors.initials: RJ
authors.lastname: Wilson
authors.affiliation: Department of Earth and Environmental Sciences, University of St Andrews St Andrews, UK ; School of Mathematics and Statistics, University of St Andrews St Andrews, UK.
authors.forename: Cristina M
authors.initials: CM
authors.lastname: Evans
keywords: Rhône; climate change; oxygen isotopes (δ18O); soil moisture; tree rings; water partitioning