Competition between broom snakeweed and sand dropseed
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Broom snakeweed (Gutierrezia sarothrae (Pursh) Britt. & Rusby) is a suffrutescent shrub which infests miUions of hectares of grazing land in the western U.S. It competes with desirable grasses, reduces rangeland production and often dominates large areas of rangeland. A common associate of broom snakeweed in west Texas is sand dropseed (Sporobolus cryptandrus). This study was designed to investigate the competitive relationship between these two plants. Plant competition between perennial broom snakeweed and sand dropseed was examined in a three-year removal study in the field by imposing three treatments effects: plant removal or no removal, supplemental or no supplemental water, and distance between two plants. Results show competition between the two species. Competition limited broom snakeweed leafy stems in number and length, consequentiy reducing its aboveground biomass. However, neither surface soil water status nor distance had any significant effect on broom snakeweed growth in this experiment. No significant interactions were found between removal and surface soil water status, between removal and distance, and between distance and surface soil water status. However, there was a significant season and removal treatment interaction in 1992. It leads to the conclusion that the degree of competition effects depended on the month of year in the third year of experiment. The presence of broom snakeweed limited the growth of sand dropseed and reduced its abovegroimd biomass, basal area and number of tillers. The degree of competition effect on sand dropseed was closely related with its distance to competitor. Competition was usually more severe when the competitor grew closer. A significant interaction was found between removal and distance; however, no significcmt interaction was found between removal and water, and distance and water treatment. The significance of the interaction also depended upon the year of experiment, and the period of year. It was concluded that differences between the two species in responding to environmental stress and their phenology give broom snakeweed a competitive advantage over sand dropseed. Ecological advantages such as early rapid growth, high rate of resources use, insensitive to crowding, and tolerance of low resources enable broom snakeweed to become a dominate species in the western rangeland when weather conditions are optimal for this species.