Hydrologic Controls over Water Use in a Forested Floodplain Wetland

Abstract:

Hydrologic variability is a primary driver of wetland forest physiological function and structure. Spatial variations in forest composition co-occur with hydrologic variations, generally assumed to be due to differences in tolerance to flooding and drought. To develop a process-level understanding of water table effects on wetland tree function, we measured transpiration of dominant bottomland hardwood species and analyzed how transpiration responded to spatial and temporal water table variations. Sap flow was measured with Granier-style heat dissipation probes in twenty-six trees (Celtis laevigata and Quercus lyrata) at two sites in the White River floodplain in Arkansas, USA (July – October, 2013). Hydrology at one site is controlled by headwater flooding driven by precipitation events. The other site is controlled by Mississippi River backwater flooding, where artificial levees cause high flood levels with rapid recession. Both sites have alluvial clay soils. At the backwater site, the receding water table corresponded with a steady reduction in transpiration throughout the growing season, suggesting that precipitation recharge of soil moisture did not satisfy demands despite the humid climate. In contrast, at the headwater site, precipitation events repeatedly raised the water table, which were mirrored by increases in transpiration. These results suggest preferential uptake from the free water surface over more tightly bound soil water, and that subsurface flood subsidies enable a longer effective growing season. Despite divergent growth strategies between the two monitored species, differences among the temporal patterns of transpiration depended more on microtopographic position rather than species. Recharge events by precipitation provided greater benefit for trees in microtopographic lows compared to those on ridges, evident by relatively higher sapflow rates, during the groundwater recession limb. Despite the similar species composition between sites, seasonality of growth and water use differed with topography and hydrologic forcing. Therefore, local hydrology, with evapotranspiration being a primary efflux, appears to be driven by larger-scale variation in groundwater.