Water Relations: A New Dimension for Niche Separation Between Bouteloua gracilis and Agropyron smithii in North American Semi-Arid Grasslands

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Abstract
The dynamics of the leaf water status and stomatal behavior in B. gracilis and A. smithii, 2 of the dominant grasses of the semi-arid grasslands in North America, were studied. The grasses showed remarkably different stomatal behavior. While A. smithii showed a typical behavior, B. gracilis presented maximum values of conductance before dawn and then a slow decrease until reaching a stable status around mid-morning. Leaf water potential in B. gracilis was independent of leaf conductance and closely followed the pattern of atmospheric water demand. In the case of A. smithii, both leaf conductance and atmospheric water demand exerted control upon leaf water potential. The predawn opening of stomata represented an adaptive advantage for B. gracilis allowing this species to maximize C fixation during the early morning, the time of day when, under the most frequent steppe conditions, water deficit does not impair the photosynthesis process. It is proposed that B. gracilis and A. smithii have 2 different and complementary ecological strategies. B. gracilis is adapted to a scarce and highly variable water supply and to a precipitation pattern in which small rainfall events constitute a large portion of total water input. A. smithii is adapted to a less variable water supply and to a water balance more favorable in which small precipitation events account for a smaller portion of the water input.

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