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Hydraulic redistribution in mangroves: time-lapse electrical resistivity reveals diel patterns of subsurface salt mobilization consistent with exchange of water between trees and sediments
  • Christine Downs,
  • Ken Krauss,
  • Sarah Kruse
Christine Downs
Sandia National Laboratories, Sandia National Laboratories

Corresponding Author:cdowns@sandia.gov

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Ken Krauss
U.S Geological Survey
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Sarah Kruse
University of South Florida,Massachusetts Institute of Technology, University of South Florida,Massachusetts Institute of Technology
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A 24-hour 2D time-lapse electrical resistivity imaging (ERI) survey was conducted in an altered mangrove forest on a barrier island in southeast Florida, USA, to (1) assess the method’s utility in hypersaline conditions and (2) understand how trees respond to hypersaline conditions. ERI measurements serve as a proxy for pore water salinity and saturation. Here, resistivity changes suggest a lag between the tidal cycle and changes in ground resistivity. ERI data show that overall changes within 24 hours are very small, but there is more variability in resistivity in the root zone of mangroves than in open salt flat portions along a fixed transect. Two to three hours after sunset, root zone resistivity increased from initial, midday conditions. Overnight, the root zone was less resistive than midday. By sunrise, root zone resistivity was once again higher than initial conditions. Measurements from the salt flat where roots are absent remained generally constant throughout the survey. Thus, changes in resistivity over time are inferred to reflect mangrove tree physiological influences related to diel water use. A mechanistic explanation for the decreased resistivity two hours after sunset from the re-distribution of salts to the soil around the roots is the Cohesion-Tension Theory, which suggests that trees continue water uptake after sunset to balance the pressure after leaf stomates have closed. The corresponding overnight drop in ground resistivity just prior to sunrise may be explained by redistribution of freshwater from the tree to the soil that was delayed until the early morning hours. The limited period of data acquisition limits definitive data interpretations, but the study illustrates the monitoring potential of ERI in hypersaline environments such as a mangrove forest.