Photosynthetic and growth responses of the C₃ Bromus inermis and the C₄ Andropogon gerardii to tree canopy cover

Agroforestry systems are designed to improve the 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 the 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 (A_net), stomatal conductance (g_s), 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 N content of the leaves increased with shade in both species. In addition, Tchl, SLA, N, and g_s were significantly greater in smooth bromegrass than in big bluestem at all canopy levels. Lower g_s and N, and higher A_net in big bluestem resulted in a higher water and N 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 indicate 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.