Tree responses to hummock hydrology in a forested coastal swamp

Abstract:

Microtopographic controls on variability in growth stressors, such as salinity and flooding, may be crucial to some forested wetlands, especially because wetland trees generally occupy hummocks more frequently than swales. We quantified how elevation differences of less than one meter affect sources of water uptake by individual trees in the Mississippi River Delta, Louisiana. Salinity and naturally occurring stable isotopes of water were used as tracers of hydrologic and physiologic processes. Water samples were taken from tree xylem and soil at multiple depths across a range of drought and wet conditions. Hummocks were frequently but temporarily submerged after rainfall (total 9% of the time) and tides influenced inundation (every 46.9 hours). Isotopic composition of surface water and throughfall were similar, and were temporally more variable than was groundwater in hummocks. Salinity was lower but more temporally variable in the upper hummocks than in lower hummocks. Low deuterium excess indicated evaporative concentration in the unsaturated zone. Low deuterium excess (1.9 ± 2.1‰ below the local meteoric water line) and low temporal variability in groundwater suggested long residence time and limited exchange with surface water. The δD/δ¹⁸O ratio of xylem water (3.9) matched that of pore water in the unsaturated zone, showing that the water source for trees was the upper portion of hummocks. This study indicates that trees rely on hummocks as refuge from salinity in addition to reducing flood durations.