Trajectory of ecosystem recovery in restored riparian zones in urban settingsEPA Grant Number: F5F11559
Title: Trajectory of ecosystem recovery in restored riparian zones in urban settings
Investigators: Morse, Jennifer L.
Institution: Duke University
EPA Project Officer: Hahn, Intaek
Project Period: July 1, 2005 through August 1, 2008
Project Amount: $111,000
RFA: STAR Graduate Fellowships (2005) RFA Text | Recipients Lists
Research Category: Academic Fellowships
Intact riparian zones remove nutrients, particularly nitrogen and phosphorus, from groundwater and surface runoff, thus improving water quality. Because streams and riparian zones in many urban areas are degraded and have diminished ecological functions, stream and floodplain restoration is becoming important to water quality management strategies. Understanding how riparian zones regain ecological functions following restoration is key to setting realistic performance criteria. This project will examine stream restoration sites along an age gradient study ecosystem development following disturbance and the role of restored riparian zones in processing nutrients.
This project will examine stream restoration sites along an age gradient, as a proxy for a long-term study, to address basic questions about ecosystem development following disturbance, as well as practical considerations for understanding the role of restored riparian zones in processing nutrients. By examining stream and floodplain restoration sites completed between 1989 and 2005, and comparing them to unrestored sites in non-urban areas and urban, degraded areas, I will test these general hypotheses: 1. Hydraulic connectivity is key to the recovery of ecosystem processes in restored riparian zones: successful restoration will lead to more variable water table depths and less incised channels; 2. Time since restoration will explain differences in ecosystem functions in restored riparian zones, as vegetation and soil processes begin to approach those of natural riparian zones; 3. Abiotic mechanisms of nutrient retention (e.g., P sorption) will be more important in younger restored sites, while biotic processes (e.g., microbial assimilation, denitrification, plant uptake) will be increasingly important controls on nutrient biogeochemistry in older restored sites.
The National River Restoration Science Synthesis (NRRSS), led in part by Dr. Emily Bernhardt, compiled a database of over 35,000 river and stream restoration projects nationwide. This database enabled me to identify a well-documented, large population of stream restoration sites of varying ages, and I generated a map of channel reconfiguration projects in North Carolina for which the dates of completion and project contact information are known. I will use a three-tiered design in this study: I. Hydrogeomorphic classification, II. Indexes of Ecological Function, and III. Biogeochemical Processes.
Phase I: Hydrogeomorphic Survey for Site Selection. I will conduct an extensive field survey of stream and riparian restoration sites in urban and suburban areas of North Carolina, with the specific objectives to identify sites of different ages (time since restoration) to be studied more intensively in Phases II and III. The use of a time sequence of multiple study sites with known histories has been advocated and implemented where a long-term study at one site is not feasible. I will limit the population of channel reconfiguration projects to sites on 3rd or 4th order streams in the Piedmont physiographic province, to control for inter-site variability. I will assess hydraulic connectivity between stream channels and riparian zones by measuring channel incision in restoration sites. This survey will enable me to exclude from Phase II and III sites with re-incised channels, which would lack hydraulic connectivity between the stream and riparian zone.
Phase II: Indexes of Ecological Function. Based on results from Phase I, I will focus on 12 restoration sites and 6 complementary unrestored sites for more intense characterization of ecosystem properties. Restoration sites of similar hydrogeomorphic classification within the North Carolina Piedmont (to control for major physical differences between sites) will be chosen to represent an age gradient. The specific objective of Phase II is to apply a framework for evaluating restoration of ecological function across the age gradient of sites. The five evaluative axes in this method are 1) hydrology, 2) productivity, 3) decomposition, 4) community habitat, and 5) biogeochemistry. By measuring the same variables at reference and target sites, an ecosystem response surface model can be created. The parameters I will measure for this assessment have been shown to be important in evaluating impacted forested wetlands, and will be readily applicable to riparian zones.
Phase III: Biogeochemical Processes. The goal of Phase III is to explicitly link biogeochemical processes in the soil to hydrological flowpaths in the riparian zone; this design is essential to understanding biogeochemical processes and patterns in soils. Results from Phase II will allow me to select two restoration sites (one young and one old) for finer-scale, spatially explicit studies of soil processes along hydrological flowpaths in the riparian zone.
Phase I: These results will enable an evaluation of channel incision following restoration. I expect to find that sites restored longer ago will have more stable, less incised channels than more recently restored sites, as the older sites will have had more time to establish a new equilibrium. Publication of these results will be relevant to watershed planners and engineers who design stream restoration projects.
Phase II: I anticipate that ecological functions in restored riparian zones will begin to approach patterns in non-urban reference sites with increasing age. I expect that the five ecological functions will follow different trajectories, with hydrology and decomposition recovering earlier than the plant community and biogeochemical characteristics. Productivity in a restored site would likely approach reference conditions more slowly than other parameters. These results may validate the use of ecosystem surface response models as tools for assessing the success of ecological restoration.
Phase III: I anticipate that abiotic processes (e.g., phosphorus sorption) will be more important in the younger restored site, while biotic processes (e.g., microbial assimilation, denitrification, plant uptake) will be increasingly important controls on nutrient biogeochemistry in the older restored site and the reference site.