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This study investigated the ecophysiological role of Amorpha canescens, a leguminous shrub native to the Nebraska Sandhills. Although A. canescens is an important ecological player in the Sandhills, its impacts on the surrounding plant community are poorly understood. To remedy this, two sites were selected for study at the Gudmundsen Sandhills Laboratory near Whitman, Nebraska – one with A. canescens (G-L) and one without A. canescens (G-NL). Both sites contained five representative herbaceous species: two C3 grasses (Hesperostipa comata and Koeleria macrantha), two C4 grasses (Andropogon hallii and Calamovilfa longifolia), and one forb (Helianthus pauciflorus). Plant canopy cover and aboveground biomass were characterized on both sites in June and August 2010, along with soil composition, organic matter, carbon, and nitrogen content. Seasonal trends in transpiration (E), water use efficiency (WUE), and predawn (Ψpre) and midday (Ψmid) water potentials were determined for all species in both plots at 2-week intervals from June through September. Precipitation, groundwater, plant root crown, and soil water samples were collected to determine sources of plant water uptake via stable isotope analysis and the IsoSource mixing model. The results indicate that the presence of A. canescens is favorable to C3 grasses when water is plentiful. However, under water limited conditions, the additional demands on shallow soil water coupled with increased rainfall interception from shrub canopy and litter were disadvantageous to C3 grasses. A. canescens also appeared to enhance the amount of water available deeper in the soil profile, resulting in greater overall moisture in the upper 1 m of soil. Water resource partitioning was not observed during the wetter periods in the first half of the study period, with all species predominantly using shallow soil water. However, H. pauciflorus and A. canescens switched to deeper water sources as water became limited, while C3 and C4 grasses senesced or reduced stomatal conductance to limit water loss. The ecological implications of these results are discussed.
Advisor: Tala Awada