Soil Water Extraction and Photosynthesis in Gutierrezia Sarothrae and Sporobolus Cryptandrus

Abstract

Broom snakeweed (Gutierrezia sarothrae Shinners), a C3 evergreen half-shrub, is a formidable competitor of grasses in the semiarid southwestern rangelands. Sand dropseed (Sporobolus cryptandrus (Toff.) Gray), perennial C4 bunchgrass, is the most drought resistant species in the short-grass prairie. A comparative study on soil water extraction patterns, photosynthesis, and canopy development in both species during spring-summer growing season of 1991 was conducted in pot- and field-grown plants. Sand dropseed extracts water at depths between 0 and 30 cm more effectively than broom snakeweed. In contrast, broom snakeweed can take up more water from the subsoil (30-60 cm) than sand dropseed. Photosynthesis 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 by spring drought, but that of sand dropseed was reduced. Because of greater water extraction from the wetter subsoil by broom snakeweed during drought, it can assimilate more carbon and, therefore, prevail in a competitive relationship with sand dropseed.