Ecohydrology of a Great Lakes Coastal Ridge-Swale Wetland System.

Abstract

Interactions between wetland plants and the water table influence trajectories of vegetation change and resulting community responses to climate change. The specific dynamics, however, are not well defined, in part because of complexities associated with climate, physiography, and underlying geology. In this study, the dynamic interactions of vegetation with the water table were examined in a coastal ridge-swale wetland system on Lake Huron. I modified a riparian-zone method for estimating evapotranspiration (ET) and shallow groundwater flow and applied it to this structurally and vegetatively complex site. I then explored how observed variability in wetland water balance arises through interactions between plants, physiography, and hydrogeology and examined inter-annual climatic effects. Finally, I used path analytic techniques to examine the dynamic nature of feedbacks between plant water use, indexed by ET, and water availability, indexed by soil moisture. Average daily evapotranspiration rates for the 15 wetlands in the study ranged from 5.5 mm/d (SD 1.6) to 8.1 mm/d (SD 2.5). Over the growing season, the mean ET rate was 894 mm (SD 98) in 2006 (wet year) and 924 mm (SD 89) in 2007 (dry year). Shallow groundwater flux rates associated with ET averaged 681 mm (SD 79) in 2006 and 705 mm (SD 81) in 2007. Annual climatic variability (precipitation in particular) strongly affected the causal interactions between soil water availability and plant water use. A strong positive feedback was observed in a wet year, whereas a weaker interaction was observed in a dry year, along with some indication of water limitation. Underlying geology substantially affected plant-hydrology interactions in two important ways. Sandy substrates permitted considerable water loss, systematically lowering the water table and reducing soil moisture. In swales that recharged the water table, water availability had a stronger causal effect on plant water use. Effects of vegetation may be equally important, especially when considering ecosystem response to climate change. Although soil moisture had a strong negative effect on ET, vegetation (tree species in particular) reduced soil moisture in a way that facilitated ET, especially under wet climatic conditions.