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Dominant species can act as a biotic filter in structuring plant communities by constraining the establishment and survival of subordinate species. The effect of intraspecific trait variability of dominant species on the functional response of subordinate species, however, is not well understood. We quantified intraspecific variation in four functional traits of 26 subordinate species in an experimental grassland established with two population sources (i.e. cultivars and local ecotypes) of three dominant grasses (Sorghastrum nutans, Andropogon gerardii and Schizachyrium scoparium) and three pools of subordinate species (each from one origin) within each of the dominant grass source treatments. Twenty of the 26 subordinate species exhibited intraspecific trait variability for one trait or more in response to dominant species population source, and variation among population sources of the dominant species was non‐random. Dominant grass population source affected intraspecific variability in functional traits of multiple subordinate species. Cultivar sources of the dominant grasses and some of the subordinate species that established with them had higher and generally more variable functional leaf area and leaf nitrogen content compared to local ecotypes of the dominant grasses and the subordinate species that established with them. Local ecotype sources of the dominant grasses increased leaf area based functional diversity of subordinate species. Synthesis. This study provides evidence that intraspecific trait variability in dominant species acts as a biotic filter to constrain niche availability and dimensionality affecting trait variation of subordinate species during community assembly.

Biodiversity enhances a variety of ecosystem processes, and yet the underlying mechanisms through which these relationships occur remain a critical knowledge gap. Here, we used the natural abundance of stable isotopes to measure depth of water uptake in five common grassland species (Asclepias tuberosa, Lespedeza capitata, Liatris aspera, Schizachyrium scoparium and Sorghastrum nutans) growing across an experimental grassland diversity gradient. Using this approach, we addressed the following questions: 1) does the depth‐specific provenance of water uptake differ among species and/or do interspecific differences in water source manifest with increasing community diversity? 2) Does the isotopic niche space occupied by plants change with increasing diversity? 3) Is plasticity in water uptake depth across a diversity gradient associated with functional plant responses? We found that the depth of soil water used by plants was inherently different among species when grown in monocultures. All species used less shallow soil water and more intermediate‐depth soil water in mixed assemblages than in monocultures, resulting in similar interspecific differences in water source across the diversity gradient. However, plasticity in the locations of water used were positively associated with increases in plant growth in higher diversity treatments. These results indicate that plasticity in water‐use may contribute to positive biodiversity–productivity relationships commonly observed in temperate grasslands.

The pooling of soil samples in plant-microbe interaction studies is commonly employed, but the impact of sample handling has rarely been explored experimentally. Concerns have been raised that sample pooling may reduce biological variation leading to inflated type I errors or may alter the magnitude of microbial effects observed, invalidating the results achieved. To assess the impact of inocula pooling on plant-microbe interactions, we examined the reciprocal influence of unpooled and pooled soil microbial inocula on growth of Solidago altissima and Schizachyrium scoparium, with and without inoculum sterilization. Soil pooling had no effect on the variance among replicates in either plant species. However, pooling dramatically altered the magnitude and direction of microbial impacts on plant performance. Pooling of Solidago altissima soil increased the antagonistic effects on growth of both target species. In contrast, pooling of Schizachyrium scoparium soil shifted impacts on Solidago altissima from effectively neutral to slightly positive. Pooling in this system altered both the strength and direction of plant-microbe interactions relative to unpooled soils. Therefore soil mixing should be avoided when the research goal is to determine naturally occurring interaction strengths, even within a single habitat.

Summary Flower production can comprise up to 70% of above-ground primary production in grasslands. Yet we know relatively little about how the environment and timing of rainfall determine flower productivity. Evidence suggests that deficits or additions of rainfall during phenlologically relevant periods (i.e. growth, storage, initiation of flowering and reproduction) can determine flower production in grasslands. We used long-term data from the Konza Prairie LTER to test how fire, soil topography and precipitation amounts during four phenologically relevant periods of the growing season constrain three aspects of flowering in three dominant C4 grass species. Specifically, we examined the probability of flowering, flowering stalk density and individual flowering stalk biomass for Andropogon gerardii, Schizachyrium scoparium and Sorghastrum nutans. We found that each of the three species responded to the amount of precipitation during phenologically relevant periods in unique ways. All aspects of A. gerardii flowering were sensitive to precipitation during the flowering stalk elongation period (20 June – 3 August). The probability of S. nutans flowering was partly determined by precipitation during the rapid growth phase (21 April – 4 June), whereas flowering stalk density of this species depended on rainfall during flowering stalk elongation (20 June – 3 August). In contrast, all aspects of flowering of S. scoparium were relatively independent of rainfall during any period. Our results demonstrate that three functionally similar, co-dominant C4 grass species respond differently to phenologically relevant precipitation periods. As a result, drought during any phenological window during the growing season can adversely impact biomass and flowering production of grasslands via species-specific reductions in flowering stalk density and biomass. A lay summary is available for this article.

Plant competition may mediate the impacts of herbivory on invasive plant species through effects on plant growth and defense. This may predictably depend on whether herbivory occurs above or below ground and on relative plant competitive ability. We simulated the potential impact of above- or belowground damage by biocontrol agents on the growth of a woody invader (Chinese tallow tree, Triadica sebifera) through artificial herbivory, with or without competition with a native grass, little bluestem (Schizachyrium scoparium). We measured two defense responses of Triadica through quantifying constitutive and induced extrafloral nectar production and tolerance of above- and belowground damage (root and shoot biomass regrowth). We examined genetic variation in plant growth and defense across native (China) and invasive (United States) Triadica populations. Without competition, aboveground damage had a greater impact than belowground damage on Triadica performance, whereas with competition and above- and belowground damage impacted Triadica similarly. Whole plant tolerance to damage below ground was negatively associated with tolerance to grass competitors indicating tradeoffs in the ability to tolerate herbivory vs. compete. Competition reduced investment in defensive extrafloral nectar (EFN) production. Aboveground damage inhibited rather than induced EFN production while belowground plant damage did not impact aboveground nectar production. We found some support for the evolution of increased competitive ability hypothesis for invasive plants as United States plants were larger than native China plants and were more plastic in their response to biotic stressors than China plants (they altered their root to shoot ratios dependent on herbivory and competition treatments). Our results indicate that habitat type and the presence of competitors may be a larger determinant of herbivory impact than feeding mode and suggest that integrated pest management strategies including competitive dynamics of recipient communities should be incorporated into biological control agent evaluation at earlier stages.

Summary 1. Climate change is expected to bring fewer, larger rainfall events and prolonged droughts (i.e. increased rainfall variability). Concurrently, the burning of fossil fuels and reliance on nitrogen (N) fertilizers are expected to continue to increase N availability in many ecosystems. These changes in water and N availability have the potential to alter plant community composition and structure. 2. We manipulated rainfall variability and N inputs in a restored tallgrass prairie over the course of two growing seasons. 3. Greater rainfall variability led to wetter soils throughout the majority of both growing seasons and provided punctuated relief from a severe drought that occurred during the first three months of the experiment. 4. Both rainfall variability- and fertilization-induced increases in resource availability favoured fast-growing, deeply rooted C3 forbs, particularly the dominant Solidago canadensis, at the expense of species adapted to low resource conditions, particularly C4 grasses and N-fixing forbs. 5. This change in community composition decreased plant community diversity and evenness in plots receiving both supplemental N and more variable rainfall. 6. Synthesis and applications. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable.

Seedling performance is often a limiting factor in ecological restoration. Changes in the soil microbial community generated by invasive plants contribute to seedling failure. A method to remediate invasive species-induced changes to the soil microbial community that results in increased native species seedling performance and decreased invasive species seedling performance could have a large impact on the success of many restoration efforts. In a greenhouse experiment, we first examined the changes in the soil microbial community created by invasive compared to native grasses. Then, we investigated four microbial treatments (bacterial inoculant, fungal inoculant, fungicide, and bactericide/fungicide) to remediate microbial plant–soil feedbacks (PSFs) created by invasive species Bromus inermis and Poa pratensis and increase the performance of natives Andropogon gerardii, Elymus canadensis, Pascopyrum smithii, and Schizachyrium scoparium. We found that the PSF mitigation treatments had some context-dependent utility for restoration. For example, all of the treatments decreased the performance of B. inermis and fungal inoculant decreased the performance of P. pratensis. However, no single treatment increased the performance of all natives. Fungicide increased the performance of A. gerardii and E. canadensis in soil previously occupied by B. inermis and the performance of S. scoparium in soil previously occupied by P. pratensis. If validated in the field, PSF mitigation treatments may have utility for restoration practitioners.

Question Can the timing of cutting or burning be used to selectively damage an invasive C4 grass species while limiting collateral damage to dominant native C4 grass species in a grassland community? Location A medium to short grass prairie in central Texas, US. Methods We conducted six cut and burn trials between June 2011 and January 2012 during an exceptional drought year. Target species were the native, perennial, C4 grass Schizachyrium scoparium (little bluestem) and the introduced, perennial, C4 grass Bothriochloa ischaemum (yellow bluestem). Burn trials were conducted inside a steel barrel using standardized fuel loads. To assess recovery, we counted the first cohort of new tillers that emerged after treatment and tillers that grew in the subsequent spring. We also recorded phenological status, meteorological variables and burn temperatures to determine if tiller recovery was predictable. Results Bothriochloa ischaemum responded more negatively to burning than to cutting, but these treatments were no different for S. scoparium. Both species produced the smallest number of tillers after the summer burns, and the burn date × species interaction was insignificant. On average, peak fire temperature inside tussocks was 100 °C higher in B. ischaemum than in S. scoparium. Judging from the reduction in tiller densities in undisturbed plots from spring 2011 to spring 2012, B. ischaemum was also more sensitive to drought. Multivariate models were significant and related burn recovery to peak fire temperature, among other factors, and recovery from cutting to ambient temperature on the day of cutting. In B. ischaemum, plants with higher investment in early reproductive growth suffered more burn damage. Conclusions We identified no optimal burn time that maximized discrimination between the two species. Nonetheless, the invasive species was more sensitive to both drought and fire, possibly due to higher heat exposure of growing points. Thus, burning during the summer of a drought year may constitute the sought-after treatment that selectively favours native species. Ultimately, successful management will involve fuel and temperature management combined with assessment of the relative phenologies of the target species prior to burning. This study suggests that a hot fire at a time when the invasive species has a larger investment in early reproductive growth will provide the best control.

Restoration of glades over the past 30 years, involving removal of woody cover and re‐establishment of herbaceous plant communities, has created an archipelago of habitat patches varying in age, size, and isolation. Within these glades, habitat varies in quality from edge to core. We investigated impacts of within‐ and among‐glade variation on the frequency of root colonization by arbuscular mycorrhizal fungi (AMF) for two host species, Schizachyrium scoparium and Rudbeckia missouriensis, and the plant community at large. We also conducted a soil analysis to explore the role of nutrient resources in mediating putative habitat effects. We used Akaike's information criterion (AIC) to evaluate a set of a priori models that assessed effects of glade size, isolation, age, and edge effects on AMF colonization. In community samples, AMF colonization increased while variation in AMF colonization decreased with restoration age. Edge effects also reduced plant community AMF colonization. AMF colonization was higher in R. missouriensis than in S. scoparium, but increased for both species in core habitat. Glade size had no direct effect on AMF at the plant scale, but indirectly affected overall AMF occupancy at the landscape scale via its geometric relationship to glade edge:core ratio. Hosts in small glades that are dominated by edge exhibit lower occupancy rates. The first two axes in a principal components analysis (PCA) combining soil nutrient variables explained 74% of sample variance: PC1 correlated positively with organic matter (OM), total nitrogen (N), calcium (Ca), magnesium (Mg), and potassium (K) and decreased from the edge to the core of older glades. PC2, which correlated positively with pH and negatively with available P, increased with time since restoration. While results are consistent with a role of soil properties in explaining edge and age effects, the two PCA dimensions themselves are weakly correlated with AMF colonization. Overall, our findings suggest that environmental filters associated with woody encroachment limit the colonization of otherwise suitable herbaceous hosts and create a landscape mosaic with larger, older glades serving as hotspots or reservoirs of AMF root colonization and smaller, newly restored glades as cold spots or potential sinks.

This study evaluated the reactions of seven common C4 grasses of the tallgrass prairie of the USA Great Plains to the economically important wheat pathogens Pyrenophora tritici-repentis and Gaeumannomyces graminis var. tritici (Ggt) isolated from wheat. The P. tritici-repentis isolates (race 1) were pathogenic on all grasses tested, but symptom severity was markedly low. Three of the grass species inoculated with Ggt were highly susceptible, while four species exhibited no symptoms. Because measures of connectedness can provide a proxy for population processes, connectedness was evaluated within and among the seven grass species in representative tallgrass prairie environments for all potential pathogen-sharing patterns. Andropogon gerardii was ubiquitous, so all plant species were well connected to it. Andropogon scoparius (= Schizachyrium scoparium), Sorghastrum nutans and Panicum virgatum were fairly common but specialized to particular environments. Bouteloua curtipendula was uncommon but occurred in all environments, while Buchlo€e dactyloides and Bouteloua gracilis were uncommon and only occurred in upland sites. Co-occurrence of plant species was generally not reciprocal in that, for many species pairs, species A rarely occurred without potential exposure to inoculum from species B, while species B commonly occurred without species A. The three grass species susceptible to Ggt may act as sources of inoculum for each other within tallgrass prairie, with the potential to influence fitness, and tallgrass prairie and commercial wheat ecosystems in the Great Plains also have the potential to share both pathogens.

Fire suppression has removed an important ecological force previously responsible for shaping many plant communities throughout the world. Upland areas of north-central Mississippi that have been protected from fire are now closed-canopy forests including species known to be uncommon as bearing/witness trees in upland portions of the landscape (historically off-site species) and sparse ground cover vegetation. Anecdotal evidence suggests that warm-season grasses were prevalent in the understory of these communities, which could have provided more consistent fuel. We corroborate the historic presence of these grasses by looking at their natural co-occurrence with oak regeneration (a requisite of self-replacing stands of oaks found historically). Restoration of these communities has typically focused on burning and off-site tree thinning. Utilizing a restoration experiment implementing these treatments, we found significantly reduced understory leaf litter in treatment areas. To test which variables associated with restoration treatments were most important for the survival of these grasses, we measured the effect of leaf litter removal and its interaction with environmental conditions on the survival of transplanted shoots. Survival of little bluestem increased with decreasing canopy density and decreasing leaf litter. Leaf-litter removal did not increase survival, nor did it interact with either pre-treatment leaf litter depth or canopy density. These results show that little bluestem benefits from conditions expected historically: increased light and possibly fire.

Arbuscular mycorrhizal fungi (AMF) have numerous effects on temperate grassland ecosystems, but prairie restorations are frequently located in sites with depauperate AMF communities. In this greenhouse study, four native species (Schizachyrium scoparium, Elymus canadensis, Monarda punctata, and Aster ericoides) and an invasive grass (Bromus inermis) were grown in unsterilized field soils and treated with two types of commercial AMF inoculum. Inocula were applied at one and two times the manufacturers' suggested rate. Soil was collected from a meadow enrolled in the Conservation Reserve Program (CRP), and from an active agricultural field. Inoculum addition had no effect on biomass or percent colonization by AMF for any grass species, regardless of soil type. Inoculum type significantly affected Aster biomass and percent colonization, although pairwise comparisons of treated individuals and controls were not significant. The overall lack of effectiveness of the commercial inocula may reflect the small number of propagules added, even when used at twice the recommended rate. Higher rates of fungal colonization in all three grasses and increased biomass in the native grasses were observed in individuals grown in the CRP soil. Plants were also colonized by dark septate endophytic fungi; for Schizachyrium, endophyte colonization was significantly greater in tilled than CRP soil. Our results indicate that an existing soil fungal community promotes colonization by AMF more than the addition of commercial inocula, and that soil characteristics associated with land use history significantly affect the growth of native species in a restoration setting.

Little bluestem, Schizachyrium scoparium (Michx.) Nash, is a native grass that has been shown to have high levels of genetic variation for traits such as biomass yield, disease resistance, plant height, leafi ness, maturity, and seed yield. If high levels of genetic variation exist for these traits, it is likely that it would exist for other traits as well. Thus, the objectives were to describe the variation in traits that contribute to canopy morphology within and among little bluestem parental lines, an F 1 plant population, and selected F 1 populations. Plants were classifi ed as not upright (NU), upright compact (UC), or upright open (UO) on the basis of their shape. As expected, all parental lines expressed considerable variation for frequencies of discrete morphological traits and for continuous morphological traits. The F 1 plant population was characterized by 12.2% NU plants, 22.4% UC plants, and 65.4% UO plants. The selected populations were fi xed for several discrete variables, including growth form, lack of lodging, and little or no leaf rust. To fully exploit the genetic variability of little bluestem, it would be necessary to obtain ecotypes from throughout its natural range and breed and select for new genetic variation that could be captured through recurrent selection. Little bluestem is an ideal species for breeding new cultivars and/or germplasm lines for pasture and rangeland renovation, roadside revegetation, wildlife habitat, and recreation areas.

Pinus palustris (longleaf pine) savannas depend on contiguous grass cover to facilitate frequent surface fires. Enhanced flowering, seed production, and germination may be linked to season of fires. We assessed the effect of month of prescribed fire (February, April, May, and July) on percentage of plants with flowering culms (FCs%), FC/area, FC/plant, seed production, and germination for five warm seasons, fall-flowering grasses. Multivariate analysis indicated the response of flowering and fruiting to burn month varied among the grass species. The dominant species, Sporobolus junceus and Schizachyrium scoparium var. stoloniferum, had greater numbers of most flowering characteristics when burnt during April, May, and July. Aristida purpurascens had increased FC/plant after May and July burns. In contrast, Ar. mohrii had the fewest FC/plant and seeds/FC when burnt in July. Germination was greatest (26–60%) for Ar. purpurascens. Seeds collected following July burns for Ar. purpurascens and Ar. ternarius and after May burns for S. junceus were within the highest germination values recorded. Germination of Sc. scoparium var. stoloniferum was very low after February and July burns (≤5%). With Ar. mohrii, only seed collected following February (2%) and April burns (3%) germinated. April, May, and July fires increased seed production of dominant matrix grasses, thus facilitating the potential for recruitment of these species and facilitating seed collection from potential donor sites for ground-layer restoration projects. Varying prescribed fire burn month captured variation in flowering characteristics among these grasses.

Dominance of warm-season grasses modulates tallgrass prairie ecosystem structure and function. Reintroduction of these grasses is a widespread practice to conserve soil and restore prairie ecosystems degraded from human land use changes. Seed sources for reintroduction of dominant prairie grass species include local (non-cultivar) and selected (cultivar) populations. The primary objective of this study was to quantify whether intraspecific variation in developing root systems exists between population sources (non-cultivar and cultivar) of two dominant grasses (Sorghastrum nutans and Schizachyrium scoparium) widely used in restoration. Non-cultivar and cultivar grass seedlings of both species were isolated in an experimental prairie restoration at the Konza Prairie Biological Station. We measured above- and belowground net primary production (ANPP and BNPP, respectively), root architecture, and root tissue quality, as well as soil moisture and plant available inorganic nitrogen (N) in soil associated with each species and source at the end of the first growing season. Cultivars had greater root length, surface area, and volume than non-cultivars. Available inorganic N and soil moisture were present in lower amounts in soil proximal to roots of cultivars than non-cultivars. Additionally, soil NO3–N was negatively correlated with root volume in S. nutans cultivars. While cultivars had greater BNPP than non-cultivars, this was not reflected aboveground root structure, as ANPP was similar between cultivars and non-cultivars. Intraspecific variation in belowground root structure and function exists between cultivar and non-cultivar sources of the dominant prairie grasses during initial reestablishment of tallgrass prairie. Population source selection should be considered in setting restoration goals and objectives.

Grasses can be important regulators of species diversityand ecosystem processes in prairie systems. Although C 4 grasses are usually assumed to be ecologically similarbecausetheyareinthesamefunctionalgroup,theremaybe important differences among species or between seedsources that could impact restorations. I tested whetherC 4 grass species identity, seed source, or grass speciesrichness scales to influence aboveground net primaryproductivity (ANPP), resistance to weed invasion, orestablishment of subordinate prairie species during res-toration. Plots in western Iowa, United States, wereplanted with equal-sized transplants of one of five com-mon grass species (Panicum virgatum L., Sorghastrumnutans (L.) Nash, Andropogon gerardii Vitman, Schiza-chyrium scoparium (Michx.) Nash, andBouteloua curti-pendula(Michx.) Torrey) either from local seed or fromcultivar seed sources. These plots were compared toplots containing all five species in mixture and to non-planted plots. Differences in ANPP were foundamong species but not between cultivars and nonculti-vars or between monocultures and mixtures. Panicumvirgatum, S. nutans,andS. scoparium were more pro-ductive than A. gerardii and B. curtipendula.Weedinvasion was much higher when plots were not plantedwith grasses. Schizachyrium scoparium allowed greaterestablishment of subordinant prairie species than allother focal grass species. There were two separate mech-anisms by which grasses suppressed prairie species estab-lishment either (1) by growing tall and capturing lightor (2) by quickly filling in bare space by spreading hori-zontally through rhizome growth in short species. Theseresults suggest that high ANPP can be found with non-cultivar plantings during the first 2 years after plantingand that subordinate species establishment is most likelywhen shorter bunchgrasses such as S. scoparium aredominant.Key words: biodiversity-ecosystem functioning, biofuels,C

The flowering of grasses is a process critical to plant population dynamics and genetics, herbivore performance, and human health. To better understand the climate factors governing grass flowering, we analyzed the patterns of culm production over 25 years for three perennial tallgrass prairie species at Konza Prairie in Kansas, USA. The three species (Andropogon gerardii, Sorghastrum nutans, and Schizachyrium scoparium) all utilize the C4 photosynthetic pathway and were measured annually at the same locations for the past 25 years in an annually burned watershed. Culm production of all three species increased with higher growing-season soil moisture and precipitation but differed in their responses to water availability at different times during the growing season. Relative to Andropogon, Sorghastrum responded more to precipitation early in the growing season, and Schizachyrium responded more to precipitation late in the growing season. Flowering by each species also revealed a threshold relationship with late-season soil moisture at approximately 1 m depth, which likely is a proxy for season-long water balance. Although flowering can be influenced by conditions antecedent to the current growing season, neither soil moisture nor precipitation during the previous year influenced flowering over the 25-year period. Flowering culm production averaged 9% and 7% of total graminoid aboveground net primary production (ANPP) in the uplands and lowlands, respectively. Interannual variation in ANPP correlated only with Sorghastrum flowering, suggesting a predominant role of the species in ANPP responses to climate.

Dominant species play crucial roles in determining plant community structure and ecosystem function. Cultivars of the dominant prairie grasses are widely used in prairie restoration and are selected for characters such as high biomass production, increased reproductive output, and stress tolerance. Genetic differences exist between cultivar and non-cultivar population sources of dominant tallgrass prairie species, which may have implications for plant performance in prairie restoration. We measured net photosynthesis (Anet), stomatal conductance (gs), and water use efficiency (WUE) in cultivar and non-cultivar dominant tallgrass prairie species Andropogon gerardii Vitman, Sorghastrum nutans (L.) Nash, and Schizachyrium scoparium (Michx.) Nash in both a greenhouse experiment and an experimental tallgrass prairie restoration. We found indicators of enhanced physiological performance (higher Anet, gs, and/or WUE) in cultivar population sources of all three dominant grass species relative to non-cultivars. For A. gerardii, cultivars exhibited higher Anet and WUE than non-cultivars. For S. nutans, cultivars exhibited higher gs, whereas non-cultivars showed higher WUE. Lastly, cultivars of S. scoparium showed higher WUE than non-cultivar population sources. Our results show that population selection of dominant species in restoration can have consequences for plant performance, which may have implications for competitive interactions that affect community structure (i.e. diversity) and ecosystem function (i.e. aboveground net primary production) during the reassembly of prairie systems.

Disturbances such as burning or grazing maintain the herbaceous nature of eastern tallgrass prairie. These disturbances are also known to affect the relative abundance of warm-season (C4) and cool-season (C3) grasses in native prairie. Although burning is a commonly used tool, the utility of livestock grazing to manage restored prairie is less understood. We established five monocultures and one mixture of C4 grass species of the eastern tallgrass prairie in southern Wisconsin. To examine their persistence under high-intensity, short-duration summer grazing, we estimated cover of several functional groups and C4 species over a 6-year period (2000 through 2006) in a randomized complete block design. After a 2-year establishment phase (1998-1999), bison were rotated through paddocks two or three times annually during late June, July, or early August. All C4 grasses declined over time but at different rates depending on the species. Switchgrass (Panicum virgatum) decreased at the lowest rate, whereas Little bluestem (Schizachyrium scoparium) cover declined faster than Big bluestem (Andropogon gerardii), Indiangrass (Sorghastrum nutans), and Sideoats grama (Bouteloua curtipendula), whose rates of decline were not significantly different from each other. Succession followed a predictable trajectory with annual grasses initially colonizing interstitial space among C4 grasses, followed by legumes, which ultimately gave way to exotic C3 forage grasses. The focal C4 grasses remained the dominant functional group 8 years postseeding, but recolonization by non-native C3 grasses increased over the study period.

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium ,C 4 grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, 11.5 1C) in both monoculture and tree-grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in Aarea (� 56% and � 66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (� 44%, � 64%), while J. virginiana showed increased Aarea under normal drought (113%) and a small decrease in Aarea when exposed to intensified summer drought (� 10%). Following individual rainfall events, mean increases in Aarea were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of Aarea of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, Aarea recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in Aarea following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C4 grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.

We evaluated how three co-occurring tree and four grassland species influence potentially harvestable biofuel stocks and above- and belowground carbon pools. After 5 years, the tree Pinus strobus had 6.5 times the amount of aboveground harvestable biomass as another tree Quercus ellipsoidalis and 10 times that of the grassland species. P. strobus accrued the largest total plant carbon pool (1375 g C m-2 or 394 g C m -2 yr), while Schizachyrium scoparium accrued the largest total plant carbon pool among the grassland species (421 g C m -2 or 137 g C m -2 yr). Quercus ellipsoidalis accrued 850 g C m -2 , Q. macrocarpa 370 g C m -2 , Poa pratensis 390 g C m -2 , Solidago canadensis 132 g C m -2 , and Lespedeza capitata 283 g C m -2 . Only P. strobus and Q. ellipsoidalis significantly sequestered carbon during the experiment. Species differed in total ecosystem carbon accumulation from -21.3 to + 169.8 g C m -2 yr compared with the original soil carbon pool. Plant carbon gains with P. strobus were paralleled by a decrease of 16% in soil carbon and a nonsignificant decline of 9% for Q. ellipsoidalis. However, carbon allocation differed among species, with P. strobus allocating most aboveground in a disturbance prone aboveground pool, whereas Q. ellipsoidalis, allocated most carbon in less disturbance sensitive belowground biomass. These differences have strong implications for terrestrial carbon sequestration and potential biofuel production. For P. strobus, aboveground plant carbon harvest for biofuel would result in no net carbon sequestration as declines in soil carbon offset plant carbon gains. Conversely the harvest of Q. ellipsoi- dalis aboveground biomass would result in net sequestration of carbon belowground due to its high allocation belowground, but would yield lower amounts of aboveground biomass. Our results demonstrate that plant species can differentially impact ecosystem carbon pools and the distribution of carbon above and belowground.

Little bluestem [Schizachyrium scoparium (Michx.) Nash] was the most important dominant species of uplands in the tallgrass prairie. Presently, it is a dominant species, most significantly on coarse-textured soils, in the mixed-grass prairie region of the United States and Canada. This wide geographical adaptation to dry soils suggests little bluestem has potential for biomass production in areas unsuitable for most other grass species in the North American steppe. Primary objectives of this research were to (i) describe the morphology and determine the distribution of biomass among main stem and axillary components of little bluestem, and (ii) determine genetic variation for morphology and biomass production in 'Camper' little bluestem. Morphological analysis was conducted on tillers collected from natural populations in South Dakota and Montana and from swards of Camper and 'Badlands' ecotype in eastern South Dakota. Sixteen genotypes of Camper were evaluated for three years at two locations for determination of genotypic variation for biomass and biomass components. Quadratic models explained relationships between phytomer position and leaf and internode lengths and mass of individual phytomers of natural and selected populations. Genotypic variation occurred for phytomers per tiller, tillers per plant, mass per tiller, axillary branches per tiller, and biomass (range: 102-387 g plant -1 ). However, the correlation between locations for biomass was not significant for genotypes due to genetic variation for phenotypic plasticity. High frequency of survival over two winters for all genotypes indicated Camper has adequate winter hardiness to serve as a source population for cultivar development for biomass in the northern Great Plains.

I compared growth and arbuscular mycorrhizal fungal (AMF) colonization of two prairie grasses (Wild rye [Elymus canadensis] and Little bluestem [Schizachyrium scoparium]), an early- and a late-dominating species in prairie restorations, respectively, grown in soil from restored prairies of differing age, soil characteristics, and site history. There were no consistent patterns between restoration age and soil inorganic nutrients or organic matter. The oldest restoration site had higher soil mycorrhizal inoculum potential (MIP) than 2- and 12-year-old restorations. However, MIP did not translate into actual colonization for two species grown in soils from the three restorations, nor did MIP relate to phosphorus availability. There were significant differences in root mass and colonization among Wild rye plants but not among Little bluestem plants grown in soils from the three restorations. Wild rye grown in 2-year-old restoration soil had significantly higher AMF colonization than when it was grown in soils from the 12- and 17-year-old restorations. Wild rye grown in 2-year-old restoration soil also had higher colonization than Little bluestem grown in 2- and 12-year-old restoration soils. Little bluestem had no significant correlations between shoot biomass, root biomass or colonization, and concentrations of soil P, total N, or N:P. However, for Wild rye, total soil N was positively correlated with root mass and negatively correlated with colonization, suggesting that in this species, mycorrhizae may affect N availability. Collectively, these results suggest that soil properties unrelated to restoration age were important in determining differences in growth and AMF colonization of two species of prairie grasses.

Species diversity is thought to stabilize functioning of plant communities. An alternative view is that stability depends more on dynamics of dominant species than on diversity. We compared inter-annual variability (inverse of stability) of aboveground biomass in paired restored and remnant tallgrass prairies at two locations in central Texas, USA. Data from these two locations were used to test the hypothesis that greater richness and evenness in remnant than restored prairies would reduce variability in aboveground biomass in response to natural variation in rainfall. Restored prairies were chosen to be similar to paired remnant prairies in characteristics other than species diversity that affect temporal variability in biomass. Variability was measured as the coefficient of variation among years (square root of variance/mean; CV), where variance in community biomass equals the sum of variances of individual plant species plus the summed covariances between species pairs. Species diversity over five years was greater by a factor of 2 or more in remnant than restored prairies because richness and evenness were greater in remnant than restored prairies. Still, the CV of community biomass during spring and CV of annual biomass production did not differ consistently between prairie types. Neither the sum of species covariances nor total community biomass differed between prairies. Biomass varied relatively little in restored compared to remnant prairies because biomass of the dominant species in restored prairies (the grass Schizachyrium scoparium) varied less than did biomass of other dominant and sub-dominant species. In these grasslands, biomass response to natural variation in precipitation depended as much on characteristics of a dominant grass as on differences in diversity.

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...

Little bluestem [Schizachyrium scoparium (Michx.) Nash] is one of the most widespread native grasses in North America. Little is known about the genetic diversity of this species in natural populations and in seed collections. The amplified fragment length polymorphism (AFLP) technique was applied to assess the comparative genetic diversity of six natural populations of little bluestem in Manitoba and Saskatchewan and their corresponding seed collections. Five AFLP primer pairs were employed to screen a total of 180 samples representing about 15 tillers per population and 15 seeds per collection, and 158 polymorphic AFLP bands were scored for each sample. Analyses of these scored bands revealed that >91% of the total AFLP variation was present within the natural populations and within the seed collections. The among-population and among-collection variation components, although relatively small (7-9%), were statistically significant from zero. Comparisons of AFLP profiles between the seed and tiller samples revealed the seed samples had fewer polymorphic bands, higher average band frequencies, and more bands with extremely high or low frequencies. A significant association of AFLP variation with geographical origin was detected in the seed, but not the tiller samples. These results indicate collecting seeds may not be as effective as collecting tillers in sampling genetic diversity from natural populations for the improvement of little bluestem germplasm for rangeland seeding.

Caucasian old world bluestem (OWB) [Bothrichloa bladhii (Retz) S.T. Blake] has been widely introduced in the southern and central Great Plains as a perennial warm-season grass for forage and soil conservation. As a result of its widespread introduction, it has escaped into some native rangelands. Once established in native pastures, observations suggest that Caucasian OWB may mature earlier than native vegetation and may be avoided by grazing animals in mixed rangelands, thus altering grazing distribution and over utilizing native species. More advanced morphological development in grasses is also associated with lower forage quality and lower palatability. This study was conducted to examine if morphological development of Caucasian OWB and native perennial warm-season grass species differs. Monoculture stands of Caucasian OWB, big bluestem (Angropogon gerardii Vitman), little bluestem [Schizachyrium scoparium (Michx.) Nash], and side-oats grama [Bouteloua curtipendula (Michx.) Torr.] were established in the spring of 2000. After the initiation of first growth in 2002 and 2003, vegetation was hand clipped each week for 8 wk and quantified for morphological stage of development according to the Nebraska staging method. In 2002, a drought season, Caucasian OWB had a greater mean stage weight (MSW) than the native species during Weeks 3 through 7 (P < 0.001). In 2003, Caucasian OWB began the season at a similar morphological stage as the native species, but had a rate of development that was 5 to 11 times greater than the native species. The last 3 wk of the sampling period, MSW of Caucasian OWB was greater than all three native species (P < 0.001). With more advanced morphological development in Caucasian OWB than the native grasses, declining forage quality could affect grazing preference and distribution patterns in mixed swards of Caucasian OWB and native species. The introduction of Caucasian OWB into native rangelands may have impacts that have yet to be understood.

Structural attributes of the C4, perennial bunchgrass Schizachyrium scoparium in restored prairies may be affected by the time since restoration. One hundred plants each in 8-, 17-, and 23-year-old restored prairies and a native Texas Blackland prairie were assessed for the presence/absence and diameter of a hollow crown (i.e., dead center portion), degree of fragmentation, plant height, and tiller density. Structural attributes of S. scoparium plants were generally (1) different between recent (8 years) and older (17 and 23 years) restored prairies (2) similar between the 17- and 23-year-old restored prairies, and (3) more similar between the 8- and 17-year restored prairies and the native, remnant prairie than between the 23-year restored prairie and the native prairie. Plants were shorter in restored prairies, regardless of time since restoration, than in the native prairie. Mean basal area of plants was 80163% greater in the 17- and 23-year restored prairies compared with the native and 8-year restored prairies. Percentage of hollow crowns and fragmentation was smallest in the 8-year restored prairie, largest in the 17and 23-year restored prairies, and intermediate in the native prairie. Tiller density exhibited inverse second-order polynomial decreases with increasing plant basal area for all prairies. In contrast to tiller density, diameter of hollow crowns increased logarithmically with increasing plant basal area. Structural attributes of S. scoparium in restored prairies changed predictably with age, despite growing in different communities.

We used differences in soil carbon δ 13 C values between forested sites and grasslands dominated by the C 4 grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ 13 C was measured at (1) sites with long-term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ 13 C of soil under long-term grassland was -24.1‰ at 0 to 2 cm depth and -23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long-term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ 13 C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ 13 C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ 13 C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ 13 C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes.

Feedback between plants and arbuscular mycorrhizal (AM) fungi can affect species diversity in plant and fungal communities. Feedback depends on (1) some specificity between plants and fungi and (2) fungi exhibiting specificity either improving (positive feedback) or decreasing (negative feedback) host performance relative to other fungi. Associations between AM fungi and plant species in a serpentine grassland dominated by Andropogon gerardii, Schizachyrium scoparium, Sorghastrum nutans, and Sporobolus heterolepis were examined, and their performance consequences were evaluated. Specificity was determined from AM fungal spore abundance under plants in the field and from trap cultures established by inoculating greenhouse plants with field-collected roots containing fungal material. Seven AM fungal species were unevenly distributed among plant species, with differences in total spore numbers and evenness. In the field, Gigaspora gigantea exhibited specificity to Sporobolus compared to Andropogon and Sorgha...

Summary 1. Aristida stricta and Schizachyrium scoparium are C 4 perennial bunchgrass species native to Pinus palustris savannas in the south-eastern USA. Species differences in growth rate, tissue nutrient content and distribution suggest that A. stricta is more stress-tolerant ( sensu Grime) than S. scoparium . In addition, A. stricta retains leaves all year, whereas S. scoparium is winter deciduous. Based on these observations, we wanted to determine (1) whether root longevity was higher for A. stricta as compared to S. scoparium ; and (2) whether the leaf habits of these species had consequences for seasonal patterns of root production and death. 2. Using in situ rhizotrons, we recorded root number and length of these species over the course of 2·5 years on flat, transparent 50 〈 50 cm windows. Kaplan‐Meier (productlimit) analysis was used to produce survival functions and to estimate median lifespans for the right-censored root longevity data. These survival functions were compared using log-rank χ 2 statistics, and a proportional hazards model was used to examine the effects of season and species on the survival of each cohort. Final demographic variables were analysed using anova . 3. The median lifespan of A. stricta was 777 days whereas that of S. scoparium was 374 days. This significant difference in median lifespan was consistent between species for all cohorts in both years. Also, coincident with winter leaf senescence, root production nearly stopped for S. scoparium whereas A. stricta produced roots all year. This is probably due to the winter photosynthetic capacity of A. stricta . Root death did not exhibit any seasonal pattern in either species. A pulse of roots produced in apparent response to the 2000 drought was evident for S. scoparium but not for A. stricta . This cohort, however, rapidly died during the dormant season, probably due to the lack of winter photosynthesis in that species. 4. Aristida stricta and S. scoparium exhibited root production, death and longevity patterns that were predicted on the basis of ecological and leaf habit differences between them. These findings suggest species-specific differences in performance and element cycling in these nutrient-poor soils.

I investigated the separate and joint effects of herbivory (grazing) and com- petition on six grass species and on their distributions across the landscape. In a factorial field experiment, cattle were excluded from half the subplots, and neighboring plants were removed from half the subplots. Transplants of each of the species were planted in plots on hillsides and in flat areas, and the entire experiment was replicated in two pastures. The effects of grazing were negative and were proportional to the ungrazed height of each species. In the absence of grazing and competition, the flat plots were more favorable than the hillside plots. Grazing reversed the relative favorableness of flat and hillside plots, most likely because grazing was more intense in the flat plots due to cattle preference for flatter ground. The removal of neighboring plants always had a positive effect, indicating competition from neighboring plants. The relative impact of competition from pre-existing plants was generally greater in the flat plots than in the hillside plots. The relative impacts of grazing were generally greater in the absence of competing plants, and the relative impacts of competition were generally greater in the absence of grazing. The most likely explanation for this antagonistic interaction between the effects of herbivory and competition is an indirect positive effect of grazing that arose from a reduction in competition experienced by a target plant due to the defoliation of its neighbors. The results of this study, when combined with the known distributions of the six grass species in this region, indicate that herbivory, rather than competition or the physical environment, controls the distribution of at least five of the six species across the landscape. For example, Schizachyrium scoparium grew well in ungrazed flat subplots but very poorly in grazed flat subplots; grazing, not the physical environment or competition, accounts for the absence of S. scoparium from most flat sites in this region. Competition modulates the effects of 'herbivory and edaphic factors on landscape distributions but does not control the distribution of any of the six species.

We studied primary succession on mobile tropical coastal sand dunes over an 8-yr period. Every six months, we monitored changes in species composition and sand movement in permanent quadrats located on the windward slopes, the arms and the crests. Our results indicate that sand movement decreased over time but was significantly higher on the slopes and crests than on the arms. In all cases, there were seasonal fluctuations in sand movement which increased during the period with strong northerly winds and decreased during the rainy season. Sand movement was significantly correlated with species distribution. Plant cover and species richness increased at all three locations. Diversity increased on the slope, decreased on the arms and remained unchanged on the crest. However, the equitability values indicated the dominance of a few species, especially at the end of the study period. Temporal trends and species turnover rates differed among locations. Species turnover occurred first on the arms (1994), then on the crests (1998) and lastly on the slopes (at the end of the study period). In all cases the tall grass, Schizachyrium scoparium var. littoralis, became dominant over the endemic legume, Chamaecrista chamaecristoides var. chamaecristoides. Similar to temperate dunes, primary succession on tropical sand dunes was spatially and temporally heterogeneous.

summary We conducted two experiments to investigate the expression of shade avoidance in response to low ratios of red to far-red irradiation (R:FR) in the C % perennial grass Schizachyrium scoparium at two levels of above-ground (photosynthetic photon flux density (PFD) 400‐700 nm) and below-ground (soil volume) resource availability and in plants of two ages. Young plants showed greater sheath and ramet height in response to low R:FR and old plants showed reduced ramet initiation in experiments one and two, respectively, but both responses were not expressed simultaneously in either group. Growth of all ramet variables, including ramet initiation, was suppressed in small as opposed to large soil volumes. By contrast, architectural variables, but not ramet initiation, were greater for plants grown in low as opposed to ambient PFD. However, expression of shade avoidance to low R:FR was not significantly aected by either level of PFD or soil volume. We must conclude that the levels of resource availability provided do not modulate shade avoidance in this perennial grass. These results demonstrate that S. scoparium is capable of expressing shade avoidance in response to low R:FR, but inconsistent juvenile ramet initiation and architectural responses in plants of dierent ages and phenological stages of development indicate that the shade avoidance response might not be expressed consistently throughout the life of plants.

originated in the USA and led to cultivar development of these species. Blue grama and little bluestem are Inadequate seed supplies of adapted blue grama [Bouteloua gracioutcrossing species (Anderson and Aldous, 1938; Rielis (H.B.K.) Lag. ex Steud.] and little bluestem [Schizachyrium scoparium (Michx.) Nash.] cultivars limits the use of these two species in gel, 1941; Miller, 1967) in which populations with high the northern Great Plains of Western Canada. This study examined levels of genetic diversity are found across a broad geovariation in seed yield traits of 11 blue grama and 14 little bluestem graphical range. McGinnies et al. (1988) found that blue plant collections obtained from southern Manitoba, Canada to facili- grama plants from a single pasture in Colorado showed tate development of improved germplasm of these species. Measure- high levels of variability in number and height of reproments of seed yield traits including harvested air-dried biomass, culm ductive culms, plant biomass, basal diameter, and anthenumber, seedhead number, seed yield, caryopsis weight, and kernel sis date. Coffin and Lauenroth (1992) found that differindex were taken from randomized complete block spaced-plant nurs- ences in seed yield of blue grama between range sites eries. Tests were conducted for 2 yr at Winnipeg and 1 yr at Carman, in north-central Colorado were greatly influenced by MB, Canada. Collections within both species differed significantly for soil texture and animal grazing. McMillan (1956) evaluall traits. Significant collection 3 year (Winnipeg) and collection 3 location interactions were present for biomass and seed yield, and ated 16 little bluestem ecotypes in Kansas and found many of the seed yield-component traits. While significant, the interac- differences in morphology and seed yield between poputions resulted in few changes in rank among collections. Generally, lations obtained from sites ranging from North Dakota the most northern collection in both species showed earlier anthesis, and Colorado to Kansas. He found that the local Kansas produced less biomass, and had lower seed yield than more southern ecotypes produced the highest seed yield; northern ecocollections. These findings indicate that indigenous plant collections types from near Waterton, ND showed earlier anthesis of blue grama and little bluestem show high levels of genetic diversity and low seedset; and southern ecotypes with late matufor biomass, seed yield, and seed yield components, which provide ration succumbed to autumn frosts. Selection in blue opportunity for development of adapted cultivars with enhanced for- grama has improved different seed yield traits such as age and seed production capability for the region. culms per plant (Lovington and Hachita), spikes per culm (Lovington), spikelets per spike (Hachita), and fertility (PM-K1482) (Wilson et al., 1981). Similar stud

In restoration experiments it is imperative to consider the study of mechanisms of how species are maintained and preserved in a system. This paper reports on the results of a field experiment examining the growth and survival of seedlings of Schizachyrium scoparium, a dominant perennial bunchgrass member of the Quercus havardii (sand shinnery oak) communities of semiarid western Texas, on mounds of displaced soil produced by Sylvilagus auduboni (rabbit). The central question posed is: does environmental heterogeneity created by small mammals influence seedling survival and growth? The specific questions addressed are: (1) Does seedling survival, growth, and nutrient uptake vary when grown on mounds, off-mound soils, and artificially created mounds?; (2) What is the influence of the microbial and litter components of mound soils on seedling survival?; and (3) In communities where animal disturbances create environmental heterogeneity and may impact seedling establishment, is it possible to artificially create mounds that could serve a similar function? Results show that characteristics of mound soils increase seedling survival, shoot and root biomass, root length, number of tillers, mycorrhizal infection, and nutrient uptake more in plants grown on mounds than off mounds. Both the microbial and litter components of mound soils are essential components of this effect. Artificial mounds generated from soils associated with the herbaceous community were more similar to intact rabbit mounds than artificial mounds generated from soils associated with the oaks. The results indicate that rabbits produce rich patches (both nutrient and microbial) favorable to the growth of seedlings of the dominant bunchgrass, and point to the potential importance of rabbit disturbances in shaping the dynamics of this plant community. Thus, rabbit-generated disturbances produce environmental heterogeneity in the sand shinnery oak community, similar to that produced by harvester ants in this community as shown by an earlier study. Small animal activity that results in soil displacement, and influences soil characteristics, may indirectly contribute to the persistence of certain plant species within a community. Thus, disturbances may well operate in semiarid communities to produce nutrient and microbe rich microsites which may function to maintain diversity.

Summary 1 Intraclonal regulation of the C4 caespitose perennial grass Schizachyrium scoparium was evaluated in response to various levels of above-ground (radiation intensity) and below-ground (soil volume) resource availability in the field for three successive growing seasons. We reasoned that the relative plasticity of clonal growth in response to various levels of resource availability may provide insight into the mechanism of intraclonal regulation. 2 Six naturally occurring clones were randomly assigned to each of five treatments: (i) unrestricted soil volume (control); (ii) large soil volume in which roots were confined to a cylinder of soil with a radius four times that of the clone; (iii) small soil volume, twice the clone radius; (iv) large soil volume with radiation intensity reduced to 55% of ambient; and (v) small soil volume with reduced radiation intensity. 3 Ramet recruitment increased within the first season following root confinement in large soil volumes, while ramet and leaf growth exhibited less plasticity. The effects of a reduction in radiation intensity were smaller, and, in contrast, increased ramet and leaf growth, but did not modify ramet recruitment. 4 The pattern and magnitude of ramet mortality in clones confined to the large soil volume were similar to those of clones in unrestricted soil volume, indicating that intraclonal regulation was sufficient to reduce ramet recruitment and thus density to within the carrying capacity established by resource availability. 5 Similar growth and ramet demography of clones in the unrestricted and small soil volumes suggest that unrestricted clones normally access resources from a soil volume similar to that of the small soil volumes. 6 Rapid clonal growth in response to root confinement to large soil volumes, resource acquisition from a relatively small soil volume that contains the zone of resource accumulation, and the capacity for rapid expression of ramet plasticity provide circumstantial evidence that individual caespitose clones may respond to resource accumulation in soils beneath them.

1 In this study the potential role of competition in influencing the distribution of three displaced native perennial grasses across complex gradients of plant productivity and species composition was investigated in Michigan old-fields. To do this plant removal and propagule addition experiments were conducted at nine old-field sites to examine the effects of living plant neighbours and litter on seedling establishment and growth of target species in relation to community biomass. 2 For two target species, Andropogon gerardi and Schizachyrium scoparium, living plant neighbours suppressed establishment from seed at most sites, and suppressed the growth of transplants at all sites. 3 Plant litter strongly inhibited the seedling establishment of both Andropogon and Schizachyrium at sites of high community biomass and litter accumulation, but had little impact on the growth rate of transplants at any of the sites. 4 The total suppressive effect of the plant community on seedling establishment and transplant growth of both Andropogon and Schizachyrium increased in magnitude in a non-linear fashion with community biomass. These effects increased in magnitude more rapidly across sites of low to medium biomass than sites of medium to high biomass. 5 The results suggest that these native grasses may be restricted to low productivity habitats within this landscape because of strong competitive interference with establishment by the existing vegetation in the most productive sites.

Random amplified polymorphic DNA (RAPD) markers were used to provide estimates of the comparative genetic variation within and among four native populations of Schizachyrium scoparium. Genotypes were collected from high- and low-fertility sites in both New Jersey (forest biome) and in Oklahoma (grassland biome), USA, and propagated in the greenhouse. Four oligonucleotide primers, 10 bp in length, produced a total of 60 RAPD markers, with the minimum marker difference between any two individuals being 14 markers. Euclidean metric distances were calculated among all individuals, and the analysis of molecular variance (AMOVA) technique was used to apportion the total genetic variation among individuals within populations, populations within fertility levels, populations within biomes, fertility levels, and biomes. Even though most genetic variation resided within populations, statistically significant differences were detected between populations within each biome. Furthermore, genetic distances between high and low fertility levels within biomes were equal to or greater than biome distances. Therefore, in this wide-ranging and highly variable species, RAPD analysis suggests that local site differences in fertility and ecological history can promote genetic differentiation equal to or greater than geographical differentiation.

Plant-soil feedbacks can alter N cycling rates in terrestrial ecosystems, but the mechanistic relationship between species characteristics, soil properties, and N dynamics is unclear. Plant species may affect patterns of soil aggregation, which can affect soil C and net N mineralization. This mechanism was examined in two common garden experiments : one containing five tree species (European larch [Larix decidua Miller], red oak [Quercus rubra L.], red pine [Pinus resinosa Ait.], white pine [Pinus strobus L.], and Norway spruce [Picea abies (L.) Karst]) and one containing six grass species (big bluestem [Andropogon gerardi Vitm.], indiangrass [Sorghastrum nutans (L.) Nash], prairie sandreed [Calamovilfa longifolia (Hook) Scrib.], switchgrass [Panicum virgatum L.], little bluestem [Schizachyrium scoparium (Michx.) Nash.], and sideoats grama [Bouteloua curtipendula (Michx.) Torr.]). The grass monocultures are burned annually. Soils were wet sieved into four size classes (>2000, 250-2000, 53-250, and 2000 μm) C concentration ranged from 15 to 26 g kg -1 , and was lowest for Norway spruce and red oak (P = 0.07). Aggregate WMD correlated weakly (and negatively) with potentially mineralizable N (r = -0.57) and in situ net N mineralization (r = -0.67), but positively (again weakly) with potentially mineralizable C (r = 0.49). Grass species had no effect on aggregate-size distribution or organic matter concentration in spite of twofold differences in root biomass and threefold differences in N cyding rates. Species-induced changes in soil aggregation explained little of the variation in whole-soil C and N cycling rates, and are therefore unlikely to be an important mechanism explaining species effects on ecosystem processes.

1 A series of three experiments was conducted with three C4 perennial grasses, Panicum virgatum (short rhizomes forming distinct clones), Schizachyrium scoparium (caespitose) and Bouteloua gracilis (caespitose), to determine whether intraclonal regulation was characterized by interdependence, independence or a combination of both processes. Specific hypotheses tested were: (i) all ramets within individual clones are physiologically integrated; (ii) ramet hierarchies (several connected generations) within individual clones are capable of acquiring soil nitrogen equally from localized nutrient-rich pulses; and (iii) nitrogen is retained within individual hierarchies through allocation to sequentially developed ramet generations. 2 Mean 15N atom percent excess values within labelled ramet hierarchies were 10–21-fold greater than in immediately adjacent ramet hierarchies, and 105–137-fold greater than in the remaining ramet hierarchies within clones for all three species. This pattern of 15N distribution demonstrates that physiological integration does not occur among all ramets within individual clones and that ramet hierarchies function as integrated physiological individuals. 3 Ramets in closer proximity to a 15N pulse in soils acquired disproportionately greater amounts of the isotope compared to more distant ramets within clones for all three species. Inequitable resource acquisition among ramet hierarchies establishes the potential for asymmetric intraclonal competition below-ground and the development of size inequalities among hierarchies. 4 Approximately 21.7%, 3.5%, 1.7% and 2.3% of the initial 15N pool was allocated from the first ramet generation to the second and third (spring cohort), third (autumn cohort) and fourth generations, respectively. However, the magnitude of nitrogen allocation from parent to juvenile ramet generations was relatively consistent, with 17–32% of the 15N pool within parental ramets allocated to juvenile ramets. Intergenerational 15N allocation in S. scoparium clones therefore demonstrates a high degree of interdependence within individual ramet hierarchies. 5 Clones of the rhizomatous and two caespitose grasses investigated functioned as assemblages of autonomous ramet hierarchies, but ramet generations within individual hierarchies were effectively integrated. Consequently, intraclonal regulation in these temperate, perennial grasses is characterized by both ramet interdependence and independence. The occurrence of ramet independence throughout much of the life history of perennial grasses demonstrates the ecological importance of clonal fragments to genet and population maintenance and emphasizes that this may be the most relevant scale to investigate further the ecology of this important and widely distributed group of clonal plants.

We investigated the effects of prescribed fire, herbicide treatment, and sod removal on the eradication of exotic grasses and the establishment of native plant species in 24 experimental restoration plots in three razed residential sites within the boundary of Indiana Dunes National Lakeshore. During 1992-1995, herbicide treatment and sod removal decreased the combined cover of Poa pratensis (Kentucky blue grass) and Agropyron repens (quackgrass) significantly (from 82% to 13%, and 85% to 8%, respectively), whereas fire did not suppress such exotic lawn grasses. In 1993, several opportunistic species, represented by Cyperus spp. (umbrella sedges), Digitaria sanguinalis (crab grass), and Ambrosia artemisiifolia (common ragweed), filled the gaps left by the removal of lawn grasses. For the same period, Detrended Correspondence Analysis revealed a clear vegetation divergence between the control-fire plots and the herbicide-sod removal plots. While Poa pratensis and Agropyron repens continued to dominate the control and fire plots, the planted native species, represented by Schizachyrium scoparium (little blue-stem), Sorghastrum nutans (Indian grass), Rudbeckia hirta (black-eyed Susan), and Monarda punctata (horsemint), began to dominate in the herbicide and sod removal plots from 1994. In both herbicide and sod removal plots, the ground cover of grasses (68%) was much higher than the forbs (10%). The herbicide plots, where exotic species were removed but nitrogen-rich top soils were not removed, showed a higher diversity of planted native species than the sod removal plots (where both exotic species and top soils were removed) and the control-fire plots (where neither was removed). This finding suggests that an optimum but not excessive concentration of soil nitrogen is needed to support a maximum species diversity in such infertile substrate as sandy soil. In addition, the decrease in potassium in all plots, regardless of treatment, suggests that potassium may become a limiting factor for our restored native vegetation.

Plants with the C4 photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO2 is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C4 grasses would respond more to elevated CO2 treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO2 concentration of either 350 or 700 µl l−1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO2 (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO2 was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO2 (Ci) curves revealed that leaf photosynthesis was not saturated at 350 µl l−1 CO2 in any of these C4 grasses. Assimilation of elevated CO2-grown A. gerardii was higher than in plants grown in ambient CO2. In contrast, B. gracilis grown in elevated CO2 displayed lower A, a trait more commonly reported in C3 plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:Ci curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO2. Our study shows that C4 grasses have the capacity for increased growth and photosynthesis under elevated CO2 even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO2 for B. gracilis.

While stomatal closure usually limits photosynthesis during drought, our previous results suggest that drought-related decreases in foliar nitrogen concentration (N L ) limit photosynthesis during recovery from drought in prairie grasses. Here we estimate the importance of decreases in N L to decreased photosynthetic capacity (PS cap ) during drought and a subsequent recovery period in three perennial C 4 prairie grasses. PS cap (O 2 evolution at light and CO 2 saturation) decreased 69 to 78% during drought in these grasses, and full recovery of PS cap required 8 to 12 days, until younger leaves were expanded or older leaves were repaired, depending on species. Decreases in N L explained 38 to 51% of the loss of PS cap during drought and accounted for 51 to 69% of the total loss of PS cap integrated over the post-drought recovery period. N-related loss of PS cap appeared to result more from decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (4.1.1.31), and other soluble photosynthetic enzymes, than from decreases in thylakoid N-containing compounds. Decreases in quantum yield of 02 evolution and F V /F m (variable-to-maximum fluorescence of dark-adapted leaves) during drought were small, so we assumed that little damage to photsystem II (PSII) and thylakoid membrane function occurred. Further, F o (minimum F) decreased or remained unchanged, dark F o was greater than light F o , and decreases in photochemical quenching (the fraction of oxidized PSII) were reversed within 1-3 days after drought. Therefore, prolonged increases in non-photochemical quenching (q n ; thermal dissipation of excess light energy) during and after drought were indicative of protective downregulation and were likely associated with disproportionate loss of soluble photosynthetic proteins during drought. In support of this, post-drought recovery of q n paralleled recovery of N L and PS cap . Thus, in C4 prairie grasses, loss of PS cap during drought is largely the result of decreases in shoot N L and of associated protective downregulation, decreasing carbon assimilation for 1-2 weeks after drought.

In response to a paucity of data on the chemical toxicity of uranium (U) to plants, a factorial experiment employing five U concentrations (0, 50, 500, 5000, 25,000 mg kg{sup -1}) and three moisture regimes (low, medium, and high) was performed using three native grasses. Buchloe dactyloides (buffalograss; mid/late-seral), Schizachyrium scoparium (little bluestem; late-seral), and Aristida purpurea (purple threeawn; early/mid-seral) were grown in monocultures and as a mixture of two species under all combinations of U and moisture levels. This design allowed for the analysis of U effects, as well as possible interactions with moisture stress. Several measures of plant health and viability were made, including: percent emergence, plant survival, shoot biomass, and number and weight of inflorescences. Decreases in plant biomass, fecundity, and long-term survivability were observed only at the highest U level (25 000 mg kg{sup -1}). No significant differences (P < 0.05) between the U treatment levels were observed in terms of seedling emergence and survival. Drought stress also negatively impacted survival and biomass, but acted independently of U stress. 18 refs., 4 figs., 3 tabs.

Aboveground production and shoot and root litter decomposition rates were measured in amended coarse taconite tailing plots over the second and third growing seasons of a 3-yr study. Plots were seeded with a mixture of native prairie species that included Andropogon gerardii Vitm., Schizachyrium scoparium (Michx.) Nash, Elymus canadensis L., Bromus kalmii Gray, and Lespedeza capitata Michx. Reclamation treatments included inoculation with indigenous mycorrhizal fungi, three rates of fertilizer, and three rates of composted yard waste. Aboveground plant biomass in plots amended with 44.8 Mg ha -1 compost increased from 150 g m -2 in the second season to 330 g m -2 in the third season. The litterbag technique was used to assess the influence of reclamation treatments on the decomposition of plant litter. Shoot litter placed on the surface lost 60 to 70% of its original mass after 15 mo and was unaffected by treatment during the first season but was slower in plots amended with 44.8 Mg ha -1 composted yard waste in the second season. Buried root tissue lost 40% of its original mass after 15 mo and was unaffected by treatment. By significantly increasing plant biomass and reducing litter decomposition rate in tailing, additions of 44.8 Mg ha -1 compost should result in the continued accrual of soil organic matter and begin to meet reclamation goals to establish self-sustaining native plant communities on tailing deposits.

Few empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data. Both MAP and MAT were significantly related to annual above-ground net primary production (ANPP). MAP explained 54% to 89% of the variation in ANPP of two C4 short-grasses, Bouteloua gracilis and Buchloe dactyloides, and two C4 tall-grasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19% to 41% of the variation in ANPP of two C4 grasses, B. gracilis and B. dactyloides, and 41% to 66% of the variation in ANPP of two C3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short-grasses, tall-grasses, C3, C4). Our empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.

In tallgrass prairie, high plant species diversity results not from a large number of grass species, but from a large number of forb (nongrass, herbaceous) species. Forbs exhibit morphological, life history, and ecophysiological characteristics that contrast sharply with those of the dominant C4 grasses. Success of the subdominant forbs varies strongly with topographic position and burning regime, and landscape scale patterns of abundance are well documented. But comparatively little is known about the mechanisms determining these patterns in persistent tallgrass prairie forbs. To elucidate these mecha- nisms, (1) leaf-level physiological characteristics of the dominant C4 grass, Andropogon gerardii, and four co-occurring C3 forbs were measured in response to natural and exper- imentally manipulated gradients of N availability, and (2) seasonal light environments of forbs in contrasting topographic positions and burning regimes and their morphological and physiological responses in these environments were compared to determine whether resource availability and utilization patterns contributed to patterns of forb distribution and abundance. The effects of burning regime and topographic position on maximum rate of photosynthesis (A) and stomatal conductance to water vapor (g) measured at the leaf level were not consistent with patterns of forb abundance. Nitrogen did not appear to limit forb physiological processes, even though increased N availability resulted in higher tissue N concentrations and greater biomass. There was no consistent increase in A or decrease in g in response to fertilization. However, A at low light levels was as much as 67% higher in fertilized Vernonia baldwinii and A. gerardii compared to unfertilized plants. Greater light availability to forbs in the canopy was associated with lower grass biomass production in uplands compared to lowlands and in unburned compared to burned sites. Forbs did not appear to adjust morphologically (leaf area and plant height) to different light environments at different sites. As a result, as much as 90% of forb leaf area in the burned lowland was displayed in low light, whereas as little as 30% of forb leaf area was in low light in the uplands at midseason. Estimates of potential whole-plant carbon uptake, based on leaf area distribution relative to available light and A as a function of light availability, agreed well with patterns of forb abundance and production. Differences in light availability may ac- count for much of the variability in forb abundance related to burning regime and topo- graphic position by limiting carbon gain in forbs more in burned lowlands than in other sites.

Herbivore-induced shifts in species composition have been documented from grasslands throughout the world, but the mechanism(s) of species replacement remains largely unexplored. An experiment was conducted in a transplant garden, on the campus of Texas AM however, tiller number per plant was suppressed by B. saccharoides, but not by S. leucotricha or conspecific neighbors. Defoliation of S. scoparium plants, but not neighbors, negatively impacted the late-seral plants. Selective defoliation of S. sco- parium plants significantly reduced tiller variables of mean mass, leaf blade area, and leaf number, but did not significantly reduce plant variables including mean basal area, tiller number, or annual shoot production. Defoliation of both S. scoparium plants and neighbors increased annual shoot production, mean basal area per plant, mean tiller leaf area, leaf number, tiller mass, stomatal conductance to H20 vapor, and plant xylem pressure potential in comparison with S. scoparium plants grown with comparable, nondefoliated neighbors. An increase in both plant and tiller variables in defoliated S. scoparium plants grown with uniformly defoliated neighbors establishes that replace- ment of a late-seral dominant is not driven by a greater relative expression of herbivory tolerance of mid-seral species. These results collectively suggest that the late-seral dominant, S. scoparium, pos- sesses a greater competitive ability and a comparable or greater degree of herbivory tolerance than the mid-seral species that comprise the community. Therefore, the initial hypothesis was rejected. It can be inferred that the alternative mechanism, selective herbivory of the late-seral dominant, is the dominant mechanism contributing to species replacement. Herbivore-induced modifications of competitive interactions are most like- ly to drive species replacement in grasslands characterized by high and consistent resource availability. This may partially explain why condition and trend analysis was developed and initially implemented in the true and mixed prairie associations of North America and why it is widely used by rangeland managers in these grasslands.

This study was designed to identify the critical life-history stages and ecological processes responsible for variation in rates of tree invasion in utility rights-of-way in the Hudson Valley of New York. We used repeated censuses to estimate natural rates of tree seedling establishment, growth, and survival in the 12 major plant communities found along rights-of-way in the region. An individual-based model was used to integrate our field data and calculate measures of the rate of tree invasion in each plant community. The most common tree invaders were species with large, wind-dispersed seeds (i.e., Acer rubrum and Fraxinus americana) although these were rarely the most common tree species in the forests bordering the corridors. In narrow ({approx}30-50 m wide) rights-of-way there was little relationship between seedling density and distance to the nearest seed sources; however, in wide corridors, seed dispersal clearly limited tree invasion. Annual variation in seed production in adjacent forests was also a major factor in the high spatial and temporal variability in tree seedling establishment. Growth rates of newly established seedlings did not vary significantly among the different communities; however, seedling growth rates increased dramatically once the seedlings emerged above shrub or herb-dominated canopy. Thus, the duration ofmore » competition (i.e., the number of years required by a seedling to emerge above the shrub or herbaceous canopy) appears to have been more important than the initial intensity of competition in determining variation among communities in resistance to tree invasion. As a result, shrub communities generally had high resistance to invasion. Among herbaceous communities, the highest resistance to invasion occurred in communities dominated by the grass Schizachyrium scoparium (little bluestem) on poor soils. 37 refs., 6 figs., 5 tabs.« less