Recent Land Cover History and Nutrient Retention in Riparian Wetlands
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Environmental Management (2009) 44:62–72 DOI 10.1007/s00267-009-9313-9 Recent Land Cover History and Nutrient Retention in Riparian Wetlands Dianna M. Hogan Æ Mark R. Walbridge Received: 20 February 2008 / Accepted: 30 April 2009 / Published online: 30 May 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Wetland ecosystems are profoundly affected by Introduction altered nutrient and sediment loads received from anthro- pogenic activity in their surrounding watersheds. Our Riparian wetlands occupy transition zones along streams objective was to compare a gradient of agricultural and and rivers between upland and aquatic environments, and urban land cover history during the period from 1949 to have the potential to provide ecosystem services that 1997, with plant and soil nutrient concentrations in, and include water quality improvement (nutrient and sediment sediment deposition to, riparian wetlands in a rapidly removal and retention), biodiversity and habitat support, urbanizing landscape. We observed that recent agricultural flood mitigation, and groundwater discharge and recharge land cover was associated with increases in Nitrogen (N) and (Johnston 1991; Walbridge 1993; Vought and others 1994; Phosphorus (P) concentrations in a native wetland plant Zedler 2003). Variations in current and historic land use species. Conversely, recent urban land cover appeared to (LU) and land cover (LC) in the surrounding watershed alter receiving wetland environmental conditions by can affect wetland hydrology and nutrient and sediment increasing the relative availability of P versus N, as reflected loads (Soranno and others 1996; Wickham and others in an invasive, but not a native, plant species. In addition, 2000; Winter and Duthie 2000; Horner and others 2001; increases in surface soil Fe content suggests recent inputs of Groffman and others 2003; Houlahan and Findlay 2004), terrestrial sediments associated specifically with increasing potentially compromising a wetland’s ability to provide urban land cover. The observed correlation between urban important ecosystem services. land cover and riparian wetland plant tissue and surface soil Both agricultural and urban LUs are potential sources nutrient concentrations and sediment deposition, suggest and can facilitate the transport of excess nutrients [i.e., that urbanization specifically enhances the suitability of Nitrogen (N) and Phosphorus (P)] and sediments to aquatic riparian wetland habitats for the invasive species Japanese systems via erosion, leaching, and runoff, significantly stiltgrass [Microstegium vimenium (Trinius) A. Camus]. impacting receiving wetland, stream, or downstream eco- system health and sustainability (Johnston 1991; Carpenter Keywords Nitrogen Á Phosphorus Á Sediment Á and others 1998; Reinelt and others 1999; Foley and others Iron Á Land cover Á Land use Á Riparian wetland Á 2005). A legacy of past agricultural LU can result in excess Microstegium vimenium Á Japanese stiltgrass Á nutrient concentrations in soil surface horizons that may be Invasive plant species released as sediment from soil disturbance due to agricul- tural activities such as tilling or grazing or conversion to urban LU (del Campillo and others 1999). In urban D. M. Hogan (&) watersheds, the primary sources of excess nutrients are Eastern Geographic Science Center, US Geological Survey, lawn and garden fertilizers, leaky sewer lines, home septic 12201 Sunrise Valley Drive, MSN 521, Reston, VA 20192, USA systems, and effluents from waste treatment plants (Schu- e-mail: [email protected] eler 1987; Paul and Meyer 2001), and sedimentation is M. R. Walbridge promoted by construction activities and LU that reduce Agricultural Research Service, USDA, Beltsville, MD, USA vegetative cover and increase erosion. Further, nutrient and 123 Environmental Management (2009) 44:62–72 63 sediment transport are facilitated in urban watersheds due Methods to increased impervious surface cover (e.g., building footprints, paved surfaces, etc.) (Schueler 1994; Paul and Riparian Wetland Selection and Analysis Meyer 2001), and stormwater management infrastructure (including road culverts and curb and gutter systems). Twenty-one potential forested riparian wetland study sites Stormwater management often inadvertently directs urban in Fairfax Co., VA, were identified from National Wetlands runoff away from historic flow patterns and directly into Inventory (NWI) maps and verified in the field, and then streams or stormwater management facilities (Schueler ranked based on percent impervious surface cover (ISC) in 1987; Paul and Meyer 2001), and may therefore reduce the surrounding watershed, as an index of degree of hydrologic and nutrient inflows to natural wetlands. As the urbanization (Arnold and Gibbons 1996). Impervious sur- landscape and flow patterns are altered, ecosystem services face cover data were obtained by delineation of 1997 aerial performed by riparian wetlands are reduced, and water photography, and provided by the Fairfax County Depart- quality in receiving aquatic ecosystems is likely to decline ment of Public Works and Environmental Services (Reinelt and others 1999; Groffman and others 2003; (DPWES). Sites were categorized as either: (1) slightly Hogan and Walbridge 2007). urbanized (1.0–6.1% ISC); (2) moderately urbanized The alterations in hydrology and water chemistry (8.6–24.4% ISC); or (3) highly urbanized (24.8–37.6% resulting from watershed agricultural or urban LU can ISC). These categories are representative of the suburban degrade wetlands, making them susceptible to the coloni- gradient that existed in 1997 in Fairfax County, VA, a zation and spread of invasive plant species (Ehrenfeld and suburb of Washington, DC that has experienced rapid Schneider 1993; Baldwin 2004). Invasive plants threaten population growth over the past 65 years. Four riparian biological diversity and can alter natural ecosystem func- wetlands were randomly selected from each category for a tions through their impacts on community structure and the total of 12 study sites (Fig. 1). Wetlands were delineated in vigor of native species (Ehrenfeld and Schneider 1993; Gallaher 2000; Sakai and others 2001; Gibson and others 2002; Simberloff and others 2005). The distribution of the invasive grass, Microstegium vimenium (Japanese stilt- grass), has been correlated with disturbed landscapes, wetlands, and elevated soil nutrient concentrations (Sakai and others 2001; Gibson and others 2002). Japanese stilt- grass has the ability to alter resource allocation to maxi- mize growth and reproduction, sometimes forming a monoculture to the exclusion of other species (Gallaher Piney Run 2000; Claridge and Franklin 2002). Colvin Run Here we seek to improve the understanding of the con- Snakeden Branch sequences of LU change on riparian wetlands and the Little Difficult ecosystem services they provide, by separating the effects South Fork of previous agricultural LC history from those associated Bear Branch with the more recent effects of urbanization. We used Fairfax County, VA, a rapidly urbanizing area just west of Washington, DC, as a model, and compared recent (1949– Daniels Run 1997) LC with nutrient (N and P) concentrations in tissues Fairfax County of a native and an invasive plant species and in surface soils, Holmes Run Virginia and with surface soil Fe chemistry, as indices of eutrophi- USA cation and hydrologic modification respectively. The two Wolf Run plant species were Acer rubrum L. (red maple), a common Sandy Run 1 native deciduous tree that thrives in many different land- Sandy Run 2 forms, soil conditions, moisture, and light regimes (Abrams Elk Horn 1998), and Microstegium vimenium (Trinius) A. Camus (Japanese stiltgrass), an annual shade-tolerant C4 grass that has invaded disturbed understory habitats along stream- Fig. 1 Map of the United States with Fairfax County marked by the banks and in forests in the eastern United States since its black dot. The twelve study sites within Fairfax County are shown; black squares represent wetland location and shaded areas represent introduction in 1919 (Kartesz 1994; Horton and Neufeld the wetland watersheds. Watershed area and land cover are given in 1998; Gibson and others 2002). Table 2 123 64 Environmental Management (2009) 44:62–72 the field using US Army Corps of Engineers guidelines oven drying *40 g soil aliquots at 90°C to a constant (USACE 1987); all riparian wetland study sites are char- mass. Sodium hydroxide-extractable P, an estimate of P acterized by comparable hydrophytic vegetation, hydric associated with soil Al and Fe minerals, was estimated by soils, and wetland hydrology. Study sites occurred on either the methods of Bache and Williams (1971) and Hedley and somewhat poorly drained Chewacla (fine-loamy, mixed, others (1982). Phosphorus extracts were analyzed colori- active, thermic Fluvaquentic Dystrudepts), or poorly metrically as orthophosphate by the method of Murphy and drained Hatboro/Codorus (fine-loamy, mixed, active, mesic Riley (1962), using a Technicon II Autoanalyzer (Bran and Fluvaquentic Endoaquepts/Fluvaquentic Dystrudepts) soils Luebbe Inc. 1989, Method Number 696-82W, 1B). Total N (Porter and others 1963; EFRD 1990; Anonymous 2004). and P were determined using the same procedure as for Samples of red maple (Acer rubrum L.) leaf tissue, and plant tissues. Soil non-crystalline (amorphous), crystalline, Japanese stiltgrass aboveground biomass (Microstegium and total Fe content were estimated by the method of Darke vimineum (Trinius) A. Camus)