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.Key PointsWater 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
Highlights
An important challenge in ecohydrology is identifying the direct controls exerted by water or its absence on vegetation
We present isotopic data for 50 trees, which we subdivide by species, as well as by floodplain elevation, gravel elevation, and soil depth
In order to quantify how atmospheric conditions might affect our isotopic values in tree ring cellulose, we investigated the role of interannual differences in relative humidity (RH), temperature (T), and barometric pressure by first extracting daily values of these climate variables for the 3 month growing season, 15 May–15 August
Summary
An important challenge in ecohydrology is identifying the direct controls exerted by water or its absence on vegetation. It is important to separate the influences of climatically driven water availability from local, site-based physical factors and anthropogenic impacts to the water cycle. This is possible using trees that record hydrologic signatures in their annual growth rings. It could be used to better understand the recent history of partitioning between water storage reservoirs (e.g., vadose versus phreatic zones). This would support drainage basin water management, as well as restoration of river flows and of forest resources. We use historical time series of tree ring oxygen isotopes, ring width chronologies, instrumental climate data, highresolution topography, and soil depth measurements to identify climatic, anthropogenic, and physical controls on spatial and temporal variability of water sources used by streamside trees and their influence on annual tree growth within this major river floodplain
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