Herbivore‐induced shifts in species composition have been documented from grasslands throughout the world, but the mechanism(s) of species replacement remains largely unexplored. An experiment was conducted in a transplant garden, on the campus of Texas A&M University, to test the hypothesis that greater herbivory tolerance of mid‐seral species, rather than selective herbivory of a late‐seral dominant, is the dominant mechanism contributing to species replacement in a mesic grassland. Individual plants of a dominant, late‐seral, perennial grass, Schizachyrium scoparium, were grown in competition with conspecifics and two other perennial grasses (Bothriochloa saccharoides and Stipa leucotricha) known to increase in relative abundance in response to intensive grazing by domestic herbivores in the southern true prairie. Uniform defoliation of both S. scoparium and neighbors was imposed to test the relative expression of herbivory tolerance, while selective herbivory was imposed by defoliating S. scoparium plants, but not neighbors. After four successive defoliations, mean annual shoot production and cumulative shoot production of S. scoparium plants were suppressed to a greater extent by conspecific neighbors than by either of the two mid‐seral neighbors. Mean basal area of S. scoparium plants was not significantly suppressed by either intra‐ or interspecific neighbors; however, tiller number per plant was suppressed by B. saccharoides, but not by S. leucotricha or conspecific neighbors. Defoliation of S. scoparium plants, but not neighbors, negatively impacted the late‐seral plants. Selective defoliation of S. scoparium plants significantly reduced tiller variables of mean mass, leaf blade area, and leaf number, but did not significantly reduce plant variables including mean basal area, tiller number, or annual shoot production. Defoliation of both S. scoparium plants and neighbors increased annual shoot production, mean basal area per plant, mean tiller leaf area, leaf number, tiller mass, stomatal conductance to H2O vapor, and plant xylem pressure potential in comparison with S. scoparium plants grown with comparable, nondefoliated neighbors. An increase in both plant and tiller variables in defoliated S. scoparium plants grown with uniformly defoliated neighbors establishes that replacement of a late‐seral dominant is not driven by a greater relative expression of herbivory tolerance of mid‐seral species. These results collectively suggest that the late‐seral dominant, S. scoparium, possesses a greater competitive ability and a comparable or greater degree of herbivory tolerance than the mid‐seral species that comprise the community. Therefore, the initial hypothesis was rejected. It can be inferred that the alternative mechanism, selective herbivory of the late‐seral dominant, is the dominant mechanism contributing to species replacement. Herbivore‐induced modifications of competitive interactions are most likely to drive species replacement in grasslands characterized by high and consistent resource availabilit...

Increasing attention, resources and efforts are being focused on the conversion of weedy dominated rangelands back to perennial plant communities that resemble predisturbance communities in form, function and composition. A study was conducted in 1998 and replicated again in 1999 to determine whether native plants could be established through "assisted succession" -manipulating a cheatgrass-dominated area to perennial plant domination, then to native or near -native diversity. Cheatgrass dominated rangeland that had been successfully revegetated with crested wheatgrass (Agropyron cristatum [L.] Gaertner) was seeded with native species. Another area dominated by cheatgrass, but without crested wheatgrass, was also seeded. Four seedbed preparation methods were investigated: tilling, harrowing, application of a herbicide, and no treatment. Four different seeding methods were used in the 2 areas and 4 seedbed preparation techniques: drilling, broadcasting, a broadcast -cover method, and no seed. Seeding was done in February, and data were collected in mid -summer each year. Native grasses and shrubs emerged in greater numbers on treatments established on the crested wheatgrass matrix than on those established on the cheatgrass matrix. Perhaps in general, but especially in years with normal or below average precipitation, the assisted succession approach proved successful for restoration of native sagebrush-grassland steppe from cheatgrass range.Key Words: cheatgrass, crested wheatgrass, rangeland reseeding, restoration, sagebrush-steppe Natural succession has been suggested as a model for restoration and management of disturbed plant communities (Luken 1990, Hironaka 1994, Jones 1997, 1998, Brown and Amacher 1999. In wildland restoration, 3 basic components of succession may be used to determine the suitability of land for intervention: site availability, species availability, and species performance (Rosenberg and Freedman 1984, Pickett et al. 1987, Luken 1990, Sheley et al. 1996. Each of these may be initiated, modified, or curtailed by restorationists with the aim of affecting the trajectory of succession.The difficulty of seeding native species directly into areas dominated by cheatgrass (Bromus tectorum L.) and other exotic annuals is well documented (Campbell and Swain 1973, Pellant 1990, Allen 1995, Beckstead et al. 1995, Monsen and McArthur 1995 nar si las plantas nativas pudieran ser establecidas a través de "sucesión asistida ", esto es, manipulando un área dominada por "Cheatgrass" para que dominen las plantas perennes y alcanzar una diversidad igual o cercana a la original. Un pastizal dominado por "Cheatgrass" que había sido revegetado exitosamente con "Crested wheatgrass" (Agropyron cristatum [L.] Gaertner) se sembró con especies nativas. Otra área dominada por "Cheatgrass "pero sin "Crested wheatgrass" también fue sembrada. Se investigaron cuatro métodos de preparación de la cama de siembra: Labranza, barbecho, aplicación de un herbicida y no tratamiento. En las dos áreas se usaron cuatro méto...

Invasive plants are a common problem in the management and restoration of degraded lands in the semiarid western United States, but are often not the primary focus of restoration ecologists. Likewise, restoring native vegetation has not been a major concern of weed scientists. But trends in the literature demonstrate increasing overlap of these fields, and greater collaboration between them can lead to improved efficacy of restoration efforts. Succession and ecosystem development are the products of complex interactions of abiotic and biotic factors. Our greatest restoration and invasive plant management successes should result when we take advantage of these natural processes. Recent shifts in management objectives have generated approaches to directing plant community development that utilize species that are strong competitors with invasive species as a bridge to the establishment of native perennial vegetation. Soil water and nutrient characteristics and their interactions can affect desired and undesired plant species differentially and may be manipulated to favor establishment and persistence of desired perennial plant communities. Selection of appropriate plant materials is also essential. Species assemblages that suppress or exclude invaders and competitive plant materials that are well adapted to restoration site conditions are important keys to success. We provide guidelines for restoration based on the fundamental ecological principles underlying succession. Knowledge of the complex interactions among the biotic and abiotic factors that affect successional processes and ecosystem development, and increased collaboration between weed scientists and restoration ecologists hold promise for improving restoration success and invasive species management.

Cheatgrass (Bromus tectorum L.) is a winter annual weed that presents a serious obstacle to rangeland restoration in the Intermountain West. The objective of this study was to evaluate factors regulating the size and persistence of cheatgrass carryover seed banks on semiarid sites in western Utah. We prevented current-year seed production in each of four habitats, then tallied emerging seedlings over the next 4 yr. Two iterations of the study were conducted during consecutive years. One year before initiation of each iteration, we estimated seed rain at each site. Above-average precipitation in 1998-1999 resulted in relatively high seed rain (13 942 seeds ? m 22) for the first iteration, whereas seed rain for the second iteration averaged only 3 567 ? m 22 because of drought conditions in 1999-2000. Mean total number of seedlings emerging from carryover seeds for the first and second iterations were 1 304 and 270 seedlings ? m 22. Seedling emergence from carryover seed was positively correlated with production-year seed rain (R 2 5 0.69). The fraction of seed rain that carried over tended to be lower when precipitation the year following production favored fall emergence of the transient seed bank. First-year emergence of carryover seeds averaged 96% of total emergence, whereas third-year emergence averaged , 1% and was zero for six of eight cases. Carryover seeds persisted somewhat longer at the xeric black greasewood site than at more upland sites. Our study shows that cheatgrass seeds rarely persist beyond the second carryover year even on semiarid sites. Emergence from the carryover seed bank can be predicted from site attributes and precipitation patterns in previous years. Resumen La espiguilla colgante (Bromus tectorum L.) es una pasto invernal que presenta un serio obstáculo en la restauración de los pastizales en el oeste inter montano. El objetivo de este estudio fue evaluar los factores que regulan el tamañ o y la persistencia del banco de reserva de semillas de la espiguilla colgante en los sitios semiáridos en el oeste de Utah. Nosotros evitamos la producción de semillas del corriente añ o en cada uno de los quatro hábitats, y luego los juveniles que crecieron en los próximos 4 añ os. Las dos iteraciones del estudio fueron llevadas a cabo durante añ os consecutivos. Nosotros estimamos la lluvia de semillas en cada sitio un añ o previo al inicio de cada iteración. La precipitación sobre el promedio en el 1998 al 1999 resultó en una lluvia de semilla relativamente alta (13 942 semillas ? m 22) para la primera interacción mientras que la lluvia de semilla para la segunda iteración promedio solo 3 567 semillas ? m 22 debido a las condiciones secas en el 1999 al 2000. La media del número total de juveniles emergentes de las semillas de reserva para la primera y segunda de las iteraciones lo fue 1 304 y 270 juveniles ? m 22. El juvenil emergente de las semillas de reserva fue correlacionado positivamente con la producción anual de lluvia de semillas (R 2 5 0.69). La fracción de la lluvia de semillas que se...

Increasing native diversity of cheatgrassdominated rangeland through assisted succession Item Type text; Article