Browsing by Author "Stewart, Rebecca M."

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    Effects of instream processes, discharge, and land cover on nitrogen export from southern Appalachian Mountain catchments
    Webster, Jackson R.; Stewart, Rebecca M.; Knoepp, Jennifer D.; Jackson, C. Rhett (2019-01-15)
    Catchments with minimal disturbance usually have low dissolved inorganic nitrogen (DIN) export, but disturbances and anthropogenic inputs result in elevated DIN concentration and export and eutrophication of downstream ecosystems. We studied streams in the southern Appalachian Mountains, USA, an area dominated by hardwood deciduous forest but with areas of valley agriculture and increasing residential development. We collected weekly grab samples and storm samples from nine small catchments and three river sites. Most discharge occurred at baseflow, with baseflow indices ranging from 69% to 95%. We identified three seasonal patterns of baseflow DIN concentration. Streams in mostly forested catchments had low DIN with bimodal peaks, and summer peaks were greater than winter peaks. Streams with more agriculture and development also had bimodal peaks; however, winter peaks were the highest. In streams draining catchments with more residential development, DIN concentration had a single peak, greatest in winter and lowest in summer. Three methods for estimating DIN export produced consistent results. Annual DIN export ranged from less than 200 g ha(-1) year(-1) for the less disturbed catchments to over 2,000 g ha(-1) year(-1) in the catchments with the least forest area. Land cover was a strong predictor of DIN concentration but less significant for predicting DIN export. The two forested reference catchments appeared supply limited, the most residential catchment appeared transport limited, and export for the other catchments was significantly related to discharge. In all streams, baseflow DIN export exceeded stormflow export. Morphological and climatological variation among watersheds created complexities unexplainable by land cover. Nevertheless, regression models developed using land cover data from the small catchments reasonably predicted concentration and export for receiving rivers. Our results illustrate the complexity of mechanisms involved in DIN export in a region with a mosaic of climate, geology, topography, soils, vegetation, and past and present land use.
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    A global synthesis reveals biodiversity-mediated benefits for crop production
    Dainese, Matteo; Martin, Emily A.; Aizen, Marcelo A.; Albrecht, Matthias; Bartomeus, Ignasi; Bommarco, Riccardo; Carvalheiro, Luisa G.; Chaplin-Kramer, Rebecca; Gagic, Vesna; Garibaldi, Lucas A.; Ghazoul, Jaboury; Grab, Heather; Jonsson, Mattias; Karp, Daniel S.; Kennedy, Christina M.; Kleijn, David; Kremen, Claire; Landis, Douglas A.; Letourneau, Deborah K.; Marini, Lorenzo; Poveda, Katja; Rader, Romina; Smith, Henrik G.; Tscharntke, Teja; Andersson, Georg K. S.; Badenhausser, Isabelle; Baensch, Svenja; Bezerra, Antonio Diego M.; Bianchi, Felix J. J. A.; Boreux, Virginie; Bretagnolle, Vincent; Caballero-Lopez, Berta; Cavigliasso, Pablo; Cetkovic, Aleksandar; Chacoff, Natacha P.; Classen, Alice; Cusser, Sarah; da Silva e Silva, Felipe D.; de Groot, G. Arjen; Dudenhoeffer, Jan H.; Ekroos, Johan; Fijen, Thijs; Franck, Pierre; Freitas, Breno M.; Garratt, Michael P. D.; Gratton, Claudio; Hipolito, Juliana; Holzschuh, Andrea; Hunt, Lauren; Iverson, Aaron L.; Jha, Shalene; Keasar, Tamar; Kim, Tania N.; Kishinevsky, Miriam; Klatt, Bjorn K.; Klein, Alexandra-Maria; Krewenka, Kristin M.; Krishnan, Smitha; Larsen, Ashley E.; Lavigne, Claire; Liere, Heidi; Maas, Bea; Mallinger, Rachel E.; Martinez Pachon, Eliana; Martinez-Salinas, Alejandra; Meehan, Timothy D.; Mitchell, Matthew G. E.; Molina, Gonzalo A. R.; Nesper, Maike; Nilsson, Lovisa; O'Rourke, Megan E.; Peters, Marcell K.; Plecas, Milan; Potts, Simon G.; Ramos, Davi de L.; Rosenheim, Jay A.; Rundlof, Maj; Rusch, Adrien; Saez, Agustin; Scheper, Jeroen; Schleuning, Matthias; Schmack, Julia M.; Sciligo, Amber R.; Seymour, Colleen; Stanley, Dara A.; Stewart, Rebecca M.; Stout, Jane C.; Sutter, Louis; Takada, Mayura B.; Taki, Hisatomo; Tamburini, Giovanni; Tschumi, Matthias; Viana, Blandina F.; Westphal, Catrin; Willcox, Bryony K.; Wratten, Stephen D.; Yoshioka, Akira; Zaragoza-Trello, Carlos; Zhang, Wei; Zou, Yi; Steffan-Dewenter, Ingolf (AAAS, 2019-10)
    Human land use threatens global biodiversity and compromises multiple ecosystem functions critical to food production. Whether crop yield-related ecosystem services can be maintained by a few dominant species or rely on high richness remains unclear. Using a global database from 89 studies (with 1475 locations), we partition the relative importance of species richness, abundance, and dominance for pollination; biological pest control; and final yields in the context of ongoing land-use change. Pollinator and enemy richness directly supported ecosystem services in addition to and independent of abundance and dominance. Up to 50% of the negative effects of landscape simplification on ecosystem services was due to richness losses of service-providing organisms, with negative consequences for crop yields. Maintaining the biodiversity of ecosystem service providers is therefore vital to sustain the flow of key agroecosystem benefits to society.
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    Land Cover Influences on Stream Nitrogen Dynamics During Storms
    Stewart, Rebecca M. (Virginia Tech, 2012-05-14)
    Previous studies on the effects of land cover influence on stream nitrogen have focused on base flow conditions or were conducted specifically within urbanized or primarily agricultural watersheds. While these studies have shown relationships between land cover and nitrogen, this relationship and the scale of influence could change during storms. The purpose of my study was to understand how land cover influences nitrogen in streams during storms. This was address using nine watersheds within the Little Tennessee Basin in North Carolina. While this basin is primarily forested, the nine watersheds have mixed agricultural, built, and forest land cover. Land cover influences were addressed through nitrogen concentration/discharge patterns, nitrogen concentration relationship to land cover, and comparison of storm and base flow nitrogen concentrations over time. Weekly base flow samples and samples from six storm were collected in 2010-2011. Total dissolved nitrogen (TDN), nitrate (NO??), dissolved organic nitrogen (DON), and ammonium (NH?⁺) concentrations were compared among sites. During most storms, DON peaked before the peak of the discharge while NO?? peaked after the peak of the storm. This suggest that DON could be coming from a near stream source or surface runoff while NO?? could be from longer pathways such as subsurface flow or from sources further away on the watershed. NO?? concentration varied among sites, while DON concentration varied more between base flow and storm samples. Examining the different landscape scales from 200-m local corridor, 200-m stream corridor, and entire watershed, watershed land cover was the best predictor for all the nitrogen concentrations. Agricultural and built combined best predicted TDN and NO??, while agricultural land cover was a better predictor of DON. For storms, nitrogen concentrations did not show seasonal patterns but was more related to discharge. Nitrogen concentration increased with discharge during storms and the more intense and longer storms had higher TDN and NO?? concentrations. However, conflicting seasonal trends were seen in monthly base flow. The more forested watersheds had high NO?? during the summer and low NO?? in the winter. For sites with higher NO??, the seasonality was reversed, with higher winter NO?? concentration. The least forested site had relatively constant nitrogen through the year at base flow and concentration decreased for most storms. Further studies on storms and nitrogen transport are needed to understand better the seasonal patterns of nitrogen input during storms.