Water relations: a new dimension for niche separation between Bouteloua gracilis and Agropyron smithii in North American semi- arid grasslands.

While A. smithii showed a typical stomatal behaviour B. gracilis presented maximum values of conductance between 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. With A. smithii, both leaf conductance and atmospheric water demand exerted control upon leaf water potential. The predawn opening of stomata represents an adaptive advantage for B. gracilis allowing this species to maximize carbon fixation during the early morning, the time of day when under the most frequent steppe conditions water deficit does not impair the photosynthesis process. B. gracilis and A. smithii have two 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 favourable in which small precipitation events account for a smaller portion of the water input.-from AuthorsNatural Res. Ecology Lab., Colorado State Univ., Fort Collins, CO 80523, USA.