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BackgroundBelow-ground bud banks have experienced much recent interest due to discoveries that they (1) account for the majority of seasonal population renewal in many communities, (2) are crucial to regeneration following disturbance, and (3) have important consequences for plant population dynamics and plant and ecosystem function across a number of habitats.ScopeThis review presents an overview of the role of bud banks in plant population renewal, examines bud bank life history, summarizes bud bank traits and their potential ecological implications, synthesizes the response of bud banks to disturbance, and highlights gaps to guide future research. The characteristics and life history of buds, including their natality, dormancy, protection and longevity, provide a useful framework for advancing our understanding of bud banks. The fate of buds depends on their age, size, type, location, and biotic and abiotic factors that collectively regulate bud bank dynamics. A bud bank can provide a demographic storage effect stabilizing population dynamics, and also confer resistance to disturbance and invasion. Regeneration capacity following disturbance is determined by interactions among the rates of bud natality, depletion and dormancy (meristem limitation), and the resources available to support the regeneration process. The resulting response of plants and their bud banks to disturbances such as fire, herbivory and anthropogenic sources determines the community’s regenerative capacity.ConclusionsVegetation responses to environmental change may be mediated through changes in bud bank dynamics and phenology. Environmental change that depletes the bud bank or prohibits its formation likely results in a loss of vegetation resilience and plant species diversity. Standardization of bud sampling, examination of bud banks in more ecosystems and their response to environmental variation and disturbance regimes, employment of stage-structured bud bank modelling and evaluation of the cost of bud bank construction and maintenance will benefit this expanding field of research.

PREMISE OF THE STUDY: Tiller recruitment from the belowground bud bank of caespitose grasses infl uences their ability to monopolize local resources and, hence, their genet fi tness. Diff erences in bud production and outgrowth among tiller types within a genet and among species may explain co-occurrence of caespitose grasses. This study aimed to characterize genet bud-bank and tiller production and dynamics in two co-occurring species and compare their vegetative reproductive strategies. METHODS: Bud-bank and tiller dynamics of Hesperostipa comata and Nassella viridula , dominant C 3 caespitose grasses in the northern mixed-grass prairie of North America, were assessed throughout an annual cycle. KEY RESULTS: The two species showed similar strategies, maintaining polycyclic tillers and thus creating mixed-age genet bud banks comprising multiple bud cohorts produced in diff erent years. Vegetative tillers produced the majority of buds, whereas fl owering tillers contributed little to the bud bank. Buds lived for at least 2 yr and were maintained in multiple developmental stages throughout the year. Because bud longevity rarely exceeded tiller longevity, tiller longevity drove turnover within the bud bank. Tiller population dynamics, more than bud production per tiller, determined the diff erential contribution of tiller types to the bud bank. Nassella viridula had higher bud production per tiller, a consistent annual tiller recruitment density, and greater longevity of buds on senesced and fl owering tillers than H. comata . CONCLUSIONS: Co-occurring C 3 caespitose grasses had similar bud-bank and tiller dynamics contributing to genet persistence but diff ered in bud characteristics that could aff ect genet longevity and species coexistence.

Because most shoot recruitment in perennial grasses occurs from belowground axillary buds, bud dynamics determine plant population dynamics and meristem limitation to population growth. Therefore, grassland vegetation responses to environmental change or disturbance may be influenced by interspecific differences in bud banks and the patterns and environmental controls of bud development and demography. We examined bud bank dynamics in Andropogon gerardii and Dichanthelium oligosanthes in tallgrass prairie by enumerating and classifying buds throughout 15 months to determine whether grass buds live for multiple years and accumulate; whether bud natality, dormancy and outgrowth are synchronous or variable; and whether bud bank dynamics differ between these co-occurring species. Andropogon gerardii (a C4 species) maintained a larger dormant bud bank, showed synchrony in bud development and transition to tiller, and its buds lived for multiple years. Thus, multiple previous years’ bud cohorts contributed to recruitment. By contrast, D. oligosanthes (a C3 species) maintained a smaller dormant bud bank, had asynchronous bud development with active buds present year-round, and its buds lived for 1 year. Buds played different roles in the dynamics of each species, allowing A. gerardii to over-winter and recruit new spring tillers and D. oligosanthes to survive and recruit new tillers following summer dormancy. These differences in bud bank age structure, phenology, and dynamics between these species suggest greater demographic buffering and time-lag effects in A. gerardii populations. Interspecific differences in bud bank structure and dynamics may explain and help predict grassland responses to environmental change.

In arid to semi-arid grasslands and savannas, plant growth, population dynamics, and productivity are consistently and strongly limited by soil water and nutrient availability. Adaptive traits of the root systems of grasses in these ecosystems are crucial to their ability to cope with strong water and/or nutrient limitation and the increasing drought stress associated with ecosystem degradation or projected climate change. We studied 18 grass species in semi-arid savanna of the Kalahari region of Botswana to quantify interspecific variation in three important root system traits including root system architecture, rhizosheath thickness and mycorrhizal colonization. Drought- tolerant species and shorter-lived species showed greater rhizosheath thickness and fine root development but lower mycorrhizal colonization compared to later successional climax grasses and those characteristic of wetter sites. In addition, there was a significant positive correlation between root fibrousness index and rhizosheath thickness among species and a weak negative correlation between root fibrousness index and mycorrhizal colonization.These patterns suggest that an extensive fine root system and rhizosheath development may be important complementary traits of grasses coping with drought conditions, the former aiding in the acquisition of water by the grass plant and the latter aiding in water uptake and retention, and reducing water loss in the rhizosphere. Within species, both rhizosheath development and mycorrhizal colonization were significantly greater in a wet year than in a year with below-average precipitation. The observed patterns suggest that the primary benefit of rhizosheath development in African savanna grasses is improved drought tolerance and that it is a plastic trait that can be adjusted annually to changing environmental conditions. The functioning of mycorrhizal symbiosis is likely to be relatively more important in infertile savannas where nutrient limitation is higher relative to water limitation.

Plant responses to herbivory are complex. In grasses, relative growth rate (RGR), seed, and vegetative reproduction, resource allocation, and architecture vary differentially and often nonlinearly with grazing intensity. High grazing tolerance may be achieved through compensatory photosynthesis and leaf growth, or through demographic mechanisms such as activation of a belowground dormant bud bank. This study assessed the relationship between grazing frequency and responses of Schizachyrium scoparium (little bluestem) in a tallgrass prairie, and examined the roles of tiller growth, reproduction, and bud (meristem) populations in its persistence under grazing. Genets were subjected to varying simulated grazing frequencies for a period of 2 years. Strong differential responses were observed among plant traits. RGR, biomass, and flowering showed strong nonlinear reductions in response to increasing clipping frequency, with no evidence of threshold effects. However, meristem density was unaffected, and plants maintained a large bud bank across all clipping treatments. Tiller natality decreased initially, but increased with >4 clippings, suggesting that declines in tiller RGR are partially offset by increasing tiller natality, and that variation in genet size is driven more by demography than by variation in individual tiller growth. Increased grazing frequency also resulted in differential activation of buds at different positions (emerging within vs. outside the subtending leaf sheath), explaining the shift to a more prostrate growth form observed in many caespitose grasses under persistent grazing. Thus, although this grass species lacks the capacity for compensatory foliage re-growth, the maintenance of a large dormant bud bank and the differential activation of buds in different positions contribute to its grazing tolerance and avoidance, respectively, and its long-term persistence in grazed grasslands.

Periodic fire, grazing, and a variable climate are considered the most important drivers of tallgrass prairie ecosystems, having large impacts on the component species and on ecosystem structure and function. We used long-term experiments at Konza Prairie Biological Station to explore the underlying demographic mechanisms responsible for tallgrass prairie responses to two key ecological drivers: fire and grazing. Our data indicate that belowground bud banks (populations of meristems associated with rhizomes or other perennating organs) mediate tallgrass prairie plant response. Fire and grazing altered rates of belowground bud natality, tiller emergence from the bud bank, and both short-term (fire cycle) and long-term (>15 year) changes in bud bank density. Annual burning increased grass bud banks by 25% and decreased forb bud banks by 125% compared to burning every 4 years. Grazing increased the rate of emergence from the grass bud bank resulting in increased grass stem densities while decreasing grass bud banks compared to ungrazed prairie. By contrast, grazing increased both bud and stem density of forbs in annually burned prairie but grazing had no effect on forb bud or stem density in the 4-year burn frequency treatment. Lastly, the size of the reserve grass bud bank is an excellent predictor of long-term ANPP in tallgrass prairie and also of short-term interannual variation in ANPP associated with fire cycles, supporting our hypothesis that ANPP is strongly regulated by belowground demographic processes. Meristem limitation due to management practices such as different fire frequencies or grazing regimes may constrain tallgrass prairie responses to interannual changes in resource availability. An important consequence is that grasslands with a large bud bank may be the most responsive to future climatic change or other global change phenomena such as nutrient enrichment, and may be most resistant to exotic species invasions.

Growth of tallgrass prairie plants, many of which maintain substantial bud banks, can be limited by nitrogen (N), water, and/or light. We hypothesized that tallgrass prairie plants respond to increases in N through demographic effects on the bud bank. We tested the effects of a pulse of N on (1) bud bank demography, (2) plant reproductive allocation, and (3) ramet size. We parameterized matrix models, considering each genet as a population of plant parts. Nitrogen addition signifi cantly impacted bud bank demography in two subdominant species of bunchgrass: Sporobolus heterolepis (a C 4 grass) and Koeleria macrantha (a C 3 grass), but had no effect on the size of individual ramets. Emergence from the bud bank and ramet population growth rates ( λ ) were signifi cantly higher in S. heterolepis genets that received supplemental N. Nitrogen addition also affected the bud demography of K. macrantha , but N addition decreased rather than increased λ . Prospective and retrospective demographic analyses indicated that bud bank dynamics were the most important demographic processes driving plant responses to nutrient availability. Thus, the variation in productivity in these tallgrass prairie species is driven principally by the demography of the bud bank rather than by the physiology and growth of aboveground tillers. Improved understanding of bud bank dynamics may lead to improved predictive models of grassland responses to environmental changes such as altered N deposition and precipitation.

Recruitment, establishment and survivorship of seed- and vegetatively-derived shoots were quantified biweekly in annually burned and infrequently burned tallgrass prairie to investigate the contributions of seed and vegetative reproduction to the maintenance and dynamics of tallgrass prairie plant populations, the demography of seedlings and ramets, and the influence of fire on the demography of grasses and forbs. Clonally produced grass and forb ramets comprised >99%of all established shoots present at the end of the growing season, whereas established seedlings accounted for

Summary 1 A glasshouse 32P-labelling study examined arbuscular mycorrhizal (AM)-mediated transfer of phosphorus between individuals of two tallgrass prairie species, an obligately mycotrophic grass (Sorghastrum nutans Vitm.) and a facultatively mycotrophic forb (Artemisia ludoviciana Nutt.). 2 Regardless of which species served as donor, 32P was transferred between both intra- and interspecific neighbours via AM mycelia. However, nutrient transfer via AM fungi was not uniform between neighbouring species. 3 Conservative estimates indicate that interplant transfer via AM fungi accounted for >50% of the total 32P acquisition by S. nutans, but accounted for only 20% of 32P uptake into A. ludoviciana. 4 While this study did not show conclusively that a common mycelial network acted as a conduit for nutrient transfer, it clearly demonstrated that mycorrhizae facilitated transfer. 5 The results indicate that differential movement of plant resources via AM mycelium may be a mechanism whereby a dominant, highly mycotrophic grass extends competitive advantage over a less mycotrophic, subdominant forb species in grasslands.

Three semi-arid savanna grasses in Botswana (Stipagrostis uniplumis, Eragrostis lehmanniana, and Aristida stipitata) were sampled to quantify their belowground bud banks during the dormant season and to estimate their relative allocation to vegetative and sexual reproduction. Bud banks of these African perennial caespitose grasses were also compared with four perennial caespitose grasses of semi-arid North American grasslands. The three African grasses each maintained approximately two buds per tiller and showed a high percentage (88–99%) of tillers producing seed. Only E. lehmanniana produced new aerial tillers from axillary buds at elevated nodes on the stem as well as from the belowground bud bank. Compared with species of North American grasslands, these African grasses produced fewer belowground buds but showed a much higher percentage of tillers producing seed. These patterns indicate relatively greater belowground meristem limitation, lower allocation to vegetative reproduction (tillering) and higher allocation to seed reproduction in these African grasses, although studies of more species are needed to assess the generality of this pattern. The management of savannas in ways that favour the maintenance of a reserve population of belowground buds may increase the ability of grasses to respond to pulses of resource availability, increase their compensatory growth capacity following grazing or drought, and decrease the invasibility of these plant communities by exotic species, whereas maintaining allocation to sexual reproduction may be important for conserving genetic variation and enhancing their capacity to adapt to environmental change. Resume On a recolte des echantillons de trois herbes de savane semi-aride au Botswana (Stipagrostis uniplumis, Eragrostis lehmanniana et Aristida stipitata) pour pouvoir quantifier les reseaux de bourgeons souterrains pendant la saison dormante et estimer leur evolution future relative vers la reproduction vegetative ou sexuee. Les reseaux de bourgeons de ces herbes africaines cespiteuses et perennes ont aussi ete compares a quatre herbes cespiteuses des prairies semi-arides d'Amerique du Nord. Les herbes africaines avaient environ deux bourgeons par tige et presentaient un fort pourcentage (88–99%) de tiges produisant des semences. Seul E. lehmanniana produisait de nouvelles pousses aeriennes a partir de bourgeons axillaires des nœuds situes plus haut sur la tige ainsi qu’a partir du reseau de bourgeons souterrains. Comparees aux especes des prairies nord-americaines, ces herbes africaines produisent moins de bourgeons souterrains, mais presentent un pourcentage beaucoup plus eleve de tiges produisant des graines. Ces schemas indiquent une limitation relativement plus grande du meristeme souterrain, une reproduction moins vegetative (moins de tiges) et plus orientee vers la production de semences chez ces herbes africaines, bien qu'il faille etudier plus d'especes pour evaluer la generalite de ce pattern. La gestion des savanes de facon a favoriser le maintien d'une population de reserve de bourgeons souterrains pourrait accroitre la capacite des herbes de repondre aux variations de la disponibilite des ressources, augmenter leur capacite de croissance compensatoire en reponse au pâturage ou a la secheresse et diminuer le potentiel d'envahissement de ces communautes vegetales par des especes exotiques, tandis que le maintien de la reproduction sexuee peut etre important pour conserver la variete genetique et augmenter leur capacite de s'adapter aux changements environnementaux.

Summary • In perennial grasslands, the below-ground population of meristems (bud bank) plays a fundamental role in plant population dynamics. Here, we tested the ‘meristem limitation hypothesis’ prediction – that bud banks increase along an increasing precipitation/productivity gradient in North American grasslands – and assessed the seasonal dynamics of bud banks. • We sampled bud and stem populations quarterly at six sites across a 1100 km gradient in central North America. • Bud banks increased with average annual precipitation, which explained 80% of the variability between the sites. In addition, seasonal changes in grass bud banks were surprisingly similar across a 2.5-fold range in precipitation and a 4-fold range of productivity: densities peaked in March, decreased in June and increased slightly in September. • Increasing meristem limitation may constrain vegetation responses to inter-annual changes in resources. An important consequence of this is that biomes with large bud banks may be the most responsive to environmental change. If meristem limitation represents an important constraint on productivity responses to environmental variability, then bud banks must be considered in developing predictive models for grassland responses to environmental change.

Comparisons between how bison and cattle grazing affect the plant community are understood poorly because of confounding differences in how the herbivores are typically managed. This 10-year study compared vegetation changes in Kansas (USA) tallgrass prairie that was burned and grazed season-long at a moderate stocking rate by either bison or cattle. We held management practices constant between the herbivores and equalized grazing pressure by matching animals so that the total body mass in all pastures was similar each year. Trends in species cover and diversity indices in the bison and cattle pastures were compared with ungrazed prairie that also was burned annually. We found that little bluestem (Schizachyrium scoparium) cover decreased over time in bison pastures, and big bluestem (Andropogon gerardii) cover increased over time in cattle pastures. Grazing by either herbivore increased the canopy cover of annual forbs, perennial forbs, and cool-season graminoids, but both annual and perennial forb cove...

Complex relationships occur among plants, mycorrhizal fungi, and herbivores. By altering plant nutrient status, mycorrhizas may alter herbivory or plant tolerance to herbivory via compensatory regrowth. We examined these interactions by assessing grasshopper preference and plant growth and fungal colonization responses to herbivory under mycorrhizal and non-mycorrhizal conditions within tallgrass prairie microcosms. Mycorrhizal symbiosis increased plant regrowth following defoliation, and some strongly mycotrophic plant species showed overcompensation in response to herbivory when they were mycorrhizal. Although grasshoppers spent more time on mycorrhizal plants, herbivory intensity did not differ between mycorrhizal and non-mycorrhizal plants. Aboveground herbivory by grasshoppers significantly increased mycorrhizal fungal colonization of plant roots. Thus mycorrhizas may greatly benefit plants subjected to herbivory by stimulating compensatory growth, and herbivores, in turn, may increase the development of the symbiosis. Our results also indicate strong interspecific differences among tallgrass prairie plant species in their responses to the interaction of aboveground herbivores and mycorrhizal symbionts.

Mycorrhizal symbiosis is a key factor influencing aspects of grassland and savanna structure and functioning including plant growth, competition, population and community dynamics, and responses to fire and herbivory. This study assessed the effects of fire on mycorrhizal symbiosis and root system architecture (RSA) in South African savanna grasses. Eighteen grass species were sampled across contrasting fire frequency treatments in the Kruger National Park experimental burn plots. All eighteen species studied were highly colonized by arbuscular mycorrhizal fungi (AMF). Both mycorrhizal symbiosis and RSA were strongly affected by fire, with an increase in AMF colonization and a decrease in root branching and fine root development with decreasing fire frequency. Greater water limitation in frequently burned savanna may result in greater fine root development, thus reducing plant dependency on AMF for acquisition of soil resources. Reduced mycorrhizal colonization in frequently burned savanna may also be driven by higher phosphorus : nitrogen ratios, or indirect effects related to higher grazing intensities in frequently burned sites.

We examined the effects of fire regime and bison activity on the plant communities of active bison wallows and the surrounding grazing lawns at Konza Prairie Biological Station in northeastern Kansas, USA. In both mid-June and late July the grazed sites had higher species richness and more vegetation cover than the wallow edges regardless of fire regime. The percent cover of most dominant perennial species was significantly higher on grazing lawns than in wallows. Annual species and exotic species had significantly higher cover in wallows than in grazing lawns and in annually burned sites compared to those burned at a 4-y interval. Overall, treatment effects on community structure and individual species abundance were stronger in the June. However, in July there was significantly more bare ground wallows around sites burned at a 4-y interval, suggesting increased wallowing activity at these sites. This finding suggests a strong effect of fire regime on seasonal bison activity, which further indicates the importance of multiple interacting disturbances for generating local- and landscape-level vegetation patterns in tall grass prairie.

Rhizome meristem populations were sampled in tallgrass prairie to quantify the size, grass : forb composition, and temporal and spatial variability of the soil bud bank and to compare fire effects on bud bank and seed bank composition. Soil cores (10.5 cm diameter, 15 cm deep) were collected from replicate annually and infrequently burned tallgrass prairie sites, and intact rhizomes and rhizome buds were censused. Bud bank densities ranged from approximately 600 to 1800 meristems/m(2) among sites and had high spatial and seasonal variability. In annually burned prairie, the total bud bank density was two-fold greater and the grass : forb meristem ratio was more than 30-fold greater than that of infrequently burned prairie. These patterns are opposite those observed in soil seed banks at this site. The rhizome population in annually burned prairie was 34% larger than the established aboveground tiller population. By contrast, the bud bank density in unburned prairie was significantly lower than aboveground stem densities, indicating possible belowground meristem limitation of stem density and net primary production on infrequently burned prairie. The patterns observed in this study suggest that the densities and dynamics of tallgrass prairie plant populations, as well as their response to disturbance (e.g., fire and grazing) and climatic variability, may be mediated principally through effects on the demography of belowground bud populations. Patterns of seed reproduction and seed bank populations have little influence on short-term aboveground population dynamics of tallgrass prairie perennials.

The effects of different intensities of cattle grazing on theaboveground growth, reproduction, and abundances of three palatable forbs werestudied in native tallgrass prairie. Populations of Asterericoides, Ruellia humilis, andAmorpha canescens were sampled at peak flowering duringthe1993–1995 growing seasons in four annually-burned sites varying incattle stocking density [ungrazed, low, moderate, high]. The threeforbs exhibited reduced shoot growth and/or reproduction under moderate toheavy grazing, and in no case did grazing increase any measure of plantperformance. Ruellia showed reduced shoot height andbiomass, percentage of stems flowering, and reproductive biomass in response tograzing. Aster showed decreases in shoot biomass andheightwith grazing. Amorpha showed no change in shoot orreproductive biomass, but a decrease in percent of flowering stems and inreproductive allocation with grazing. Patterns in the percentage of stemsgrazedindicated generally high but variable palatability among these species. Bycontrast, the three species showed inconsistent population response to grazing.Abundance (frequency) of all three species indicated no short-term changebetween years in response to grazing intensity. Responses of these speciesdiffer considerably from those of most other perennial tallgrass prairie forbsthat are unpalatable, unconsumed, and increase in performance (e.g. size,abundance) due to release from competition from the dominant grasses underungulate grazing. The results demonstrate that immediate aboveground growth andreproductive responses of established adults to grazing are not good predictorsof grazer effects on population abundances 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 2yr 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.

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced. The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.

Our objective was to evaluate the effects of soil disturbances created by the plains pocket gopher (Geomys bursarius) on the structure of a tallgrass-prairie plant community. We predicted that soil mounds and burrows would provide sites for the establishment of subordinate plant species, thereby increasing regional plant community richness in this ecosystem that is highly dominated by perennial C4 tallgrasses. Our results, however, revealed that plant species richness and biomass were temporarily decreased or unaffected in areas disturbed by gophers. Moreover, the species found locally on mounds and burrows were a subset of the dominant plants present in the undisturbed plant community and graminoids were more frequent on disturbances than forbs. Our results indicate that perennial graminoids predominate in the rapid recovery of vegetation on pocket gopher mounds and burrows. This preempts the establishment of less frequent forbs and, contrary to our predictions, decreases plant community richness.

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.

Tallgrass prairie sites subjected to 10 y of annual burning, mowing, nitrogen (N) fertilization or phosphorus (P) fertilization and untreated reference sites were studied to examine effects of these management practices on arbuscular mycorrhizal (AM) symbiosis. Spring burning of native prairie field plots significantly reduced AM fungal species diversity, while increasing spore abundance. This increase in total spore number was due to a general increase in most of the 17 fungal species present. In general, the management treatments had larger effects on the richness component of diversity than on the evenness of AM species abundances. Burning and mowing had no significant effects on AM fungal colonization of roots or extraradical mycorrhizal hyphae (EMH) development. However, nitrogen fertilization significantly increased root colonization and EMH development, and P amendment decreased EMH development. There was no significant effect of fertilizer amendment on AM spore abundance, fungal species d...

In grassland ecosystems, symbiotic associations between plants and mycor- rhizal fungi are widespread and have important influences on the life histories, demography, and species interactions of plants, and on belowground ecosystem processes. To assess the consequences of the symbiosis at the plant community level, we conducted a 5-yr field experiment in tallgrass prairie to investigate the influence of arbuscular mycorrhizal fungi on plant species composition, relative abundances, and diversity. Replicate plots in which mycorrhizal fungi were suppressed with benomyl application every two weeks during each growing season, were compared to nontreated mycorrhizal control plots on six watershed units at the Konza Prairie in northeastern Kansas. Benomyl successfully reduced mycor- rhizal colonization to

1 Designs for greenhouse studies of interactions between plants are reviewed and recommendations for their use are provided. 2 Papers published over a 10-year period showed the replacement series design to be the most popular, especially in studying crop–weed interactions. Fifty per cent of the studies involved only two species, although studies testing the interaction between different genotypes of only a few species were also popular. 3 Limitations imposed by the choice of design, the variables measured, and the analysis used on the range of inferences that may be validly drawn from the experiment are frequently not well understood or appropriate for the questions that appear to be addressed. One example is the failure to distinguish the outcome of competition (the long-term outcome of interaction) and the effects of species on each other. 4 Studies in which only final yield is measured are severely limited as to the inferences which may be drawn. Effects due to interspecific interaction during the course of the experiment cannot then be separated from pre-existing differences, and interpretation may be biased towards species whose individuals were initially larger. In addition, measurements at several times are necessary to understand the changing dynamics of species interaction. 5 Simple pair-wise mixtures can assess the effect of treatment factors on the outcome of competition. Replacement series and related diallel designs generally produce results that may be size-biased even when initial interspecific differences are known. Additive designs (including target–neighbour designs), despite confounding density with species proportions, offer considerable scope for addressing mechanistic questions about interspecific interactions. Designs that allow response surface analysis can avoid many of the problems inherent in the other methods, but all need to be adjusted for initial interspecific differences. Designs for multiple species experiments are still largely untested, although several designs have been used. At the level of the individual plant, hexagonal fan designs permit study of the effects of varying the spatial pattern, and the densities and the relative proportions of interacting species, but suffer from lack of independence and lack of randomization.

Symbiotic associations between plants and arbuscular mycorrhizal fungi are ubiquitous and ecologically important in many grasslands. Differences in species responses to mycorrhizal colonization can have a significant influence on plant community structure. The growth responses of 36 species of warm- and cool-season tallgrass prairie grasses and 59 tallgrass prairie forbs to arbuscular mycorrhizal (AM) fungal colonization were assessed in greenhouse studies to examine the extent of interspecific variation in host-plant benefit from the symbiosis and patterns of mycorrhizal dependence among host plant life history (e.g., annual, perennial) and taxonomic (e.g., grass, forb, legume, nonlegume) groups and phenological guilds. There was a strong and significant relationship between phenology of prairie grasses and mycorrhizal responsiveness, however this relationship was less apparent in forbs. Perennial warm-season C(4) grasses and forbs generally benefited significantly from the mycorrhizal symbiosis, whereas biomass production of the cool-season C(3) grasses was not affected. The root systems of the cool-season grasses were also less highly colonized by the AM fungi, as compared to the warm-season grasses or forbs. Unlike the native perennials, annuals were generally not responsive to mycorrhizal colonization and were lower in percentage root colonization than the perennial species. Plant growth responsiveness and AM root colonization were positively correlated for the nonleguminous species, with this relationship being strongest for the cool-season grasses. In contrast, root colonization of prairie legumes showed a significant, but negative, relationship to mycorrhizal growth responsiveness.

Forb populations were sampled on Kansas tallgrass prairie to examine the effects of native (bison) and domestic (cattle) ungulates on plant growth, reproduction, and species abundances. Five locally and regionally abundant native tallgrass prairie perennials, Baptisia bracteata, Oenothera speciosa, Vernonia baldwinii, Solidago missouriensis, and Salvia azurea, were selected for study. Replicate watershed-level treatments included three grazing regimes (ungrazed, grazed by cattle, and grazed by bison), and two spring fire frequencies (annually burned and burned at 4-yr intervals). The results show that forb responses to ungulates in tallgrass prairie are complex and vary significantly among plant species, ungulate species, fire regimes, and plant life history stages. Some forbs (e.g., B. bracteata, O. speciosa, and V. baldwinii) increased in growth and reproduction in grazed sites, indicating competitive release in response to selective grazing of the dominant warm-season matrix grasses. Forbs that reduced performance in grazed sites are likely negatively affected by disturbances generated by ungulate nongrazing activities, because none of the forbs studied were directly consumed by bison or cattle. Large grazers had no detectable effect on the frequency of plant damage by other herbivores or pathogens. Significant effects of grazers on patterns of flowering and seed production were not congruent with their effects on population densities, indicating that variation in sexual reproduction plays a minor role in regulating local population abundances. Furthermore, the native and domestic ungulates differ significantly in their effects on forb growth and reproduction.

Experimental microcosms (40 X 52 X 32 cm) containing an assemblage of eight tallgrass prairie grass and forb species in native prairie soil were maintained under mycorrhizal (untreated control) or mycorrhizal-suppressed (fungicide-treated) conditions to examine plant growth, demographic, and community responses to mycorrhizal symbiosis. The fungicide benomyl successfully reduced mycorrhizal root colonization in the fungicide-treated microcosms to only 6.4% (an 83% reduction relative to mycorrhizal controls). Suppression of mycorrhizas resulted in a 31% reduction in total net aboveground plant production and changes in the relative production of C4 and C3 plants. The C4 tallgrasses Andropogon gerardi and Sorghastrum nutans produced less plant biomass in the fungicide-treated microcosms, and had a greater ratio of reproductive to vegetative biomass. Cool-season C3 grasses, Koeleria pyramidata and Poa pratensis accumulated more biomass and were a significantly greater proportion of total community biomass in mycorrhizal-suppressed microcosms. Forbs showed variable responses to mycorrhizal suppression. The two legumes Amorpha canescens and Dalea purpurea had significantly lower survivorship in the fungicide-treated microcosms, relative to the controls. The results confirm the high mycorrhizal dependency and growth responsiveness of dominant prairie grasses, and indicate that differential growth and demographic responses to mycorrhizal colonization among species may significantly affect plant productivity and species relative abundances in tallgrass prairie.

While the effects of drought and grazing are often studied separately, these disturbances co-occur in grasslands worldwide and interactively influence population, community, and ecosystem processes. The effects of drought and grazing on the belowground bud bank may dictate the trajectory of community recovery because new shoots arise from belowground buds after disturbance in perennial grasslands. We therefore investigated the separate and interactive effects of severe drought and grazing on the belowground bud bank and aboveground vegetation in the tallgrass prairie of northeast Kansas, USA. Contrary to our expectations, we observed changes in community structure and declines in species richness both above and below ground in response to drought and grazing. We also hypothesized that drought would reduce bud bank density of all taxonomic groups, but found that grass bud and shoot densities remained constant across all drought and grazing treatment combinations. While sedge and forb bud and shoot densities were reduced by drought, only sedge bud density declined to a greater extent when grazed under drought conditions. Live rhizome biomass did not vary by treatment and was highly correlated with bud bank density, suggesting that bud demography is tightly linked to the production and senescence of rhizomes. Despite the effects of drought and grazing on aboveground net primary productivity and community structure, our work suggests that grasses stabilize tallgrass prairie plant communities because their rhizomes and associated buds persist through co-occurring disturbances.

We examined plant tolerance of gall—insect attack and gall—insect performance in rosinweed (Silphium integrifolium, Asteraceae) and its apical meristem galler Antistrophus silphii (Hymenoptera: Cynipidae). Gall densities were varied in field rosinweed populations, while gall densities, water, and nutrients were varied for rosinweed in an experimental garden. Field plants grew under prevailing resource and competitive conditions, but garden plants grew free from competition, so gall—insect impacts, rosinweed regrowth, and gall—insect performance were observed under widely different growing conditions. Seasonal measures of rosinweed growth and leaf physiology, and end—of—season measures of biomass, reproduction, gall—wasp emergence, growth, sex ratios, and parasitism were made for both experiments. Rosinweed poorly tolerated Antistrophus gall damage in the field. Galls reduced plant height, leaf area, and inflorescence production. Rosinweed diverted biomass to stems, but produced no regrowth from axillary meristems. In the garden, rosinweed was much more tolerant of Antistrophus gall damage. Galls initially reduced plant height and leaf area, but axillary meristems grew profusely after gall formation, producing nearly all galled plant inflorescences and more than replacing leaf area initially lost to gall formation. Water— and nutrient—supplemented rosinweed were most tolerant of gall damage, experiencing little loss of total biomass or reproductive output. Field rosinweed failed to mount a tolerance—enhancing regrowth response because galls, resource availability, and competition combined to constrain axillary meristem growth. Gall—wasp performance was largely independent of rosinweed tolerance. Emergence, growth, sex ratios, and parasitism were comparable in field and garden, and only slightly affected by resource availability. Gall—insect performance may be buffered from environmental variation, disconnecting plant and herbivore population dynamics. Rosinweed's poor tolerance of gall damage may typify forb responses to herbivory in highly competitive grassland plant communities.

Interplant nutrient transfer may be an important ecological process in grasslands, and may significantly influence plant neighborhood interactions. We investigated the potential for phosphorus transfer between the dominant grass Andropogon gerardii and several neighboring plant species in tallgrass prairie via a field 32P04 labelling experiment. The mean amount of 32p received from donor shoots differed significantly among neighboring species and decreased with increasing distance from the donor. In general, forbs and cool-season C3 grasses received more labelled 32P than warm-season C4 grasses. Phosphorus transfer occurred over distances up to 0.5 m. The effects of species and distance on movement of phosphorus changed with increasing time after labelling. The relative mass of receiver and donor shoots did not affect amounts of 32p transfer. A benomyl fungicide treatment, applied to suppress mycorrhizal activity, likely did not affect existing vegetative hyphae and did not affect the amount of 32P transferred. These studies demonstrate that: (1) phosphorus is transferred among neighboring species in tallgrass prairie plant communities, (2) phosphorus may be transferred over significantly greater distances than reported in other grasslands, and (3) there is differential transfer among co-occurring species. Hypothesized mechanisms accounting for these patterns in tallgrass prairie include mycorrhizal hyphal interconnections and/or extensive and differential root and rhizosphere overlap among neighboring species.

 Premise of the Study: Perennial grasses maintain aboveground tiller populations through vegetative reproduction via belowground buds and sexual reproduction via seed. The maintenance of a bud bank has important demographic consequences for perennial grasses. A tradeoff between these reproductive modes would be expected for a plant with limited resource availability. However, the ontogeny of the tiller could affect its ability to allocate between these two modes of reproduction.  Methods: Vegetative bud production and dynamics and tiller production were examined biweekly through an annual cycle on vegetative and fl owering tillers of Andropogon gerardii .  Key Results: Andropogon gerardii maintains a large reserve of dormant buds. Although vegetative and fl owering tillers had similar bud phenology, fl owering tillers produced larger numbers of buds of larger size, and transitioned a larger proportion of their buds to tiller, than did vegetative tillers. Therefore, a negative consequence of sexual reproduction on vegetative reproduction was not evident at the tiller level. A size threshold for fl oral induction likely exists that results in fltillers having more buds per tiller than vegetative tillers. The increased bud outgrowth of fltillers could be a result of their larger bud size or weaker apical dominance as compared to vegetative tillers.  Conclusions: Plant development can place signifi cant constraints on tradeoffs between the reproductive modes in perennial grasses and could affect their plasticity in plant reproductive allocation. Differences in developmental phenology and bud production between fl owering and vegetative tillers may infl uence grass responses to environmental changes such as altered precipitation regimes or resource availability.

Clones of the perennial grass Panicum virgatum were studied on the Konza Prairie in northeast Kansas to determine the effects of neighbors, nutrient availability, and physiological integration on ramet population dynamics and clonal growth and architecture. Opposite halves of established clones in the field were subjected differentially to treatments including neighbor removal and nitrogen addition, with intact or severed rhizome connections between halves. Neighborhood competition strongly influenced clone architecture and expansion rates. Removal of neighbors resulted in a >95% increase in radial clone expansion, intraclonal ramet densities, ramet population growth rates, ramet biomass, and percent of stems flowering, averaged over a 4-year period relative to halves or clones with intact competitors. Plant responses suggest that effects of interclonal neighbors are mediated through alteration of the light environment in the clone canopy and water availability. Addition of nitrogen did not affect lateral spread or clone structure, but resulted in significant increases in ramet size, flowering, and seed production. ANOVA revealed no significant effect of rhizome severing or treatment x severing interactions, suggesting that the size of the integrated physiological unit is much smaller than clone size and/or that physiological integration had no effect on clone responses to environmental heterogeneity at the scale of the diameter of established clones.

Insect herbivory can have important effects on plant life histories and architecture. We quantified the impact that a cynipid gall wasp, Antistrophus silphii, had on growth, reproduction, and biomass allocation patterns of Silphium integrifolium growing in the tallgrass prairie of northeastern Kansas. Experimentally galled individual Silphium shoots (ramets) had reduced shoot growth, leaf and flower head production, and delayed flowering compared to gall-free control shoots. Gall formation completely halted normal apical growth in 65% of the shoots. Galling did not affect individual flower head weight, the numbers of achenes per flower head or achene weight. Silphium plants (genets) with a high proportion of galled shoots had lower total biomass, a lower proportion of total biomass allocated to flower heads, higher allocation to leaves, but no change in allocation to stems or rhizome. High gall densities reduced the number of flower heads per plant and shortened the time between flower head initiation and maturity. An adaptive interpretation of these results would be that the survivorship and future performance of galled Silphium may be promoted by maintaining allocation to rhizome. However, reduced shoot growth and delayed reproduction in galled Silphium may weaken its competitive ability and reduce pollination success, so that any adaptive advantage to Silphium's allocation responses to galls may be outweighed by disadvantages from its growth and flowering phenology responses. We conclude that a more parsimonious interpretation of these results is that gall-induced allocation changes are due to architectural constraints placed by galls on meristem activity, rather than to any adaptive response on the part of the plant.

Ecosystem responses to physical or chemical stress may vary from changes in single organisms to alteration of the structure and function of the ecosystem. These responses to stress cannol be predicted exactly. Ecosystems repeatedly exposed to physical and/or chemical stress can be used to study the separate and combined environmental effects of stress. Such studies also allow the development of procedures to select test systems for the analysis of stress in ecosystems. A preliminary field survey of six military training sites at Fort Riley, Kansas, USA, was conducted to identify and verify ecological test systems for evaluating ecosystem responses to physical and/or chemical stress. Comparisons of these data with data collected concurrently from Konza Prairie Research Natural Area reference sites showed that soil microarthropods, some species of macroarthropods, small mammals, and native earthworm species were negatively affected by stress. In contrast, plant species diversity, plant foliage biomass, soil mycorrhizae, and many soil characteristics were within the boundaries of nominal variations observed on “pristine” Konza Prairie. Introduced European earthworms appeared to be positively affected by training activities. This study provided a test of systematic procedures to support impact analysis, ecological toxicology, and ecosystem risk assessments.

A 6-year study was design ed to measure the influences of stocking rate and grazing system on performance of cow-calf pairs grazing tallgrass prairie. This paper summarizes the initial 3 years. Late-season rest-rotation was compared to continuous grazing over low, moderate, and high stocking rates. No differences (P>.10) were observed in body weight of cows or calves as a result of grazing system or stocking rate. However, calf weaning weight tended (P=.20) to be greater with continuous grazing than with late season rest-rotation. Cow body condition score was unaffected (P>.10) by stocking rate or grazing system. Conception rates were also similar between stocking rates and grazing systems. This preliminary information suggests that application of a late-season rest-rotation grazing system will support cowcalf performance comparable t o that with a continuous system at a similar stocking rate.

In tallgrass prairie plant communities, new shoots are recruited from belowground bud banks, often in response to disturbance. We explored the contribution of belowground bud banks to grassland stability when perturbed by severe drought. We sought to quantify changes in bud bank density and demography, assess the contribution of the bud bank to aboveground net primary productivity, and compare shifts in above- and belowground plant community structure in response to drought. We experimentally reduced precipitation 76% from ambient levels for two years, and compared responses of drought plots to ambient and irrigated controls during the two years of drought and two years of recovery. We measured belowground bud bank density and aboveground shoot density, canopy cover, and aboveground net primary productivity. While aboveground net primary productivity and C4 grass flowering shoot density declined during drought, bud bank density was insensitive to drought. Rapid resilience of the aboveground plant community following cessation of drought appears to be mediated by the resistance of bud banks to drought. Shoot density, but not bud bank density, was positively correlated with total aboveground net primary productivity. Furthermore, proportional reductions in aboveground net primary productivity increased as shoot density increased. Plant community structure shifted both above- and belowground in response to precipitation treatments. Our results suggest that the response of perennial grasslands to drought may be mediated by the stable belowground bud bank, with shifts in abundance of individual species maintaining overall bud bank density and ecosystem function.

The effects of mycorrhizal symbiosis on seedling emergence, flowering and densities of several grasses and forbs were assessed in native tallgrass prairie and in sown garden populations at the Konza Prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed with the fungicide benomyl. Flowering and stem densities of the cool-season grass, Dichanthelium oligosanthes, sedges (Carex spp.), and the forb Aster ericoides were higher in non-mycorrhizal (benomyl-treated) than in mycorrhizal plots and the magnitude of these differences was significantly affected by burning. Mycorrhizae significantly enhanced flowering of the warmseason grasses Andropogon gerardii and Sorghastrum nutans in burned prairie, but not in unburned sites. These patterns suggest that mycorrhizal effects on the dynamics of cool-season graminoid and forb populations are likely to be mediated indirectly through effects of the symbiosis on the competitive dominance of their neighbors. Seedling emergence rates of the cool-season C3 grasses Elymus canadensis and Koeleria cristata were significantly reduced in the benomyl-treated plots, whereas benomyl treatment had no significant effect on seedling emergence of the warm-season C4 grasses A. gerardii and Panicum virgatum. The forbs showed variable responses. Seedling emergence of Liatris aspera was greater under mycorrhizal conditions, but that of Dalea purpurea was unaffected by mycorrhizal treatment. These results show that effects of mycorrhizal symbiosis on the population dynamics of co-occurring prairie plants vary significantly both among species and among different life history stages within species. The results also indicate that mycorrhizas and fire interact to influence competitive interactions and demographic patterns of tallgrass prairie plant populations.

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (Ψ). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem Ψ than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher Ψ, g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and Ψ, but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO2 concentration relationships were consistent with the explanation that increased Ψ allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low-Ψ limitations on non-stomatal components of A. Our working hypothesis is that galls increase Ψ and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased Ψ and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.

Responses to clipping and bison grazing in different environmental contexts were examined in two perennial grass species, Andropogon gerardii and Panicum virgatum, on the Konza Prairie in northeastern Kansas. Grazed tillers had lower relative growth rates (RGR) than clipped tillers following defoliation but this difference was transient and final biomass was not affected by mode of defoliation. Grazed tillers of both species had higher RGR throughout the season than ungrazed tillers, resulting in exact compensation for tissue lost to defoliation. However, A. gerardii tillers which had been grazed repeatedly the previous year (1988) had reduced relative growth rates, tiller biomass and tiller survival in 1989. This suggests that the short-term increase in aboveground relative growth rates after defoliation had a cost to future plant growth and tiller survival. In general, the two species had similar responses to defoliation but their responses were altered differentially by fire. The increase in RGR following defoliation of A. gerardii was relatively greater on unburned than burned prairie, and was influenced by topographic position. P. virgatum responses to defoliation were similar in burned and unburned prairie. Thus grazing, fire, and topographical position all interact to influence tiller growth dynamics and these two species respond differently to the fire and grazing interaction. In addition, fire may interact with grazing pattern to influence a plants' grazing history and thus its long-term performance.

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 nearest-point 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 × grazing system interactions, indicating that ani...

Complex interactions among mycorrhizal fungi, plants, and herbivores occur in grasslands. Grazing of aboveground vegetation may influence plants directly and indirectly through the alteration of my...

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Grazed and ungrazed sites subjected to different fire frequencies were sampled on the Konza Prairie 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 richness, equitability, and spatial heterogeneity) in sites of contrasting fire frequency. Cover and frequency of cool-season graminoids (e.g. Poa pratensis L., Agropyron smithii Rydb., Carex spp.) and some fortes (e.g. Aster ericoides [A. Gray] Howell, and Oxalis stricta L.) were consistently higher in sites grazed by bison than in ungrazed exclosures, whereas the dominant warm-season grasses (Andropogon gerardii Vitman, Sorghastrum nutans [L.] Nash, Panicum virgatum L., Schizachyrium scoparium [Michx.] Nash) and other forbs (e.g. Solidago missouriensis Nutt.) decreased in 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 in 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 in 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 related.

The perennial bunchgrass little bluestem (Schizachyrium scoparium [Michx.] Nash) was examined in a 5-yr study on tallgrass prairie to determine how fire influences its use by bison and its responses to grazing. On unburned prairie, bison grazed only 5% of the available little bluestem, selecting it only 30% as frequently as big bluestem, the dominant co-occurring species. On burned prairie, grazing frequency of little bluestem was over 3-fold greater and equal to that of its dominant neighbor. Grazing frequency of little bluestem was affected by plant size (basal area). On burned sites, plants of intermediate size classes were least abundant ( 50%). Small plants were most abundant but were grazed least frequently. Density, tiller numbers, and basal area of little bluestem were significantly greater in annually burned compared to infrequently burned sites but were decreased by > 50% in grazed compared to ungrazed sites. Grazing shifted the population size distribution toward higher frequencies of smaller individuals ( 200 cm2 basal area) individuals. In unburned prairie, little bluestem accumulates a persistent clump of standing dead tillers that appear to serve as a physical deterrent to grazing. Although burning enhances its growth, it also removes its canopy of dead tillers exposing the plant to grazers. The shift in population structure toward a high frequency of smaller (and perhaps less drought- or grazing-tolerant) individuals may contribute to the decline of little bluestem populations under persistent grazing. Thus, plant growth form, population size structure, and fire interact to influence bison grazing patterns and responses of little bluestem to grazing on tallgrass prairie.

1) A strongly obligately mycorrhiza-dependent grass, Andropogon gerardii, and a less dependent species, Elymus canadensis, were grown in intra- and interspecific combination in a target-neighbour experiment with and without mycorrhizal fungi to examine their influence on competition. 2) Mycorrhizal fungi significantly influenced the competitive effects and responses of both plant species. Strong competitive effects of Andropogon disappeared in the absence of mycorrhizas indicating that its competitive dominance in tallgrass prairie is highly dependent upon its mycorrhizal associations. The influence of mycorrhizal fungi on Andropogon responses to neighbours decreased with increasing neighbour density indicating reduced host plant benefit from mycorrhizas under crowded conditions

Fire and native large herbivore grazing were two important influences on the structure and function of North American grasslands. In 1988 and 1989 the influence of fire regime on grazing patterns of North American bison (Bison bison) was studied on the Konza Prairie in northeastern Kansas. Bison grazing was spatially and temporally nonrandom and was influenced by fire regime and local plant community composition. During the growing season, bison were observed up to 3 x more frequently than expected on watersheds burned in the spring. Summer grazing was concentrated in large watershed areas (79-119 ha) dominated by warm-season, perennial, C4 grasses. During the autumn and winter, bison grazed both burned and unburned watersheds more uniformly but grazed most intensively in areas with large stands of cool-season, C3 grasses. On a smaller spatial scale (5-10 m2), bison selected patches during the growing season with low forb cover dominated by the perennial C4 grass, Andropogon gerardii. Grazed patches were larger on frequently burned than on infrequently burned watersheds. The importance of fire history in determining patterns of bison grazing over the landscape indicates that interactions between bison grazing and fire regime may be important to the composition and spatial heterogeneity of tallgrass prairie vegetation.