Competition and niche partitioning among Pseudoroegneria spicata, Hedysarum boreale, and Centaurea maculosa

Maximizing desired plant diversity has been suggested as a means of minimiz ing non-indigenous plant invasion on rangeland by maximizing niche occupati on. Competition between 2 desired indigenous species, Pseudoroegneria spica ta (Pursh.) Love (bluebunch wheatgrass) and Hedysarum boreale Nutt. var. bo reale (northern sweetvetch), and a non-indigenous invader, Centaurea maculo sa Lam. (spotted knapweed), was quantified using growth of isolated individ uals 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 ex periments. H. boreale and C. maculosa seeding densities were 0, 200, 400, a nd 800 seeds m(-2), respectively, in the Ist experiment and 0, 400, 800, an d 1600 seeds m(-2), respectively, in the 2nd experiment. Densities were fac torially arranged. Pots were placed in an environmental chamber (12 degrees C, 12-h day length, 200 mu mol photons m(-2) s(-1) spectral light) in a ra ndomized-complete-block design. After 90 d the growth rate of P. spicata (9 2.1 mg d(-1) shoot growth) was greater than that of the 2 forts (1.6 and 5. 5 mg d(-1) for H. boreale and C. maculosa, respectively), and growth rates of the 2 forts were similar to one another. Curvilinear regression indicate d that intraspecific competition was more important in determining shoot we ight than interspecific competition. In addition, the 2 forts competed more directly with each other than with Il spicata. Competition coefficient rat ios (1.42 and 1.53 for II spicata with H. boreale and C, maculosa, respecti vely, and 1.03 for H. boreale with C. maculosa) indicated substantial parti tioning of resources between Il spicata and each of the forts. Little or no resource partitioning occurred between forts. This study suggests that inc reasing desired. plant diversity may minimize weed invasion by increasing n iche occupation.