Date of this Version
Xylem pressure potential of Bromus inermis and Andropogon scoparius was measured at predawn and at midday on the Oldfather Prairie west of Kearney, Nebraska (41° 42' N, 99° 08' W). This mixed-grass prairie is characterized by patches of B. inermis and A. scoparius growing in close proximity. Ten replicate pressure potential measurements were made weekly during the 1993 growing season. Water potential remained uniformly high and unchanged throughout the season at predawn for both species. Midday measurements were more variable and also more negative than at predawn on all but one sample date. Water potential deficit, defined as the difference between predawn and midday conditions, was larger for B. inermis (-1.56 MPa) than for A. scoparius (-0.68 MPa). Despite abnormally high rainfall (nearly 750 mm) in 1993, the data support the notion of tight coupling between A. scoparius and the water environment. The water potential deficit and extreme lows for B. inermis (-2.1 MPa) and A. scoparius (-1.4 MPa) indicate further that B. inermis may be less efficient yet more opportunistic in water use than A. scoparius. A. scoparius, by contrast, may be more efficient but also driven more by genetic cues.