| Mitigating the effects of urbanization on local hydrologic cycles has been promoted worldwide in the form of stormwater bioretention (e.g. , rain gardens), yet these practices remain largely untested with regard to their most fundamental controls: plants and soil. In this dissertation, I investigated the effect of vegetation on rain garden hydrology because 1.) plant-soil interactions are critical in determining the short and long-term function of rain gardens, and because 2.) the use of vegetation offers an ecological alternative (as opposed to mechanical engineering) for managing urban stormwater. This research used a controlled field experiment of 12 closed-system mesocosms (rain gardens) in a complete randomized block design with four vegetative treatments (control, turfgrass, prairie, and shrubs). Roof stormwater was collected on site and distributed equally among all rain gardens following natural rain events for two growing seasons (2009-2010). In the first study, I found significantly different hydrologic dynamics between vegetative treatments, explained by differences in soil structural development (via infiltration, saturated hydraulic conductivity, and soil-water retention) and stormwater input size. In the second study, I quantified differences in evapotranspiration by vegetation type during dry weather periods, which were consistent with differences in plant traits (root weight density, leaf mass, and leaf area) and antecedent soil water content. In the third study, I investigated grouping prairie species, a regionally popular vegetation type used in rain garden design, according to differences in canopy morphology and flowering phenology and demonstrate how these groupings may predict differences in plant traits, particularly among species of non-leguminous perennial forbs. This third study used a field pot experiment of 16 prairie species grown in monoculture at the rain garden experimental facility. Overall, this research improves our mechanistic understanding of the role of vegetation in urban rain gardens by providing quantitative evidence that plants alter rain garden hydrology through direct changes in soil structure and indirect changes in antecedent soil moisture via differences in rooting and evapotranspiration between stormwater events. |
