| The study was conducted on the Chihuahuan Desert Rangeland Research center, 32 km north of Las Cruces, NM. Physiological responses of creosotebush and bush muhly growing together were quantified from the summer of 1997 to the winter of 1998–99. Large and small plants of creosotebush with or without bush muhly were randomly chosen, as were bush muhly plants growing alone. In the summer of 1997, creosotebush competed with bush muhly for water as shrub plants growing alone had higher integrated photosynthesis than shrubs with bush muhly following a heavy rainfall event. Creosotebush had lower water use efficiency (wue) than bush muhly plants. In the summer of 1998, bush muhly had greater xwp than creosotebush when the soil water potential was above −30 bars. Creosotebush had higher stomatal conductance than bush muhly, with small creosotebush growing alone having the highest stomatal conductance. Defoliation of bush muhly growing under small creosotebush resulted in shrubs having higher photosynthetic activity than when the grass was undefoliated. While differences in net assimilation between shrubs and grasses was not discernible, bush muhly presented a higher wue.; While green in the fall season, bush muhly had higher xwp, lower stomatal conductance, and higher wue than creosotebush plants. Net assimilation was similar between shrubs and grass plants. Both. species responded to the availability of moisture in this season. Bush muhly growing alone had higher net assimilation than when growing under creosotebush. In the winter season, with bush muhly dry, xwp was influenced by low temperatures. Small creosotebush growing alone had higher photosynthetic activity than small creosotebush with bush muhly (p < .005). Creosotebush growing alone, both small and large plants, had higher activity than creosotebush growing with bush muhly (p < .01).; In the spring, although the stomatal conductance of cresotebush was higher, this was not reflected with a higher net assimilation, as bush muhly had the same net assimilation with less water loss. When soil water potential decrease below of −80 bars, creosotebush still showed net assimilation.; Roots of creosotebush and bush muhly were found concentrated closest to the center of the plant, declining with depth and distance from the plant center. Bush muhly had higher root biomass at the soil surface when growing under creosotebush than when growing alone, probably because of the plant age. Creosotebush growing alone had higher amount of root biomass than creosotebush with bush muhly. Creosotebush had greater root biomass in all the horizons. |
