Great Basin Naturalist


Maximizing desired plant diversity has been suggested as a means of minimizing non-indigenous plant invasion on rangeland by maximizing niche occupation. Competition between 2 desired indigenous species, Pseudoroegneria spicata (Pursh.) Löve (bluebunch wheatgrass) and Hedysarum boreale Nutt. var. boreale (northern sweetvetch), and a non-indigenous invader, Centaurea maculosa Lam. (spotted knapweed), was quantified using growth of isolated individuals and 2 three-species addition series experiments. Seeding densities of P. spicata remained constant at 0, 200, 400, and 800 seeds m−2 in both experiments. H. boreale and C. maculosa seeding densities were 0, 200, 400, and 800 seeds m−2, respectively, in the 1st experiment and 0, 400, 800, and 1600 seeds m−2, respectively, in the 2nd experiment. Densities were factorially arranged. Pots were placed in an environmental chamber (12°C, 12-h day length, 200 µmol photons m−2s−1 spectral light) in a randomized-complete-block design. After 90 d the growth rate of P. spicata (92.1 mg d−1 shoot growth) was greater than that of the 2 forbs (1.6 and 5.5 mg d−1 for H. boreale and C. maculosa, respectively), and growth rates of the 2 forbs were similar to one another. Curvilinear regression indicated that intraspecific competition was more important in determining shoot weight than interspecific competition. In addition, the 2 forbs competed more directly with each other than with P. spicata. Competition coefficient ratios (1.42 and 1.53 for P. spicata with H. boreale and C. maculosa, respectively, and 1.03 for H. boreale with C. maculosa) indicated substantial partitioning of resources between P. spicata and each of the forbs. Little or no resource partitioning occurred between forbs. This study suggests that increasing desired plant diversity may minimize weed invasion by increasing niche occupation.