Ecophysiological Responses of Herbaceous Plants to the Presence or Absence of Amorpha canescens in a Nebraska Sandhills Grassland

Authors

Jessica L. Milby, University of Nebraska-LincolnFollow

Date of this Version

12-2011

Document Type

Article

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Natural Resource Sciences, Under the Supervision of Professor Tala Awada and Professor Walter Schacht. Lincoln, Nebraska: December, 2011

Copyright (c) 2011 Jessica L. Milby

Abstract

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, g_s, 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 g_s; 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 Ψ_pre and Ψ_mid among sampling dates and species, but not between sites. The C₃ grasses exhibited the lowest (most negative) Ψ_pre and Ψ_mid. 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.

Advisers: Tala Awada and Walter Schacht