Influence of density on intermediate wheatgrass and spotted knapweed interference

Abstract

Establishing competitive plants is essential for restoring spotted knapweed infested grasslands. Revegetation attempts typically fail becanse of weed competition during the initial stages of establishment. We hypothesized that competitive interactions can be shifted from spotted knapweed to intermediate wheatgrass by increasing wheatgrass seedling density over 1,000 plants m-2. Spotted knapweed and intermediate wheatgrass were grown in addition series mixtures to assess their interference at low (0 to 1,000 plants m-2) versus high (1,000 to 10,000 plants m-2) densities. In the spring of 1995, 7 densities (0, 100, 500, 1,000, 3,000, 6,000, and 10,000 plants m-2) of each species were seeded in a factorial arrangement (49 density combinations) in a randomized-complete-block design and replicated 3 times at 2 sites in Montana. Plants were grown in pots (2,250 mm2 X 380 mm deep) for 60 days before harvesting. Regressions predicting shoot weight, root weight, total weight, leaf area, and root length were calculated using 1) low knapweed:low wheatgrass, 2) low knapweed:high wheatgrass, 3) high knapweed:low wheatgrass, and 4) high knapweed:high wheatgrass densities. Regression coefficients indicated intraspecific interference was most important in predicting intermediate wheatgrass weight at both sites. At the wet site (457 mm, annually), interspecific interference only occurred at high spotted knapweed densities. At the dry site (305 mm, annually), interspecific interference occurred at low densities. Increasing intermediate wheatgrass from low to high densities removed the effect of spotted knapweed on intermediate wheatgrass where interspecific interference occurred.