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Responses of Bromus inermis and Andropagon gerardii to various canopy levels of Fraxinus pennsylvanica.
Awada, Tala *,1, Perry, Micheal2, Schacht, Walter2, 1 School of Natural Resource Sciences, Lincoln, NE, USA2 Department of Agronomy and Horticulture, Lincoln, NE
ABSTRACT- Agroforestry systems are designed to improve efficiency of use of available resources and to increase potential site productivity. The capability of a plant to acclimate to shade when cultivated beneath trees is important in determining success of agroforestry projects. The objectives of this study were to determine the morphological, physiological and growth responses of C4 big bluestem (Andropogon gerardii Vitman.) and C3 smooth bromegrass (Bromus inermis Leyss) to various canopy levels of green ash (Fraxinus pennsylvanica Marsh) in the field, and to examine the impacts of these responses on grass yield. Net photosynthesis (Anet), stomatal conductance (gs), and dark respiration (Rd) declined in response to shade in both species, but the decline was steeper in big bluestem than in smooth bromegrass. Total chlorophyll content (Tchl), specific leaf area (SLA), and nitrogen (N) content of the leaves increased with shade in both species. In addition, Tchl, SLA, N, and gs were significantly higher in smooth bromegrass than in big bluestem at all canopy levels. Lower gs and N, and higher Anet in big bluestem resulted in a higher water and nitrogen use efficiencies in this species than in smooth bromegrass. Yield of big bluestem sharply declined with increased canopy cover, whereas yield of smooth bromegrass was not affected by canopy cover. Our results indicated that while both species were productive under various levels of green ash canopy, and showed similar ecophysiological responses to shade, smooth bromegrass acclimated better to shade than big bluestem. Therefore, the selection of species should be based on specific management goals and environmental conditions. For example the higher water and nitrogen use efficiency in big bluestem compared to smooth bromegrass may become advantageous in drier climates and where N is a limiting factor.
Key words: Andropogon gerardii, Bromus inermis , agroforestry, gas exchange, Fraxinus pennsylvanica, shade tolerance