Graduate Thesis Or Dissertation
 

The effects of nitrogen manipulations and hydrology on the establishment and competitive abilities of wetland prairie plant species (western Oregon)

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  • Many studies suggest that weedy plant species are most successful when soil nitrogen in abundant. Consequently, I used soil nitrogen manipulations to determine if altering nitrogen would affect the establishment of both weedy and native plant species in a western Oregon wetland prairie. In two studies, we added carbon amendments (sucrose and sawdust) to the soil in order to decrease available soil nitrogen, and in one study we added nitrogen (ammonium nitrate). I show that weedy grasses (Agrostis capillaris, Alopecurus geniculatus, Anthoxanthum odoratum, Holcus lanatus, and Lolium perenne) were significantly suppressed in carbon-treated plots compared to control plots. Also, native grasses (Deschampsia cespitosa and Danthonia calfornica) yielded significantly more biomass in carbon plots than controls. These results suggest that carbon additions can deter weeds and promote native grasses during wetland prairie restoration. Nitrogen-treated plots yielded significant increases in the biomass of both exotic and native grasses compared to controls. This increase in exotic grasses was expected due to the nitrophilous nature of these plants, but the increase in native grasses in these plots was unanticipated, since we expected these natives to be choked out by the weeds. We suggest that the native grasses were more abundant in nitrogen treatments compared to controls because the very high levels of nitrogen in these plots reduced competition for belowground resources, allowing both plant groups to flourish. The ratio of total native biomass to total weed biomass was high in carbon treatments across the hydrologic gradient, but was high in nitrogen treatments only in wetter plots. We used monoculture and mixture plots to study the effects of hydrology on establishment and competitive ability of several native and exotic grasses. Our results show that three of the seven species (Danthonia californica, Beckmannia syzigachne, and Phalaris arundinacea) were excluded from growing in several mixture plots along the gradient, despite their successful establishment in adjacent monoculture plots. Furthermore, the exotic grasses showed low competitive abilities along the hydrologic gradient compared to the native grasses, indicating that invasive species are not competitively superior to native species, at least at the establishment and seedling stages.
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file details DavisKimberlyJ2001.pdf 2017-08-15 公开 下载