SOIL-WATER EXTRACTION AND PHOTOSYNTHESIS IN GUTIERREZIA-SAROTHRAE ANDSPOROBOLUS-CRYPTANDRUS

Broom snakeweed (Gutierrezia sarothrae Shinners), a C3 evergreen half- shrub, is a formidable competitor of grasses in the semiarid southwest ern rangelands. Sand dropseed (Sporobolus cryptandrus (Torr.) Gray), p erennial C4 bunchgrass, is the most drought resistant species in the s hort-grass prairie. A comparative study on soil water extraction patte rns, photosynthesis, and canopy development in both species during spr ing-summer growing season of 1991 was conducted in pot- and field-grow n plants. Sand dropseed extracts water at depths between 0 and 30 cm m ore effectively than broom snakeweed. In contrast, broom snakeweed can take up more water from the subsoil (30-60 cm) than sand dropseed. Ph otosynthesis in sand dropseed was more affected by soil water deficit than was broom snakeweed, which was related to their water extraction patterns. Leaf area accumulation of broom snakeweed was not affected b y spring drought, but that of sand dropseed was reduced. Because of gr eater water extraction from the wetter subsoil by broom snakeweed duri ng drought, it can assimilate more carbon and, therefore, prevail in a competitive relationship with sand dropseed.