Influence of nutrient availability on the interaction between spotted knapweed and bluebunch wheatgrass

Centaurea maculosa (Lam.) (spotted knapweed) reduces wildlife and livestock habitat biodiversity and increases erosion. Nutrient availability to plant s may be used to accelerate succession away from spotted knapweed. Early-su ccessional plant communities often have high nutrient availability, whereas late-successional communities are often found on lower nutrient soils. We hypothesized that removal of nutrients would change the competitive advanta ge from spotted knapweed to Pseudoroegneria spicatum (bluebunch wheatgrass) (late seral). In two addition series matrices, background densities of Sec ale cereale (annual rye) and Elymus elimoides (bottle-brush squirreltail) ( 3,000 seedS/m(2)) were used to remove nutrients from the soil. In another s et of addition series matrices, nitrogen (33 kg/ha) or phosphorus (33 kg/ha ) were added to the soil. Nutrient analysis of soil and vegetation indicate d that annual rye and bottle-brush squirreltail reduced nutrient availabili ty in soils. In another matrix, neither a background density nor nutrients were added. Data were fit into Watkinson's curvilinear model to determine t he competitive relationship between bluebunch wheatgrass and spotted knapwe ed. This allowed comparison of the equivalence ratios (C) generated from ea ch addition series. The C parameters are the per-plant equivalent of bluebu nch wheatgrass or spotted knapweed and can be interpreted as the ratio of i ntra-to-interspecific competition. The C parameters are also the equivalenc e ratio of the number of spotted knapweed it takes to have equivalent effec t on bluebunch wheatgrass or the number of bluebunch wheatgrass having the equivalent effect on spotted knapweed. Without nutrient manipulation, spott ed knapweed was more competitive than bluebunch wheatgrass. The C for blueb unch wheatgrass was 0.17, indicating that 0.17 knapweed plants were competi tively equivalent to one wheatgrass. Annual rye changed the competitive bal ance in favor of bluebunch wheatgrass (C = 9.9). Addition of nitrogen, phos phorus, or the mid-seral species did not change the competitive relationshi p between the two species. This preliminary study suggests that succession from spotted knapweed to late-seral bluebunch wheatgrass community may be a ccelerated by altering resource availability.