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Seeding of non-native species following wildfires to stabilize soils and prevent erosion has become a controversial practice because it risks inhibiting tree regeneration, introducing noxious weeds, and permanently replacing native species. This paper reports the fate of non-native seeded species during 31 yean; of postfire forest,succession in northcentral Washington state, USA. In that region, catastrophic wildfires in grand fir (Abies grandis)/pine grass (Calamagrostis rubescens) associations characteristically result in a flush of lodgepole pine (Pinus contorta) regeneration that creates a dense tree monoculture. In the study, seven grass cultivars were broadcast seeded to prevent erosion, limit tree regeneration, and increase forage production for wildlife and livestock. Tree regeneration, native and non-native species cover, and above- ground biomass were evaluated in 1971, 1975, 1980, 1989, and 2001 . The seeded species quickly established dominant cover with levels of biomass production two to three times the level of native species. Density of tree regeneration was inversely correlated with perennial grass cover during the first 10 years. Between 1980 and 1 989, the seeded grasses disappeared, long before tree canopyclosure. Within 15 to 20 years native species had regained dominance, and after 30 years the last remnants of the non-native cultivars were replaced in the seeded areas by a diverse mixture of native graminoids, forbs, shrubs, and trees. In contrast, a monoculture of lodgepole pine dominated the unseeded areas. This study showed that non-native grasses seeded after wildfires do not always persist and can serve as a transition to restoring a more diverse seral community. [ABSTRACT FROM AUTHOR]

Establishing competitive plants is essential for restoring spot ted knapweed infested grasslands. Revegetation attempts typically fail because 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 andintermediate 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 factorialarrangement (49 density combinations) in a randomized-complete-blockdesign 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 interspecificinterference occurred. [ABSTRACT FROM AUTHOR]

Spotted knapweed (Centaurea maculosa Lam.) has reduced forage production, increased soil erosion, and lowered biodiversity on millions ofhectares of rangeland throughout the western United States. Objectives of this study were to quantify the interaction between picloram (4-amino-3,5,6-trichloropicolinic acid) and fertilizer on spotted knapweed density and grass yield. Four picloram rates (0.0, 0.14, 0.28, and 0.42 kg ha-1) and 4 fertilizer rates (N+P: 0.0+0.0, 10.5+12.2, 21.1+26.4, and 31.7+39.6 kg ha- 1) were applied to 3 spotted knapweed infested rangeland sites in a factorial combination arranged in a randomized-complete-block design during the spring of1994. Grass yield and spotted knapweed density were measured at peakstanding grass crop in 1994 and 1995. Data were analyzed as a split-plot in time using analysis of variance. Picloram and fertilizer did not interact to affect either spotted knapweed density or grass yield. All picloram treatments reduced spotted knapweed density to nearly zero. By 1995, all picloram treatments increased grass yield by an average of 1,500 kg ha-1. Fertilization did not affect spotted knapweed density, but the highest rates increased grass yield on those sites with a substantial residual grass understory. Combining fertilizer with picloram may enhance grass yield on sites with a residual of highly productive grasses. [ABSTRACT FROM AUTHOR]

Developing rangeland management strategies to minimize spotted knapweed (Centaurea maculosa Lam.) invasion will require understanding theeffects of intensity, frequency, and season of grazing on weed establishment. We studied the effects of hand-clipping 2-year-old Idaho fescue (Festuca idahoensis Elmer) plants 0, 30, 60, or 90%, 1, 2, or 3 times (14-day intervals) on spotted knapweed seedling emergence and growth in pots watered with 150 ml once weekly or 50 ml 3 times weekly. Pots were seeded with 5,000 spotted knapweed seeds m-2, replicated twice, and placed in a growth chamber in a completely randomized design. The experiment was repeated once. Plants were harvested after 50 days. Treatment effects on soil moisture, Idaho fescue andspotted knapweed shoot and root weight, and leaf area were compared using analysis of variance and regression analysis. At final harvest,Idaho fescue shoot weight and leaf area decreased with increasing defoliation level and frequency. Idaho fescue root weight was not affected by any treatment. A single Idaho fescue defoliation at 30% and 90% increased spotted knapweed weight and numbers per pot respectively,over those pots with undefoliated plants. The level of defoliation necessary to enhance spotted knapweed numbers was lower as defoliationfrequency increased. As defoliation level and frequency increased, soil water content increased resulting in a corresponding increase in spotted knapweed emergence and growth. [ABSTRACT FROM AUTHOR]

Community dynamics and dominance on cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) infested rangelandappear to be influenced by resource acquisition rates and duration of growth. Objectives were to determine the effects of densities, proportions, and soil depth on the growth rate and duration of growth of these species. In 6 field experiments isolated individuals, monocultures (100, 1,000, 10,000 plants m{sup}-2{end}), and mixtures (same densities arranged factorially) were grown with unrestricted and restricted (0.2- and 0.5-m) soil depths. Shoot weights were determined on 12-day intervals beginning on day 24 and ending on day 72 for plants grown with restricted soil depth and day 96 (cheatgrass) and day 108 (yellow starthistle) for plants grown in unrestricted soil. Quadratic growth curves were fit for each replication for plants grown in isolation. Linear and quadratic models were developed for plants grown in monocultures and mixtures. Simple linear regression coefficients were used as growth rates and regressed over plant density. Time of injection (an indicator of the duration of growth) for plants growing in monocultures and mixtures was calculated from quadratic models. Growth rate of yellow starthistle was about 7 times faster and duration of growth 25 days longer, than those of cheatgrass when grown in isolation without soil depth restriction. As densities were increased and/or soil depth decreased, growth rate and duration of growth were loweredand the difference between species were masked. Growth rates and duration of growth of cheatgrass and yellow starthistle appear to dependon plant density, soil depth, and available soil moisture. [ABSTRACT FROM AUTHOR]

Diffuse knapweed (Centaurea diffuse Lam.), an invasive weed, has reduced forage production and biodiversity, and increased soil erosion on over a million hectares of rangeland in the western United States. This study evaluated the effects of a single grass defoliation on establishment of diffuse knapweed seeded at 2 rates into a bluebunch wheatgrass (Pseudoroegneria spicata [Pursh.] Scribn and Smith)/needle-and-thread (Stipa comata Trin. & Rupr.) community and a crested wheatgrass (Agropyron cristatum (L.) Gaertn.) community. Six defoliation levels (0, 20,40, 60, 80, 100 percent) and 2 seeding rates (3,000, 6,000 diffuse knaweed seeds) were applied to 1 m{sup}2{end} plots in a randomized-complete-block design (n=4). Diffuse knapweed was seeded in the fall of 1992, and grasses were defoliated on 28 April 1993. The number of flowering culms and weed seedlings were counted in September1993. Densities of diffuse knapweed seedlings, juveniles, and adults, as well as plant standing crop, were determined in May 1994. Seed rate had minimal effect on diffuse knapweed density. By May 1994, densities of diffuse knapweed were about 20 and 30 plants m{sup}2{end} onundefoliated bluebunch wheatgrass and crested wheatgrass plots, respectively, indicating that defoliation is not required for this noxious weed to become established. Higher levels of grass defoliation ( greater than 60 percent), especially of bluebunch wheatgrass, enhanced diffuse knapweed establishment, indicating that moderate ( less than 60 percent) defoliation would not necessarily accelerate invasion by diffuse knapweed. [ABSTRACT FROM AUTHOR]