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Deeply-rooted shrubs are a common component of grasslands. This study investigated the ecophysiological role of Amorpha canescens, a common leguminous shrub, in Sandhills grassland, and response of herbaceous plants to the presence or absence of A. canescens. Two sites were selected for the study at the Gudmundsen Sandhills Laboratory near Whitman, Nebraska – one with A. canescens (G-L) and one without (G-NL) A. canescens. Plant canopy cover and aboveground biomass were characterized on both sites in June and August 2010. Seasonal trends in net photosynthesis (A), stomatal conductance (gs), transpiration (E), water use efficiency (WUE), and predawn (Ψpre) and midday (Ψmid) water potentials of A. canescens and five representative herbaceous species were determined at 2-week intervals between June and September. The herbaceous species included two C3 grasses (Hesperostipa comata and Koeleria macrantha), two C4 grasses (Andropogon hallii and Calamovilfa longifolia), and one forb (Helianthus pauciflorus). Differences in rates of A, gs, and WUE were species dependent and were not impacted by the presence of A. canescens. Net photosynthesis exhibited seasonal variability, increasing through the early growing season and peaking by midsummer. H. pauciflorus had the highest rates of A, E, and gs; and C4 grasses had the greatest WUE. Significant site impact (G-L vs. G-NL) was only observed in E, which was likely related to higher soil water content in the G-L site. There were differences in Ψpre and Ψmid among sampling dates and species, but not between sites. The C3 grasses exhibited the lowest (most negative) Ψpre and Ψmid. Soil nitrogen content was significantly greater on the G-L site, leading to higher photosynthetic nitrogen use efficiency (PNUE) on the G-NL site, but lower plant leaf quality. A. canescens presence had positive impacts on soil and associated plants’ nitrogen content and soil water content, but did not have consistent effects on water status or gas exchange of associated herbaceous plants. Ecological significance of the results is discussed.
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