Effects of defoliation and competition on total non-structural carbohydrates of spotted knapweed.

Abstract

Spotted knapweed (Centaurea maculosa Lam.) is replacing native bunchgrasses and interfering with resource management objectives on many ranges in the northern Intermountain Region. Herbicides, biological control agents, and fire have not successfully contained spotted knapweed. Since knapweed is grazed in some situations, effects of defoliation and competition on total nonstructural carbohydrates (TNC) of spotted knapweed plants were determined in this study. Transplanted knapweed plants were grown for 6 months in a greenhouse under 3 levels of competition with bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] Love) and 3 levels of defoliation. Total nonstructural carbohydrates (pool and concentration) were determined at the end of the experiment. Concentration and pools of carbohydrates generally varied among stems, roots, crowns, and with monthly defoliations. Importance of stems for carbohydrate storage was more evident in analyses of pools rather than concentrations. Monthly defoliations decreased carbohydrate concentrations by about 50%, and pools by about 80% respectively within stems, roots, and crowns. While competition from bluebunch wheatgrass influenced total nonstructural carbohydrates concentrations, it did not influence pools. Although frequent defoliations of spotted knapweed reduced carbohydrates, other factors probably limit the feasibility of using grazing animals to control spotted knapweed on native bunchgrass ranges in western North America.