Hydraulic redistribution in mangroves: time-lapse electrical resistivity
reveals diel patterns of subsurface salt mobilization consistent with
exchange of water between trees and sediments
Abstract
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.