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.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

Sub‐annual variability in historical water source use by Mediterranean riparian trees

“…Isotope tree-ring studies can provide detailed, historical records of the ecohydrological processes occurring during the time of growth, including the source-water uptake, for riparian forests (Alstad et al, 2008;Marshall and Monserud, 2006;Singer et al, 2013;Singer et al, 2014). This is achieved through analysis of oxygen isotope ratios within tree-ring cellulose (δ 18 O cell ), which preserves a signal from source water used by trees during growth.…”

“…Barbour et al, 2004;Roden et al, 2000) have demonstrated successes in predicting cellulose δ 18 O cell from a known source-water signature (δ 18 O sw ) by accounting for the relevant environmental and physiological parameters driving the fractionation of oxygen isotopes before the incorporation into cellulose. By accurately constraining these variables it is possible to use δ 18 O cell to determine the δ 18 O sw used during the growing season (Singer et al, 2014). Yet, such annual (whole-ring) analyses only provide an integrated signal of water-use over the entire growing season (Alstad et al, 2008;Marshall and Monserud, 2006;Singer et al, 2013;Singer et al, 2014), which may belie seasonal dynamics in water availability.…”

“…By accurately constraining these variables it is possible to use δ 18 O cell to determine the δ 18 O sw used during the growing season (Singer et al, 2014). Yet, such annual (whole-ring) analyses only provide an integrated signal of water-use over the entire growing season (Alstad et al, 2008;Marshall and Monserud, 2006;Singer et al, 2013;Singer et al, 2014), which may belie seasonal dynamics in water availability. There is, however, additional information stored in tree rings at a sub-annual resolution that could contribute to new understanding of the details of how source-water availability may change throughout the critical growing period in response to climate fluctuations and seasonal evolution of water availability (Treydte et al, 2014).…”

“…Climate controls the amount of water delivered to each floodplain reservoir and thus the rooting zone of riparian trees . Yet the availability of water to riparian forests is further modified through species-specific differences in rooting morphology and the local topographic and substrate characteristics (Singer et al, 2014). Fluctuations in the water delivered to each floodplain compartment can therefore have disproportionate impacts on trees of different functional types and physiology.…”

Speculations on the application of foliar <sup>13</sup>C discrimination to reveal groundwater dependency of vegetation, provide estimates of root depth and rates of groundwater use

“…(2006c) for a steeply rising 20 topography in the Daly River, this is unlikely to be attributable to increased elevation since the change in elevation was minimal across each entire transect (< 3 m), and even smaller near Allungra Creek where most of the change in Δ 13 C was recorded. Therefore the cause of the change in Δ 13 C with distance from the creek was most likely to be a function of the frequency with which trees receive flood water and hence the amount of recharge into the soil profile (Ehleringer and Cooper, 1988;Thorburn et al, 1994;Villeneuve et al, 2015;Singer et al, 2014). We therefore further conclude that foliar 25 Δ 13 C can be used as an indicator of access to additional water to that of rainfall, regardless of the source of that additional water (e.g., groundwater, flood recharged soil water storage, irrigation).…”