Host plant species effects on arbuscular mycorrhizal fungal communities in tallgrass prairie

Symbiotic associations between plants and arbuscular mycorrhizal (AM) fungi are ubiquitous in many herbaceous plant communities and can have large effects on these communities and ecosystem processes. The extent of species-specificity between these plant and fungal symbionts in nature is poorly known, yet reciprocal effects of the composition of plant and soil microbe communities is an important assumption of recent theoretical models of plant community structure. In grassland ecosystems, host plant species may have an important role in determining development and sporulation of AM fungi and patterns of fungal species composition and diversity. In this study, the effects of five different host plant species [Poa pratensis L., Sporobolus heterolepis (A. Gray) A. Gray, Panicum virgatum L., Baptisia bracteata Muhl. ex Ell., Solidago missouriensis Nutt.] on spore communities of AM fungi in tallgrass prairie were examined. Spore abundances and species composition of fungal communities of soil samples collected from patches within tallgrass prairie were significantly influenced by the host plant species that dominated the patch. The AM fungal spore community associated with B. bracteata showed the highest species diversity and the fungi associated with Pa. virgatum showed the lowest diversity. Results from sorghum trap cultures using soil collected from under different host plant species showed differential sporulations of AM fungal species. In addition, a greenhouse study was conducted in which different host plant species were grown in similar tallgrass prairie soil. After 4 months of growth, AM fungal species composition was significantly different beneath each host species. These results strongly suggest that AM fungi show some degree of host-specificity and are not randomly distributed in tallgrass prairie. The demonstration that host plant species composition influences AM fungal species composition provides support for current feedback models predicting strong regulatory effects of soil communities on plant community structure. Differential responses of AM fungi to host plant species may also play an important role in the regulation of species composition and diversity in AM fungal communities. Received: 29 January 1999 / Accepted: 20 October 1999     

Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie

In tallgrass prairie, plant species interactions regulated by their associated mycorrhizal fungi may be important forces that influence species coexistence and community structure; however, the mechanisms and magnitude of these interactions remain unknown. The objective of this study was to determine how interspecific competition, mycorrhizal symbiosis, and their interactions influence plant community structure. We conducted a factorial experiment, which incorporated manipulations of abundance of dominant competitors, Andropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal symbiosis using the fungicide benomyl under two fire regimes (annual and 4-year burn intervals). Removal of the two dominant C₄ grass species altered the community structure, increased plant species richness, diversity, and evenness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community structure, although plant species richness and diversity increased. Responses of individual plant species were associated with their degree of mycorrhizal responsiveness: highly mycorrhizal responsive species decreased in abundance and less mycorrhizal responsive species increased in abundance. The combination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, indicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that mycorrhizal fungi affect plant communities indirectly by influencing the pattern and strength of plant competitive interactions. Burning strongly influenced the outcome of these interactions, which suggests that plant species diversity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis. Received: 7 April 1999 / Accepted: 30 July 1999     

Long-Term Nitrogen Amendment Alters the Diversity and Assemblage of Soil Bacterial Communities in Tallgrass Prairie

Anthropogenic changes are altering the environmental conditions and the biota of ecosystems worldwide. In many temperate grasslands, such as North American tallgrass prairie, these changes include alteration in historically important disturbance regimes (e.g., frequency of fires) and enhanced availability of potentially limiting nutrients, particularly nitrogen. Such anthropogenically-driven changes in the environment are known to elicit substantial changes in plant and consumer communities aboveground, but much less is known about their effects on soil microbial communities. Due to the high diversity of soil microbes and methodological challenges associated with assessing microbial community composition, relatively few studies have addressed specific taxonomic changes underlying microbial community-level responses to different fire regimes or nutrient amendments in tallgrass prairie. We used deep sequencing of the V3 region of the 16S rRNA gene to explore the effects of contrasting fire regimes and nutrient enrichment on soil bacterial communities in a long-term (20 yrs) experiment in native tallgrass prairie in the eastern Central Plains. We focused on responses to nutrient amendments coupled with two extreme fire regimes (annual prescribed spring burning and complete fire exclusion). The dominant bacterial phyla identified were Proteobacteria, Verrucomicrobia, Bacteriodetes, Acidobacteria, Firmicutes, and Actinobacteria and made up 80% of all taxa quantified. Chronic nitrogen enrichment significantly impacted bacterial community diversity and community structure varied according to nitrogen treatment, but not phosphorus enrichment or fire regime. We also found significant responses of individual bacterial groups including Nitrospira and Gammaproteobacteria to long-term nitrogen enrichment. Our results show that soil nitrogen enrichment can significantly alter bacterial community diversity, structure, and individual taxa abundance, which have important implications for both managed and natural grassland ecosystems.     

Are two strategies better than one? Manipulation of seed density and soil community in an experimental prairie restoration

Restoration practitioners have a variety of practices to choose from when designing a restoration, and different strategies may address different goals. Knowledge of how to best use multiple strategies could improve restoration outcomes. Here, we examine two commonly suggested strategies in a single tallgrass prairie restoration experiment: increased forb sowing density and prairie soil inoculation. We designed a study with two different forb seeding densities. Within these densities, we transplanted seedlings into 1‐m² plots that had been grown in either a whole prairie soil inoculum or sterilized prairie soil. After 4 years, we found positive effects of both high forb sowing density and inoculation treatments on the ratio of seeded to nonseeded plant cover in these plots, and negative effects of both treatments on nonseeded plant diversity. No effects of either treatment were seen on seeded plant diversity. Each strategy also affected the plant community in different ways: high forb sowing density increased seeded forb richness and decreased native nonseeded plant cover, while inoculation decreased non‐native cover, and tended to increase average successional stage of the community. These effects on restoration outcome were typically independent of each other, with the result that plots with both manipulations had the most positive effects on restoration outcomes. We thus advocate the combined use of these restoration strategies, and further studies which focus on both seeding and soil community manipulation in restoration.     

Effects of the soil microbiome on the demography of two annual prairie plants

1. Both mutualistic and pathogenic soil microbes are known to play important roles in shaping the fitness of plants, likely affecting plants at different life cycle stages.
2. In order to investigate the differential effects of native soil mutualists and pathogens on plant fitness, we compared survival and reproduction of two annual tallgrass prairie plant species (Chamaecrista fasciculata and Coreopsis tinctoria) in a field study using 3 soil inocula treatments containing different compositions of microbes. The soil inocula types included fresh native whole soil taken from a remnant prairie containing both native mutualists and pathogens, soil enhanced with arbuscular mycorrhizal (AM) fungi derived from remnant prairies, and uninoculated controls.
3. For both species, plants inoculated with native prairie AM fungi performed much better than those in uninoculated soil for all parts of the life cycle. Plants in the native whole prairie soil were either generally similar to plants in the uninoculated soil or had slightly higher survival or reproduction.
4. Overall, these results suggest that native prairie AM fungi can have important positive effects on the fitness of early successional plants. As inclusion of prairie AM fungi and pathogens decreased plant fitness relative to prairie AM fungi alone, we expect that native pathogens also can have large effects on fitness of these annuals. Our findings support the use of AM fungi to enhance plant establishment in prairie restorations.

     

Mycorrhizal suppression alters plant productivity and forb establishment in a grass-dominated prairie restoration

A fundamental goal of restoration is the re-establishment of plant diversity representative of native vegetation. However, many prairie restorations or Conservation Reserve Program sites have been seeded with warm-season grasses, leading to grass-dominated, low-diversity restorations not representative of native grasslands. These dominant grasses are strongly mycotrophic, while many subordinate forb species appear to be less dependent on mycorrhizal symbiosis. Therefore, manipulating arbuscular mycorrhizal fungi (AMF) may be useful in promoting establishment and growth of forb species in grass-dominated prairie restorations. To assess the potential role of mycorrhizae in affecting the productivity and community composition of restored tallgrass prairie, we conducted a 4-year field experiment on an 8-year-old grassland restoration at the Konza Prairie in northeastern Kansas, USA. At the initiation of our study, seeds of 12 forb species varying in degree of mycorrhizal dependence were added to established grass-dominated plots. Replicate plots were treated bi-weekly with a soil drench of fungicide (Topsin-M^®) over four growing seasons and compared to non-treated control plots to assess the role of AMF in affecting plant species composition, productivity, leaf tissue quality, and diversity in restored tallgrass prairie. Topsin applications successfully reduced mycorrhizal colonization of grass roots to approximately 60–80% relative to roots in control plots. Four years of mycorrhizal suppression reduced productivity of the dominant grasses and increased plant species richness and diversity. These results highlight the importance of mycorrhizae as mediators of plant productivity and community dynamics in restored tallgrass prairie and indicate that temporarily suppressing AMF decreases productivity of the dominant C4 grasses and allows for establishment of seeded forb species.     

The role of hemiparasitic plants: influencing tallgrass prairie quality,diversity, and structure

Wood betony, Orobanchaceae (Pedicularis canadensis) and bastard toadflax, Santalaceae (Comandra umbellata) are two root‐hemiparasitic plant species found in tallgrass prairie communities. Natural resource managers are interested in utilizing these species as “pseudograzers” in grasslands to reduce competitively dominant grasses and thereby increase ecological diversity and quality in prairie restorations and urban plantings. We performed an observational field study at 5 tallgrass prairie sites to investigate the association of hemiparasite abundance with metrics of phylogenetic and ecological diversity, as well as floristic quality. Although no reduction in C₄ grasses was detected, there was a significant association between hemiparasite abundance and increased floristic quality at all 5 sites. Hemiparasite abundance and species richness were positively correlated at one restoration site. In a greenhouse mesocosm experiment, we investigated response to parasitism by P. canadensis in 6 species representing different plant functional groups of the tallgrass prairie. The annual legume partridge pea, Fabaceae (Chamaecrista fasciculata) had the greatest significant dry biomass reduction among 6 host species, but the C₄ grass big bluestem, Poaceae (Andropogon gerardii) had significantly greater aboveground biomass when grown with the hemiparasite. Overall, host species biomass as a total community was significantly reduced in mesocosms, consistent with other investigations that demonstrate influence on community structure by hemiparasitic plant species. Although hemiparasites were not acting as pseudograzers, they have the potential to influence community structure in grassland restorations and remnants.     

Inoculation with remnant prairie soils increased the growth of three native prairie legumes but not necessarily their associations with beneficial soil microbes

Restoring the diversity of plant species found in remnant communities is a challenge for restoration practitioners, in part because many reintroduced plant species fail to establish in restored sites. Legumes establish particularly poorly, perhaps because they depend on two guilds of soil microbial mutualists, rhizobial bacteria and arbuscular mycorrhizal (AM) fungi, that may be absent from restored sites. We tested the effect of soil microorganisms from remnant and restored prairies on legume growth by inoculating seedlings of Lespedeza capitata, Amorpha canescens, and Dalea purpurea with soil from 10 restored prairies and 6 remnant (untilled) prairies from southwest Michigan. We generally found support for the hypothesis that restored prairie soils lack microbes that enhance prairie plant growth, although there was variation across species and mutualist guilds. All three legumes grew larger and two legumes (Lespedeza and Amorpha) produced more nodules when inoculated with soil from remnant prairies, suggesting that low quantity and/or quality of rhizobial partners may limit the establishment of those species in restored prairies. In contrast, no legume experienced greater root colonization by AM fungi in remnant prairie soils, suggesting equivalent quantity (but not necessarily quality) of fungal partners in remnant and restored prairie soils. We detected no evidence of spontaneous recovery of the community of beneficial soil microbes in restorations. These results suggest that the absence of rhizobia, a largely overlooked component of prairie soils, could play a strong role in limiting restored prairie diversity by hindering legume establishment. Active reintroduction of appropriate rhizobial strains could enhance prairie restoration outcomes.     

In tallgrass prairie restorations,relatedness influences neighborhood-scale plant invasion while resource availability influences site-scale invasion

Attempting to control invasive plant species in tallgrass prairie restorations is time-consuming and costly, making improved approaches for predicting and reducing invasion imperative. Both biotic and abiotic factors mediate plant invasions, and can potentially be used by restoration managers to reduce invasion rates. Biotic factors such as plant species richness and phylogenetic diversity of the native community may impact invasion. Relatedness of invading species to those in recipient communities has also been shown to influence invasion success. However, the direction of this influence is variable, reflecting Darwin’s Naturalization Conundrum. Abiotic factors such as fire regime and soil factors may impact invasion by selecting against invasive species or indicating suitable habitats for them. We surveyed 17 tallgrass prairie restorations in Illinois, USA, to investigate the effects of biotic and abiotic factors on invasion by non-native plant species at two different scales. We predicted we would find support for Darwin’s Naturalization Hypothesis at the plot (neighborhood) scale with invasion by distantly related species, and find support for the Pre-adaptation Hypothesis at the site scale. We hypothesized that biotic factors would exert more influence at the neighborhood scale, while abiotic factors would be more influential at a coarser site scale. Contrary to our expectations, at the neighborhood scale we found that closely related invasive species are more likely to invade, supporting the Pre-adaptation Hypothesis. We found that native species richness and age of restoration were negatively correlated with invasion. At the site scale, soil organic matter [SOM] concentrations and heterogeneity in SOM were positively associated with the number of invasive species while pH heterogeneity was negatively associated. Restoration practitioners may be able to reduce plant invasions by increasing native species richness, and non-native species most closely related to the resident community should potentially be prioritized as those most likely to be highly invasive.     

Ecological impacts of the invasive grass Sorghum halepense on native tallgrass prairie

Invasive plants frequently have competitive advantages over native species. These advantages have been characterized in systems in which the invading species has already become well established. Surprisingly, invader impacts on native communities currently undergoing invasion are lacking from most ecological studies. In this work we document and quantify shifting patterns in plant community structure in a native ecosystem (remnant tallgrass prairie) undergoing invasion by the invasive exotic Sorghum halepense (Johnsongrass). Further, we use manipulative field and greenhouse studies to quantify impacts of potential allelochemicals contained in whole-plant S. halepense leachates on growth of the dominant native grass, Schizachyrium scoparium (Little Bluestem), and tested the inhibitory effects of the potential soil legacy of S. halepense on the native grass in the greenhouse. Plant diversity indices revealed three distinct plant communities within the remnant prairie: a native community, a densely S. halepense invaded area, and a transitional zone between the two. Dominance of the native grass, determined by relative percent cover, significantly declined with increased S. halepense invasion via rhizomatous growth. Annual global positioning system monitoring of the S. halepense invasion front was used to quantify advancement into native prairie, documented at an average rate of 0.45 m year^?1. In the manipulative field and greenhouse studies, native S. scoparium treated with invasive S. halepense leachate had significantly less biomass and fewer inflorescences than control plants. These findings indicate the prolific clonal growth in conjunction with the plant chemistry of S. halepense play a significant role in displacement of the native grass.     

Soil Properties and Spatial Processes Influence Bacterial Metacommunities within a Grassland Restoration Experiment

Metacommunity theory proposes that a collection of local communities are linked by dispersal and the resulting compositions are a product of both niche‐based (species sorting) and spatial processes. Determining which of these factors is most important in different habitats can provide insight into the regulation of community assembly. To date, the metacommunity organization of heterotrophic soil bacteria is largely unknown. Spatial variation of soil bacterial communities could arise from (1) the resource heterogeneity produced by plant communities through root exudation and/or litter inputs; (2) the heterogeneity of soil environmental properties; and (3) pure spatial processes, including dispersal limitation and stochastic assembly. Understanding the relative importance of these factors for soil bacterial community structure and function could increase our ability to restore soil communities. We utilized an ongoing tallgrass prairie restoration experiment in northeastern Kansas to assess if restoring native plant communities produced changes in bacterial communities 6 years after restoration. We further examined the relative importance of the spatial heterogeneity of plant communities, soil properties, and pure spatial effects for bacterial community structure in the old‐field restoration site. We found that soil bacterial communities were not influenced by plant restoration, but rather, by the local heterogeneity of soil environmental properties (16.9% of bacterial community variation) and pure spatial effects (11.1%). This work also stresses the idea that restoring bacterial communities can take many years to accomplish due to the inherent changes that occur to the soil after cultivation and the time it takes for the re‐establishment of soil quality.     

Legacy Effects of Oil Road Reclamation on Soil Biology and Plant Community Composition

Following the unprecedented oil drilling presently occurring in western North Dakota, thousands of kilometers of oil roads must be reclaimed to an acceptable post‐extraction condition. This study assessed the soil biological and plant communities of nine decommissioned oil roads reclaimed during three periods between 1983 and 2002 in the Little Missouri National Grasslands of western North Dakota. We hypothesized that time‐since‐reclamation would positively affect soil biological and plant communities and, consequently, success of reclamation at older sites. To assess this hypothesis, we measured soil enzyme activity, soil microbial community composition, plant community composition, and soil physical and chemical properties along a gradient extending from road‐center to adjacent native prairie for the nine roads. Time‐since‐reclamation did not affect soil and plant properties measured, indicating that older reclamations are not more similar to native prairie than reclamations occurring more recently. A strong gradient between samples from road‐center and native prairie was identified with univariate and ordination analyses, indicating that soil and plant communities of reclaimed oil roads do not resemble those of the surrounding prairie. Soil organic matter (SOM) was identified as the most significantly affected soil property, being 30% lower on reclaimed roads than prairie. The relationship between SOM, microbial community, and plant community suggests that incorporating additional SOM could hasten reclamation as a result of improving the physical environment for plants and providing a labile carbon and energy source for the soil microbial community which, in turn, will enhance the nutrient and physical conditions for plant growth.     

Floristic and Soil Organic Matter Changes after Five and Thirty-Five Years of Native Tallgrass Prairie Restoration

We studied two tallgrass prairies and adjacent restoration areas in northeast Kansas to analyze (1) the invasion of native tallgrass prairie species from native prairie source populations into replanted areas; (2) the establishment of planted prairie species five and 35 years after being sown; and (3) the effects of native prairie species on soil organic matter. For the majority of dominant species, composition differed statistically between sampled areas even though seed rain was available from the native tallgrass prairie remnants. Plant community differences were statistically different between each native prairie area and all respective restoration sites according to the Multiple Response Permutation Procedure. In addition, species richness was greatly reduced in replanted areas compared to adjacent native prairie remnants. Soil carbon isotope ratios indicated that the planting of warm-season grasses resulted in substantial replacement of old soil organic matter by the newly replanted grasses but that it did not create substantial increases of soil organic matter beyond replacement. The lack of accumulation reflects a nutrient-poor system (nitrogen-poor in particular), and the relative absence of native or introduced nitrogen-fixing plant species on the replanted areas may be a significant factor. It appears that restoration of the original highly diverse vegetation component of the tallgrass prairie ecosystem, even when aided by seeding and an adjacent prairie seed source, will occur on carbon- and nitrogen-depleted soils only over very long periods of time (perhaps centuries), if at all.     

Flower visitor communities are similar on remnant and reconstructed tallgrass prairies despite forb community differences

One common goal of habitat restoration and reconstruction is to reinstate the biodiversity found at intact reference sites. However, few researchers have examined whether these practices reinstate communities of flower‐visiting insects. This is unfortunate, as anthropogenically mediated declines in flower visitors, including bees (the primary pollinators for most terrestrial ecosystems), beetles, flies, and butterflies, have been reported worldwide. Biodiversity declines may be especially severe in North America's tallgrass prairie, a once‐vast grassland that has experienced severe destruction and degradation due to agricultural conversion. As such, we assessed the structure of forb and flower‐visiting insect communities as a whole and two subsets of the flower visitor community—bees and phytophagous beetles—across five tallgrass prairie remnants and five reconstructed prairies (former crop fields) in Kansas from 2013 to 2015. Remnant prairies had significantly higher forb diversity and differed significantly in forb composition, compared to reconstructed prairies. Despite the dissimilarities in forb community structure, there were no differences in flower visitor diversity or abundance between remnants and reconstructed prairies. However, when considered separately, bee communities exhibited significantly greater variability in composition on reconstructed prairies, likely due to the abundance of generalist bee species visiting non‐native legumes at two reconstructed prairies. Our work provides evidence that prairie habitat reconstruction is a valuable tool for reestablishing flower‐visiting insect communities and also emphasizes the considerable role that non‐native species may play in structuring grassland plant–bee interactions.     

Phylogenetically and functionally diverse species mixes beget diverse experimental prairies,whether from seeds or plugs

Phylogenetic and functional diversity are relevant for restoration planning, as they influence important ecosystem functions and services. However, it is unknown whether initial phylogenetic and functional diversity of restorations as planned and planted are maintained over time, that is, the extent to which diversity of the restoration planting is reflected in the diversity of the resulting plant community. Furthermore, in the tallgrass prairie, many restorations are planted from seed. Among-species variation in emergence and establishment affects the transition from seed mixes to realized plant communities in these restorations. We evaluated emergence and early establishment of experimental communities in a biodiversity plot experiment designed to test how phylogenetic and functional diversity influence restoration outcomes. We planted the same experimental communities starting from both seeds and plugs to assess differences in establishment. Our results suggest that phylogenetically and functionally diverse species mixes tend to produce phylogenetically and functionally diverse restored plant communities. After 3 years, experimental communities generally maintained their phylogenetic and functional diversity from seed and plug mixes to established vegetation, despite declines in species richness. While plots planted from seeds had on average 1.3 fewer species than plots planted from plugs, phylogenetic and functional diversity did not significantly differ between the two. Furthermore, most species exhibited no significant differences in percent cover when planted from seeds or plugs. Seeds are generally more cost-effective for restoration than plugs, and our results indicate these two establishment methods achieved similar biodiversity outcomes.     

Plant community responses to resource availability and heterogeneity during restoration

Availability and heterogeneity of resources have a strong influence on plant community structure in undisturbed systems, as well as those recovering from disturbance. Less is known about the role of resource availability and heterogeneity in restored communities, although restoration provides a valuable opportunity to test our understanding of factors that influence plant community assembly. We altered soil nitrogen (N) availability and soil depth during a prairie restoration to determine if the availability and/or heterogeneity of soil resources influenced plant community composition in restored grassland communities. Plant community responses to three levels of N availability (ambient, enriched by fertilization, and reduced by carbon amendment) and two levels of soil depth (deep and shallow) were evaluated. In addition, we evaluated plant community responses to four whole plot heterogeneity treatments created from the six possible combinations of soil N availability and soil depth. The soil depth treatment had little influence on community structure during the first 3 years of restoration. Total diversity and richness declined over time under annual N enrichment, whereas diversity was maintained and richness increased over time in soil with reduced N availability. Non-native species establishment was lowest in reduced-N soil in the initial year, but their presence was negligible in all of the soil N treatments by the second year of restoration. Panicum virgatum, a native perennial C₄ grass, was the dominant species in all soil N treatments by year three, but the magnitude of its dominance was lowest in the reduced-N soil and highest in enriched-N soil. Consequently, the relative cover of P. virgatum was strongly correlated with community dominance and inversely related to diversity. The differential growth response of P. virgatum to soil N availability led to a higher degree of community similarity to native prairie in the reduced-N treatment than in the enriched-N treatment. There were no differences in plant community structure among the four whole plot-level heterogeneity treatments, which all exhibited the same degree of similarity to native prairie. Diversity and community heterogeneity in the whole-plot treatments appeared to be regulated by the dominant species effect on light availability, rather than soil N heterogeneity per se. Our results indicate that a strong differential response of a dominant species to resource availability in a restored community can regulate community structure, diversity, and similarity to the native (or target) community, but the importance of resource heterogeneity in restoring diversity may be dampened in systems where a dominant species can successfully establish across a range of resource availability.     

Temporal and spatial dynamics in a northern tallgrass prairie

It is well established that plants and soil properties are interdependent. What is less known is the degree to which the distribution of plant and soil properties, in both space and time, match one another. We assessed plant community and soil conditions in permanent sample plots in a tallgrass prairie community from 2002 to 2008. Redundancy analysis showed that the dominant grass species were associated with changes in elevation, soil pH, and gravimetric water content, but not nitrogen or phosphate. Indicator species analysis suggested the community is best described by four cluster types, with half of the plots changing community type over the 6-year sampling period. These cluster types were generally spatially aggregated, with semivariograms showing a range from 80 to 346 m. This degree of spatial structuring was observed in the elevation and soil water content data, but not in measures of soil inorganic N or P. This suggests that natural plant communities do not necessarily show a strong correspondence to all soil variables, especially those that vary rapidly in time and space. The dynamic nature of vegetation patch distribution, combined with their size, may contribute to the loss of species from small tallgrass prairie reserves.     

Temporal vegetation dynamics and recolonization mechanisms on different-sized soil disturbances in tallgrass prairie

Assessing the various mechanisms by which plants revegetate disturbances is important for understanding the effects of disturbances on plant population dynamics, plant community structure, community assembly processes, and ecosystem function. We initiated a 2-yr experiment examining temporal vegetation dynamics and mechanisms of recolonization on different-sized soil disturbances created to simulate pocket gopher mounds in North American tallgrass prairie. Treatments were designed to assess potential contributions of the seed rain, soil seed bank, clonal propagation from the edges of a soil mound, and regrowth of buried plants. Small mounds were more rapidly recolonized than large mounds. Vegetative regrowth strategies were the dominant recolonization mechanisms, while the seed rain was considerably less important in maintaining the diversity of forbs and annuals than previously believed. All recolonization mechanisms influenced plant succession, but stem densities and plant mass on soil mounds remained significantly lower than undisturbed controls after two growing seasons. Because natural pocket gopher mounds are indistinguishable from undisturbed areas after two seasons, these results suggest that multiple modes of recruitment concurrently, albeit differentially, contribute to the recolonization of soil disturbances and influence tallgrass prairie plant community structure and successional dynamics.     

Competitive context alters plant-soil feedback in an experimental woodland community

Recent findings on feedback between plants and soil microbial communities have improved our understanding of mechanisms underlying the success and consequences of invasions. However, additional studies to test for feedback in the presence and absence of interspecific competition, which may alter the strength or direction of feedbacks, are needed. We tested for soil microbial feedback in communities of the invasive grass Microstegium vimineum and commonly co-occurring native plant species. To incorporate competitive context, we used a factorial design with three plant treatments (M. vimineum alone, M. vimineum with the native plant community, and the native community without M. vimineum) and two soil inoculum treatments (experimentally invaded and uninvaded soil). When competing with M. vimineum, native communities were 27% more productive in invaded than uninvaded soil. In contrast, soil type did not significantly affect M. vimineum biomass or fecundity. At the community level, these results indicate a net negative soil microbial feedback when native plants and M. vimineum are grown in competitive mixture, but not when they are grown separately. Since positive, not negative, feedback is associated with dominance and invasion, our findings do not support plant–soil feedback as a driver of invasion in this species. Our results do show that the importance of soil feedback can change with competitive context. Such context-dependency implies that soil feedback may change when competitive interactions between natives and invading species shift as invasions progress.     

Mammalian Herbivores Alter the Population Growth and Spatial Establishment of an Early-Establishing Grassland Species

Plant-herbivore interactions influence the establishment context of plant species, as herbivores alter the community context in which individual species establish, and the spatial relationship between individuals and their source population as plants invade. This relationship can be described using an establishment kernel, which takes into account movement through seed dispersal, and subsequent establishment of adults. Mammalian herbivores are hypothesized to influence plant population growth and establishment through a combination of consumption of seeds and seedlings, and movement of seeds. While the movement abilities of plants are well known, we have very few empirical mechanistic tests of how biotic factors like mammalian herbivores influence this spread potential. As herbivores of all sizes are abundant on the landscape, we asked the question, how do mammalian herbivores influence the population growth, spatial establishment, and the community establishment context of an early-recruiting native prairie legume, Chamaecrista fasciculata? We planted C. fasciculata in source populations within a four-acre tallgrass prairie restoration in plots with and without herbivores, and monitored its establishment with respect to distance from the source populations. We found that herbivores decreased population growth, and decreased the mean and range establishment distance. Additionally, C. fasciculata established more often without herbivores, and when surrounded by weedy, annual species. Our results provide insight into how the interactions between plants and herbivores can alter the spatial dynamics of developing plant communities, which is vital for colonization and range spread with fragmentation and climate change. Mammalian herbivores have the potential to both slow rates of establishment, but also determine the types of plant communities that surround invading species. Therefore, it is essential to consider the herbivore community when attempting to restore functioning plant communities.