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High disturbance frequency, low water availability, and advantageous growth mechanisms of invasive annual compared to native perennial grasses reduce native grass establishment throughout arid rangelands. Modifying seeding dispersal processes, including seeding rate and time, may increase native grass recruitment by influencing safe site occupation. A better understanding of seedling development through life history stages and the ecological processes occurring during these stages may be necessary to comprehend modified dispersal dynamics on plant community assembly. We tested the effects of spring vs. fall annual grass seeding times, adding water, and varying annual and perennial grass propagule pressure on perennial and annual grass recruitment in an eastern Oregon shrub-steppe ecosystem. Across species, survival rates were lowest between germination and emergence stages. However, perennial grass germination rates were highest when perennials were seeded with annual grasses in autumn. Perennial grass recruitment was generally low, especially when annual grass propagule pressure was higher than 150 seeds m −2 . Although, by the second growing season, perennial grasses had the highest density when perennials were seeded with annuals in autumn and water was added. Consequently, modifying native perennial grass dispersal, like priority autumn seeding and increasing propagule pressure, should produce higher perennial grass recruitment across arid lands. [ABSTRACT FROM AUTHOR]

The annual grass Bromus tectorum has invaded millions of hectares in western North America and has transformed former perennial grass and shrub-dominated communities into annual grasslands. Fire plays a key role in the maintenance of B. tectorum on the landscape but the type of disturbance responsible for initial invasion is less well understood. We conducted an experiment in a perennial shrub/grass/forb community in eastern Idaho, USA to examine the roles of plant community and soil disturbance on B. tectorum emergence and establishment prior to state-changing fires. Our experiment consisted of a plant community disturbance treatment where we (1) removed the shrub component, (2) removed the grass/forb component, or (3) removed all shrubs, grasses, and forbs. We followed this treatment with seeding of B. tectorum onto the soil surface that was (1) intact, or (2) disturbed. Each experimental plot had an associated control with no plant community disturbance but was seeded in the same manner. The experiment was replicated 20 times in two sites (high and low aboveground biomass). We measured emergence by counting seedlings in late spring and establishment by counting, removing, and weighing B. tectorum individuals in mid-summer. We also examined the influence of plant community disturbance on the soil environment by measuring extractable NH4+ and NO3- four times each summer. Soil disturbance greatly influenced the number of B. tectorum individuals that emerged each spring. Plant community disturbance, specifically disturbance of the grass/forb component, increased N availability in the late growing season and biomass of B. tectorum the following summer. We conclude that soil disturbance and plant community disturbance interact to promote the initial invasion of B. tectorum in Intermountain West valley ecosystems. [ABSTRACT FROM AUTHOR]

Ecologists are searching for models, frameworks, and principles that provide a bridge between theory and the practice of restoration. Successional management has been proposed as a useful model for managing and restoring invasive-plant-dominated rangeland because it provides a framework in which ecological processes can be manipulated by managers to achieve a desired plant community. Successional management identifies three general causes of succession (site availability [disturbance], species availability [colonization], and species performance) and suggests that managers address the ecological process influencing each general cause in a coordinated fashion to direct plant community dynamics. We tested successional management using various techniques to restore invasive-weed-dominated rangeland. Our hypothesis was that successively modifying the factors influencing the causes of succession in an integrated fashion would favor the establishment and abundance of native grasses over singularly applied treatments. Thus, we anticipated that the majority of responses to multiple treatments would be explained by higher order interactions, especially in the final year of the study (2004). To test this hypothesis; we used a model system within a Festuca campestris/Pseudoroegneria spicata habitat among pothole wetlands dominated by Centaurea maculosa and Potentilla recta, two invasive species. We used three herbicide treatments (none, 2,4-D, and picloram) to influence species performance; two seeding methods (imprinting, i.e., creating a small depression and broadcasting, and no-till drilling) to influence disturbance; three seeding rates (977, 1322, and 1557 seeds/m2) to influence colonization; and two cover crop treatments (with and without Triticum aestivum) to influence soil N and favor native grasses. Treatments were factorially arranged and replicated four times in a randomized complete block design in 2001 and sampled in 2002 and 2004. As predicted, plant response to treatments was dominated by two- and three-way interactions in 2004. The highest seeding rate (colonization) combined with no-till drilling (disturbance) produced the highest native grass density in 2002. These effects persisted into 2004 for P. spicata. but not for F. campestris or F. idahoensis. Combining picloram with no-till drill seeding also produced a high density of P. spicata. Drill seeding at 977 seeds/m2 favored F. idahoensis density, while no-till drilling at 1322 seeds/m2 favored its biomass in 2004. F. idahoensis established well after drill seeding with a cover crop and applying 2,4-D. Herbicides reduced native forb density and/or biomass, with early season forbs being more sensitive topicloram and summer forbs being more sensitive to 2,4-D. Herbicides increased exotic grasses’ density and biomass but had no effect on native grasses. In most cases, integrating treatments that addressed multiple causes of succession favored a desired plant community. Thus, we accomplished our goal of using successional management to direct plant communities toward native desired species, but the treatments used did not improve species richness. Since naturally occurring native forbs did not respond favorably to any treatment combination, ecological restoration using successional management may best be thought of as an iterative procedure where various components and processes of the system are methodically repaired or replaced over time. [ABSTRACT FROM AUTHOR]

A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Spotted knapweed ( Centaurea maculosa). The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized complete block design replicated four times at each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration × biomass) by indigenous functional groups: grasses, shallow-rooted forbs, deep-rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallow-rooted forbs, deep-rooted forbs, grasses, or spikemoss were removed. Functional group biomass was also collected and analyzed for nitrogen, phosphorus, potassium, and sulphur. Based on covariate analyses, postremoval indigenous plot biomass did not relate to invasion by C. maculosa. Analysis of variance indicated that C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of functionally similar groups as an invader may be particularly important in invasion resistance. [ABSTRACT FROM AUTHOR]

Our objective was to determine the resource most limiting to plant growth within a semi-arid plant association. We hypothesized that the addition of nitrogen (N) would elicit an increase in biomass more than additions of water, light, phosphorus (P), potassium (K), or sulfur (S). Treatments included the addition of water, N, P, K, or S, or reduction of ambient light on two sites in southwestern Montana within the Festuca idahoensis/Agropyron spicatum plant association. Nitrogen was the only treatment that increased plant community biomass, specifically native grass biomass (about 50% greater than the control). The addition of water decreased above-ground plant community biomass. Our results suggest that N is the primary limiting resource for the dominant functional group within the F. idahoensis/A. spicatum plant association. [Copyright &y& Elsevier]