Grazed and ungrazed sites subjected to different fre frequencies were sampled on the Konza Pralle Research Natural Area in northeast Kansas after 4 years of bison grazing (1987-1991). The objective was to study effects of bison grazing on plant species composition and diversity components (plant species rlchnesq equitability, and spatial heterogeneity) in sites of contrasting fire frequency. Cover and frequency of cool-season gramlnoids (e.g. Poa pratensis L., Agropyron smithii Rydb., Carex spp.) and some forbs (e.g. Aster ericoides [A. Gray] Howell, and Oxalis stricta L.) were consistently higher ln sites grazed by bison than in ungrazed exclosures, whereas the dominant warm-season grasses (Andropogon gerardii Vitman, Sorghastrum nutans &.I Nash, Panicum virgatum L., Schizachyrium scoparium michx.] Nash) and other forbs (e.g. Solidago missouriensis Nutt.) decreased ln response to bison. Plant species diversity (H') and spatial heterogeneity in all areas sampled were significantly increased by bison. Increased heterogeneity and mean species richness ln grazed prairie (40 species per sample site) compared to ungrazed prairie (29 species per site) were likely a result of greater microsite diversity generated by bison, whereas preferential grazing of the dominant grasses and concomitant increases in subordinate species resulted in an increase in equitability of species abundances. Species/area relationships indicated greater effects of bison on plant species richness with increasing sample area. Increases in plant diversity components associated with bison grazing were generally greater in annually burned than ln 4-year burned sites. Effects of ungulate grazers on floristic diversity have important implications given recent evidence that plant species diversity and the compositional and production stability of grassland plant communities are positively rellted.

A 6 -year study was conducted in tallgrass prairie to assess the effects of grazing management (cattle stocking densities and grazing systems) on plant community composition and diversity. Treatments included sites grazed season -long (May to October) at 3 stocking densities (3.8, 2.8, and 1.8 hectares per animal unit), ungrazed control sites, and sites under a late-season rest rotation grazing system at this same range of stocking densities. Plant communities were sampled twice each season using a nearestpoint procedure. Native plant species diversity, species richness, and growth form diversity were significantly higher in grazed compared to ungrazed prairie, and diversity was greatest at the highest stocking density. This enhancement of plant species diversity under grazing was not a result of increased frequency of weedy /exotic species. There were no significant effects of grazing system on plant diversity, nor any significant stocking density x grazing system interactions, indicating that animal density is a key management variable influencing plant species diversity and composition in tallgrass prairie and that effects of animal density override effects of grazing systems. Increasing cattle stocking densities decreased the abundance of the dominant perennial tall grasses, and increased abundance of the C4 perennial mid-grasses. The frequency of perennial forbs was relatively stable across grazing treatments. Abundance of annual forbs varied among years and grazing treatments. In half of the years sampled, annual forbs showed the highest frequency under intermediate stocking density. Patterns of responses among plant groups suggest that some species may respond principally to direct effects of grazers and others may respond to indirect effects of grazers on competitive relationships or on the spatial patterns of fuel loads and fires. Thus, this study suggests that large grazer densities, fire, and annual climatic variability interact to influence patterns of plant community composition and diversity in tallgrass prairie. Effects of varying management such as stocking densities and grazing systems on plant species diversity and the relative abundances of different plant growth forms or functional groups may have important consequences for grassland community stability and ecosystem function.

Landscapes with structural heterogeneity or patchiness can support diverse and stable wildlife populations. Visual obstruction methods (i.e., Robel pole and Nudd's coverboard) are common and useful techniques for quantifying vegetation structure; however, both rely on ocular estimations, which can be highly variable between observers. Our objectives were to 1) compare measurement and observer variation for visual obstruction among the two standard methods and the digital image method we developed using a digital camera; and 2) compare the performance of the Robel pole and digital image to estimate standing crop. The mean variation across the five observers using the digital image method (6.8%) was significantly lower (P , 0.05) than both the Nudds' coverboard (32.1%) and the Robel pole (52.2). There were no significant differences among locations for the digital image method; however, there were for both the Robel pole and Nudds' cover board (P , 0.05). The digital image method provided a better estimate of standing crop (r 2 5 0.89) compared to the Robel pole (r 2 5 0.68), accounting for 21% more of the observed variation in biomass. Long-term research programs that utilize seasonal field technicians to quantify habitat structure with a visual obstruction method could benefit from implementing use of the digital image method we developed. The low measurement error observed with this technique relative to the more traditional methods compared in this study might limit yearto-year and within-year variability of habitat structure data collected by numerous technicians with a high annual turnover. Resumen Los paisajes con una hetrogeneidad estructural o con muchos parches pueden sostener poblaciones diversas y estables de fauna silvestre. Los métodos de obstrucción visual (por ejemplo, el poste de Robel y la tabla de cobertura de Nudd) son te´cnicas comunes y útiles para cuantificar la estructura de la vegetación; sin embargo, ambas dependen de estimaciones oculares, las cuales pueden ser altamente variables entre observadores. Nuestros objetivos fueron: 1) comparar las mediciones y la variación de los observadores en la obstrucción visual entre los dos me´todos estándar y el me´todo de imagen digital que desarrollamos usando una cámara digital; y 2) comparar el comportamiento de los me´todos el poste de Robe y de imagen digital para estimar la biomasa aérea. La variación media entre los cinco observadores usando el método de imagen digital (6.8%) fue significativamente menor (P , 0.05) que los me´todos de la tabla de cobertura de Nudd (32.1%) y el poste de Robel (52.2%). No hubo diferencias significativas entre localidades para el me´todo digital; sin embargo, si la hubo para los me´todos de la tabla de cobertura de Nudd y el poste de Robel (P , 0.05). El método de imagen digital proveyó una mejor estimación de la biomasa (r 2 5 0.89), comparado con el me´todo del poste de Robel (r 2 5 0.68), explicando 21% más de la variación observada en la biomasa. Los programas de investigación a largo plazo que utilizan técnic...

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More intense and frequent droughts, coupled with elevated temperatures, are projected for grasslands worldwide. Although it has been suggested that alterations in temperature and precipitation will increase the success of biological invasions, studies that combine these climate change scenarios are limited. These changes in climate may increase the success of non-native, invasive plant species directly, as these species often possess traits that are favored by variable climates, or indirectly through negative impacts on native vegetation or alterations in soil microbial communities, such as arbuscular mycorrhizal (AM) fungi. The goal of our research was to assess above- and below-ground production and mycorrhizal fungal abundance of functionally similar native and invasive non-native grass species under projected climate-change scenarios. We assessed plant biomass, intra-radical AM root colonization, and relative abundance of extra-radical fungal biomass of two native (Schizachyrium scoparium (Michx.) Nash, Pascopyrum smithii (Rydb.) Á.Löve) and two invasive (Bothriochloa ischaemum (L.) Keng, Bromus inermis Leyss.) grass species subjected to increased temperature and decreased soil water availability. Regardless of temperature or soil moisture, the invasive grasses in our study displayed greater seedling growth as compared with paired native species. Invasive grasses were also generally characterized by greater intra-radical colonization by AM fungi, compared with native species. Our data suggest that invasive grasses will continue to be problematic and successfully out-compete native grasses following increased temperatures and reduced water availability, as projected by climate-change models.