The population biology of Bromus tectorum in forests: effect of disturbance,grazing, and litter on seedling establishment and reproduction

Summary The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.     

No effect of ploidy level in plant response to competition in a common garden experiment

One of the important aspects explaining the evolutionary success of polyploid plants is that polyploids often, although not always, occupy a wider range of environments than their diploid ancestors. The two most likely explanations for this pattern are a wider plasticity in polyploid species and the existence of a range of locally adapted types. Most studies on patterns of distribution of different ploidy levels are only observational, and do not distinguish between these alternative explanations. The present study investigated the performance and plasticity of diploid and hexaploid cytotypes of a perennial plant, Aster amellus . The hexaploid plants occur in habitats with a wider range of competition intensity. Nine different populations of the species were selected: three diploid (from low competition habitats) and six hexaploid (from both low and high competition habitats). Plants were grown from seeds from these populations with and without competition in a common garden. Competition had strong effect on plant performance. There was, however, little effect of the ploidy levels and home environment. There was also almost no interaction between competition and ploidy level/home environment. The results of this study provide no support for any of the two suggested explanations for the wider range of habitats occupied by hexaploid plants. Other explanations thus must be thought. Generally, the results indicate that, although the higher plasticity of higher ploidy levels is often suggested, this may not be true. We therefore should attempt to assemble more experimental data to support or reject this assumption.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 211–219.     

Soil space and nutrients differentially promote the growth and competitive advantages of two invasive plants

Aims Invasive plants commonly occupy disturbed soils, thereby providing a stage for understanding the role of disturbance-enhanced resources in plant invasions. Here, we addressed how soil space and soil nutrients affect the growth and competitive effect of invasive plants and whether this effect varies with different invaders.Methods We conducted an experiment in which two invasive plants (Bromus tectorum and Centaurea maculosa) and one native species (Poa pratensis) were grown alone or together in four habitats consisting of two levels of soil space and nutrients. At the end of the experiment, we determined the total biomass, biomass allocation and relative interaction intensity of B. tectorum, C. maculosa and P. pratensis .Important findings Across two invaders, B. tectorum and C. maculosa, increased soil nutrients had greater positive effects on their growth than increased soil space, the effects of soil space on root weight ratio were greater than those of soil nutrients, and their competitive effect decreased with soil space but increased with soil nutrients. These findings suggest that changing soil space and nutrients differentially influence the growth and competitive advantages of two invaders. Bromus tectorum benefited more from increased soil resources than C. maculosa. Soil space and nutrients affected the biomass allocation of C. maculosa but not B. tectorum. The competitive effect of B. tectorum was unaffected by soil space and soil nutrients, but the opposite was the case for C. maculosa. Thus, the effects of soil space and nutrients on growth and competitive ability depend on invasive species identity.     

Clonal integration facilitates the proliferation of smooth brome clones invading northern fescue prairies

Predicting exotic invaders and reducing their impacts on the biodiversity and function of native ecosystems require understanding of the mechanisms that facilitate their success during key stages of invasion. We determined whether clonal growth, characteristic of the majority of successful invaders of natural areas, facilitates the proliferation of Bromus inermis (smooth brome), an exotic grass invading prairie ecosystems across the Great Plains. By manipulating the below-ground connections of proliferating rhizomes as well as the levels of soil nitrogen along the margins of clones invading northern fescue prairies in Manitoba, Canada, we hypothesized that physiological integration would most benefit ramets invading low resource environments. Severing clonal connections reduced the mass of smooth brome shoots invading native prairies and was exacerbated by the immobilization of soil nutrients with glucose. Clonal connections were equally important in the maintenance of smooth brome density and the horizontal proliferation of ramets. Our results demonstrate the role of physiological integration in the proliferation of a clonal exotic invader and may help explain the success of clonal invaders in other regions. Although integration among invading ramets suggests several possibilities for successful management, future research must continue to elucidate differences in the invasiveness of native versus exotic species as well as the persistence of clonal connections among exotic invaders.     

An Experimental Test of Resistance to Cheatgrass Invasion: Limiting Resources at Different Life Stages

Variable densities of an invasive species may represent variation in invasion resistance, due to variation in resource availability. This study determined whether low- and high-density cheatgrass (Bromus tectorum L.) patches within a shadscale-bunchgrass community of western Utah, USA, can be explained by variation in resource availability. It also explored the possible role of seed limitation and enemy pressure on invasion patterns. Two parallel field experiments were conducted:(1) increasing resources within low-density cheatgrass patches and, conversely (2) reducing resources within high-density cheatgrass patches. Treatments were applied at three life stages separately and across all stages. In low-density cheatgrass patches (assumed to represent high resistance), a disturbance that reduced soil compaction had the strongest positive effect, significantly increasing biomass by 250% and density by 104% in comparison to the control. The second strongest effect was reducing neighbors (native grasses), which significantly increased cheatgrass biomass and density. These results indicate that resources are present in low-density cheatgrass patches, but they are unavailable without disturbance and/or are exploited by competitors, and hence represent resistance to invasion. In high-density cheatgrass patches (assumed to represent low resistance), nitrogen availability was important in maintaining cheatgrass densities. Reducing nitrogen (via sucrose addition) significantly decreased density (by 37%) but not biomass. Life stages of cheatgrass were differentially affected by these resource manipulations. In addition, herbivore (primarily grasshoppers) and pathogen (head smut) pressures were documented to affect cheatgrass density, but did not explain resistance patterns. Instead, we found that differential resource availability explains the observed variation in cheatgrass density, and variation in natural resistance.     

Impacts of C4 grass introductions on soil carbon and nitrogen cycling in C3-dominated successional systems

While recent research has focused on the effects of exotic plant species on ecosystem properties, less is known about how restoring individual native plant species, differing in biomass and tissue chemistry, may impact ecosystems. We examined how three native C(4) prairie grasses affected soil C and N cycling 11 years after reintroduction into successional old-field communities dominated by non-native C(3) grasses. The species examined in this study differ in traits that are expected to influence soil C and N cycling (biomass and tissue chemistry). Thus, we hypothesized that cycling rates would decrease, thereby increasing pool sizes in soils under C(4) species compared under C(3) species. As predicted, the C(4) species had greater biomass and more recalcitrant tissue [higher C:N, acid detergent fiber (ADF):N] compared to the dominant C(3) species. The three C(4) species did not differ in tissue C:N, ADF:N, or root biomass, but Andropogon had more than twice the shoot biomass of Schizachyrium and Sorghastrum. Soils under the C(4) species did not differ in inorganic N levels, but levels were lower than in soils under the C(3) species, and soils under Andropogon had slightly lower in situ net N mineralization rates compared to those under C(3) species. We found little evidence of larger surface soil C pools under C(4) species versus C(3) species after 11 years and no differences in subsurface soil C or N among species. The C(4) species contributed a significant amount of C to both soil depths after 11 years. Our results demonstrate that C(4) species reintroduction into old-fields can alter C and N cycling on relatively short timescales, and that individual C(4) species differ in the magnitude of these effects. Improving our understanding of how species influence ecosystem properties is essential to predicting the ecosystem-level consequences of plant community alterations due to land use changes, global change, and species introductions.     

Impact of grasshoppers and an invasive grass on establishment and initial growth of restoration plant species

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.     

Fine root distribution and persistence under field conditions of three co-occurring Great Basin species of different life form

Fine roots of an annual grass, a perennial grass and a perennial shrub were examined. Based on life histories and tissue composition, we expected the greatest root persistence for the shrub and shortest for the annual grass. Roots were observed with minirhizotrons over 2 yr for number, length and diameter changes. A Cox proportional hazard regression correlated root persistence with soil water, depth, diameter and date of production. In 2001, grass roots had similar persistence times, but shrub roots had the shortest. In 2002, the annual had the longest median root persistence, the perennial grass intermediate and the perennial shrub had the shortest. All species responded similarly to the magnitude of seasonal precipitation; root numbers increased with favorable soil moisture and disappeared with drying; fewer, thinner roots at greater soil depths were found in the drier year (2001). Root persistence increased with soil moisture, diameter and earlier appearance in the spring. Plasticity in root morphology and placement was influenced by water availability, yet persistence was surprisingly contrary to expectations.     

Molecular evidence for host-adapted races of the fungal endophyte Epichloë bromicola after presumed host shifts

Abstract.— Host shifts of plant‐feeding insects and parasites promote adaptational changes that may result in the formation of host races, an assumed intermediate stage in sympatric speciation. Here, we report on genetically differentiated and host‐adapted races of the fungal endophyte Epichloë bromicola, which presumably emerged after a shift from the grass Bromus erectus to other Bromus hosts. Fungi of the genus Epichloë (Ascomycota) and related anamorphs of Neotyphodium are widespread endophytes of cool‐season grasses. Sexually reproducing strains sterilize the host by formation of external fruiting structures (stromata), whereas asexual strains are asymptomatic and transmitted via seeds. In E. bromicola, strains infecting B. erectus are sexual, and strains from two woodland species, B. benekenii and B. ramosus, are asexual and seed transmitted. Analyses of amplified fragment length polymorphism fingerprinting and of intron sequences of the tub2 and tef1 genes of 26 isolates from the three Bromus hosts collected at natural sites in Switzerland and nearby France demonstrated that isolates are genetically differentiated according to their host, indicating that E. bromicola does not form a single, randomly mating population. Phylogenetic analyses of sequence data did not unambiguously resolve the exact origin of asexual E. bromicola strains, but it is likely they arose from within sexual populations on B. erectus. Incongruence of trees derived from different genes may have resulted from recombination at some time in the recent history of host strains. Reciprocal inoculations of host plant seedlings showed that asexual isolates from B. benekenii and B. ramosus were incapable of infecting B. erectus, whereas the sexual isolates from B. erectus retained the assumed ancestral trait of broad compatibility with Bromus host seedlings. Because all isolates were interfertile in experimental crosses, asexual strains may not be considered independent biological species. We suggest that isolates infecting B. benekenii and B. ramosus represent long‐standing host races or incipient species that emerged after host shifts and that may evolve through host‐mediated reproductive isolation toward independent species.