Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

EFFECTS OF FIRE IN TALLGRASS PRAIRIE ON GROWTH AND REPRODUCTION OF PRAIRIE CONEFLOWER (RATIBIDA COLUMNIFERA: ASTERACEAE)

Effects of fire on growth and reproduction of the perennial forb Ratibida columnifera were studied on the Konza Prairie Research Natural Area in northeastern Kansas, USA. Populations were sampled in seven different tallgrass prairie watersheds that varied in fire frequency and in the number of years elapsed since the last fire. Plants from sites not burned for many years were 2.6 times larger and produced 50% more stems than plants from recently burned sites. Number of seeds per plant was also higher in long-unburned sites due to greater numbers of flower heads per plant and greater numbers of achenes produced per head. Reproductive effort (ratio of inflorescence biomass to total vegetative biomass) was 33% lower in annually burned prairie than in any of the other sites. Significant differences in the relationships of inflorescence biomass to vegetative plant biomass in burned vs. unburned sites indicated that burning causes direct changes in plant reproductive effort independent from its effects on plant size. There was no clear relationship between patterns of seed production among sites and patterns of R. columnifera abundance. Ratibida columnifera responses to fire are most likely a result of changes in the relative competitive abilities of forbs and the dominant perennial grasses due to post-fire changes in abiotic conditions rather than a result of direct effects of fire on the fate of buds and subsequent vegetative and floral development.     

Effects of bison and cattle on growth, reproduction, and abundances of five tallgrass prairie forbs

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.     

The Role of Seed and Vegetative Reproduction in Plant Recruitment and Demography in Tallgrass Prairie

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 <1%,emphasizing the rarity of successful seedling establishment and the importance of vegetative reproduction in driving the annual regeneration and dynamics of aboveground plant populations in tallgrass prairie. Most recruitment from vegetative reproduction occurred early in the growing season and was higher in annually burned than infrequently burned sites, although low levels of new stem recruitment occurred continuously throughout the growing season. Peak recruitment on annually burned prairie coincided with peak recruitment of the dominant C₄ grasses Andropogon gerardii and Sorghastrum nutans prior to prescribed spring fire, with a second peak in recruitment occurring following fire. On infrequently burned prairie, grass and forb recruitment was highest in early April and declined steadily through May. The naturalized C₃ grass, Poa pratensis, was responsible for most of the early recruitment on unburned sites, whereas A. gerardii contributed most to recruitment later in May. Infrequently burned prairie was dominated by these two grasses and contained a larger forb component than annually burned prairie. The principal demographic effect of fire was on ramet natality rather than mortality. Fire regime, plant functional group, or timing of cohort emergence before or after fire did not affect ramet survivorship. C₄ grass shoots that emerged early and were damaged by fire showed similar survivorship patterns to tillers that emerged after fire. Differences in species composition between annually burned and infrequently burned prairie are driven by fire effects on vegetative reproduction and appear to be related principally to the effect of fire and detritus accumulation on the development of belowground vegetative meristems of C₄ grasses and their emergence dynamics.     

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

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.     

The effects of fire frequency and grazing on tallgrass prairie productivity and plant composition are mediated through bud bank demography

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.     

Is leaf-level photosynthesis related to plant success in a highly productive grassland?

We addressed the question: “Are short-term, leaf-level measurements of photosynthesis correlated with long-term patterns of plant success?” in a productive grassland where interspecific competitive interactions are important. To answer this question, seasonal patterns of leaf-level photosynthesis were measured in 27 tallgrass prairie species growing in sites that differed in species composition and productivity due to differences in fire history. Our specific goals were to assess the relationship between gas exchange under field conditions and success (defined as aerial plant cover) for a wide range of species, as well as for these species grouped as dominant and sub-dominant grasses, forbs, and woody plants. Because fire increases productivity and dominance by grasses in this system, we hypothesized that any relationship between photosynthesis and success would be strongest in annually burned sites. We also predicted that regardless of fire history, the dominant species (primarily C₄ grasses) would have higher photosynthetic rates than the less successful species (primarily C₃ grasses, forbs and woody plants). Because forbs and woody species are less abundant in annually burned sites, we expected that these species would have lower photosynthetic rates in annually burned than in infrequently burned sites. As expected, the dominant C₄?grasses had the highest cover on all sites, relative to?other growth forms, and they had the highest maximum and seasonally averaged photosynthetic rates (17.6 ± 0.42 μmol m^?2 s^?1). Woody species had the lowest average cover as well as the lowest average photosynthetic rates, with subdominant grasses and forbs intermediate in both cover and photosynthesis. Also as predicted, the highest overall photosynthetic rates were found on the most productive annually burned site. Perhaps most importantly, a positive relationship was found between leaf-level photosynthesis and cover for a core group of species when data were combined across all sites. These data support the hypothesis that higher instantaneous rates of leaf-level photosynthesis are indicative of long-term plant success in this grassland. However, in contrast to our predictions, the subdominant grasses, forbs and woody species on the annually burned site had higher photosynthetic rates than in the less frequently burned sites, even though their average cover was lower on annually burned sites, and hence they were less successful. The direct negative effect of fire on plant cover and species-specific differences in the availability of resources may explain why photosynthesis was high but cover was low in some growth forms in annually burned sites.     

Plant responses to selective grazing by bison: interactions between light,herbivory and water stress

Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10°C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.     

Effects of mycorrhizae on plant growth and dynamics in experimental tall grass prairie microcosms

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.     

Belowground bud banks and meristem limitation in tallgrass prairieplant populations

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.     

Plant demographic responses to mycorrhizal symbiosis in tallgrass prairie

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.     

Interannual variability in primary production in tallgrass prairie: climate,soil moisture,topographic position,and fire as determinants of aboveground biomass

From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m to 756 g/m. Across a variety of sites, NPP was significantly related to precipitation (r = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unbumed sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unbumed sites. The lack of significant correlation between soil moisture with NPP on unbumed sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = -0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.     

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.     

Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie

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.     

Effects of Frequent Mowing on Survival and Persistence of Forbs Seeded into a Species-Poor Grassland

Many early attempts at tallgrass prairie reconstruction failed to achieve the high species diversity of remnant prairies, and instead consist primarily of C₄ grasses. We hypothesized that frequent mowing of established prairie grasses could create sufficient gaps in the aboveground and belowground environment to allow for the establishment of native forbs from seed. We studied forb seedling establishment in a 25‐year‐old prairie planting in northern Iowa that was dominated by native warm‐season grasses. In winter 1999, 23 species of native forbs were broadcast into the recently burned sod at a rate of 350 viable seeds/m². Treatment plots were mowed weekly for either one or two growing seasons, and control plots were unmowed. Mowed plots had greater light availability than controls, especially when warm‐season grasses began to flower. Overwinter seedling mortality was 3% in mowed treatments compared to 29% in the controls. Forbs in mowed plots had significantly greater root and shoot mass than those in control plots in the first and second growing seasons but were not significantly more abundant. By the fourth growing season, however, forbs were twice as abundant in the mowed treatments. No lasting negative impacts of frequent mowing on the grass population were observed. Mowing a second year influenced species composition but did not change total seedling establishment. Experimental evidence is consistent with the idea that mowing reduced competition for light from large established grasses, allowing forb seedlings the opportunity to reach sufficient size to establish, survive, and flower in the second and subsequent years.     

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

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.     

Invertebrate,not small vertebrate,herbivory interacts with nutrient availability to impact tallgrass prairie community composition and forb biomass

The effects of herbivores and their interactions with nutrient availability on primary production and plant community composition in grassland systems is expected to vary with herbivore type. We examined the effects of invertebrate and small vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass in a tallgrass prairie ecosystem. The abundance of forbs relative to grasses increased with invertebrate herbivore removals. This increase in forb abundance led to a shift in community composition, where invertebrate removals resulted in greater plant species evenness as well as a divergence in composition among plots. In contrast, vertebrate herbivore removals did not affect plant community composition or aboveground biomass. Nutrient additions alone resulted in a decrease in plant species richness and an increase in the abundance of the dominant grass, but the dominant grass species did not greatly increase in abundance when nutrient additions were combined with invertebrate removals. Rather, several subdominant forbs came to dominate the plant community. Additionally, the combined nutrient addition and invertebrate herbivore removal treatment increased forb biomass, suggesting that invertebrate herbivores suppress the responses of forb species to chronic nutrient additions. Overall, the release of forbs from invertebrate herbivore pressure may result in large shifts in species composition, with consequences for aboveground biomass and forage quality due to altered grass:forb ratios in grassland systems.     

Productivity and resistance to weed invasion in four prairie biomass feedstocks with different diversity

High‐diversity mixtures of native tallgrass prairie vegetation should be effective biomass feedstocks because of their high productivity and low input requirements. These diverse mixtures should also enhance several of the ecosystem services provided by the traditional monoculture feedstocks used for bioenergy. In this study, we compared biomass production, year‐to‐year variation in biomass production, and resistance to weed invasion in four prairie biomass feedstocks with different diversity: one species – a switchgrass monoculture; five species – a mix of C₄ grasses; 16 species – a mix of grasses, forbs, and legumes; and 32 species – a mix of grasses, forbs, legumes, and sedges. Each diversity treatment was replicated four times on three soil types for a total of 48 research plots (0.33–0.56 ha each). We measured biomass production by harvesting all plant material to ground level in ten randomly selected quadrats per plot. Weed biomass was measured as a subset of total biomass. We replicated this design over a five‐year period (2010–2014). Across soil types, the one‐, 16‐, and 32‐species treatments produced the same amount of biomass, but the one‐species treatment produced significantly more biomass than the five‐species treatment. The rank order of our four diversity treatments differed between soil types suggesting that soil type influences treatment productivity. Year‐to‐year variation in biomass production did not differ between diversity treatments. Weed biomass was higher in the one‐species treatment than the five‐, 16‐, and 32‐species treatments. The high productivity and low susceptibility to weed invasion of our 16‐ and 32‐species treatments supports the hypothesis that high‐diversity prairie mixtures would be effective biomass feedstocks in the Midwestern United States. The influence of soil type on relative feedstock performance suggests that seed mixes used for biomass should be specifically tailored to site characteristics for maximum productivity and stand success.     

Frequency and Extent of Water Limitation to Primary Production in a Mesic Temperate Grassland

The frequency and extent of water limitation to aboveground net primary production (ANPP) in a mesic grassland in NE Kansas (Konza Prairie, USA) was assessed with an 8-year irrigation experiment. Since 1991, transects spanning upland and lowland sites in annually burned, ungrazed tallgrass prairie were provided with supplemental water to satisfy evapotranspirational demands. This protocol minimized water limitations during the growing season, as well as interannual variability in water stress. Irrigation of this mesic grassland increased ANPP in 6 of 8 years by an average of 26% when compared to control transects. Although interannual variation in ANPP was greater in uplands than lowlands at nominal levels of precipitation, reducing interannual variability in water availability via irrigation eliminated topographic differences; the irrigation protocol also reduced interannual variability in ANPP by as much as 40%. The addition of supplemental water enabled us to extend the relationship between annual precipitation and ANPP in grasslands to precipitation levels (average, 1153 mm; maximum, 1346 mm) similar to those experienced by more mesic grasslands that today exist only as remnants several hundred kilometers east of Kansas. This relationship was linear (r ²= 0.81), with maximum ANPP (738 g/m²) similar to values reported for sites in Illinois and Wisconsin. After 8 years of irrigation, production of the C₃ forb component was twice that in control sites. These results indicate that water limitations in grasslands at the western edge of the presettlement extent of tallgrass prairie affect ANPP in most years and that this high frequency of water limitation may lead to greater dominance of the C₄ grasses than is seen in more eastern grassland sites. Received 18 January 2000; accepted 19 July 2000.     

Septate endophyte colonization and host responses of grasses and forbs native to a tallgrass prairie

Native tallgrass prairies support distinct dark septate endophyte (DSE) communities exemplified by Periconia macrospinosa and Microdochium sp. that were recently identified as common root symbionts in this system. Since these DSE fungi were repeatedly isolated from grasses and forbs, we aimed to test their abilities to colonize different hosts. One Microdochium and three Periconia strains were screened for colonization and growth responses using five native grasses and six forbs in an in vitro system. Previously published data for an additional grass (Andropogon gerardii) were included and reanalyzed. Presence of indicative inter- and intracellular structures (melanized hyphae, microsclerotia, and chlamydospores) demonstrated that all plant species were colonized by the DSE isolates albeit to varying degrees. Microscopic observations suggested that, compared to forbs, grasses were colonized to a greater degree in vitro. Host biomass responses varied among the host species. In broad comparisons, more grass species than forbs tended to respond positively to colonization, whereas more forb species tended to be non-responsive. Based on the suspected differences in the levels of colonization, we predicted that tallgrass prairie grasses would support greater DSE colonization than forbs in the field. A survey of field-collected roots from 15 native species supported this hypothesis. Our study supports the “broad host range” of DSE fungi, although the differences in the rates of colonization in the laboratory and in the field suggest a greater compatibility between grasses and DSE fungi. Furthermore, host responses to DSE range from mutualism to parasitism, suggesting a genotype-level interplay between the fungi and their hosts that determines the outcome of this symbiosis.