Water relations: a new dimension for niche separation between Bouteloua gracilis and Agropyron smithii in North American semi-arid grasslands.
In studies on the dynamics of leaf water status and stomatal behaviour, B. gracilis and A. smithii [Elymus smithii] showed remarkably different stomatal behaviour. While A. smithii showed a typical behaviour B. gracilis showed max. values of conductance before dawn and then a slow decrease until reaching a stable state around mid-morning. Leaf water potential in B. gracilis was independent of leaf conductance and closely followed the pattern of atmospheric water damand. In the case of A. smithii, both leaf conductance and atmospheric water demand exerted control upon leaf water potential. It was concluded that the predawn opening of stomata represented an adaptive advantage for B. gracilis, allowing this sp. to maximize carbon fixation during 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 favourable in which small precipitation events account for a smaller portion of the water input.