INFLUENCE OF DENSITY ON INTERMEDIATE WHEATGRASS AND SPOTTED KNAPWEED INTERFERENCE

Establishing competitive plants is essential for restoring spotted kna pweed infested grasslands. Revegetation attempts typically fail becaus e of weed competition during the initial stages of establishment. We h ypothesized 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 whe atgrass were grown in addition series mixtures to assess their interfe rence at low (0 to 1,000 plants m(-2)) versus high (1,000 to 10,000 pl ants 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 s eeded in a factorial arrangement (49 density combinations) in a random ized-complete-block design and replicated 3 times at 2 sites in Montan a Plants were grown in pots (2,250 mm(2) X 380 mm deep) for 60 days be fore harvesting. Regressions predicting shoot weight, root weight, tot al weight, leaf area, and root length were calculated using 1) low kna pweed:low wheatgrass, 2) low knapweed:high wheatgrass, 3) high knapwee d:low wheatgrass, and 4) high knap weed:high wheatgrass densities. Reg ression coefficients indicated intraspecific interference was most imp ortant in predicting intermediate wheatgrass weight at both sites. At the wet site (457 mm, annually), interspecific interference only occur red at high spotted knapweed densities. At the dry site (305 mm, annua lly), 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 in terference occurred.