Seed and microsite limitation in Rheum nobile,a rare and endemic plant from the subnival zone of Sino-Himalaya

Background: Species persistence, particularly in monocarpic species, depends on the successful recruitment of individuals. An understanding of the factors that limit the recruitment of rare monocarpic plant species is therefore vital for their conservation.

Aims: To identify the factors limiting the recruitment of Rheum nobile, a rare and highly specialised monocarpic giant herb endemic to the high eastern Himalayas.

Methods: Seed sowing (seeds added or not added) and seedling transplanting experiments were conducted in disturbed (vegetation removed) and undisturbed plots in the vicinity of established populations of R. nobile to explore the mechanisms of recruitment limitation. Four levels of photosynthetically active radiation (0, 15, 30 and 50 μmol m^?2 s^?1) and two sowing positions (beneath and above grass litter or moss layer) were manipulated in the laboratory to determine how ground cover limited seedling emergence.

Results: Seed addition increased seedling recruitment. Disturbance significantly increased seedling emergence and establishment. Seed germination significantly decreased with the reduction of light availability, but 31.7% of all seeds germinated in complete darkness. Seedling emergence was close to zero when seeds were sown on top of a layer of grass litter or moss, but rose to 34.5% when the seeds were sown beneath such layers.

Conclusions: Our results indicate that the recruitment of R. nobile is limited by a combination of seed and microsite availability. Therefore, in order to conserve this species, we suggest adding seeds to suitable sites and implementing soil disturbances in existing populations to create suitable microsites.     

Establishment and spread of founding populations of an invasive thistle: the role of competition and seed limitation

Successful plant invasions require both the founding and local spread of new populations. High plant densities occur only when founding plants are able to disperse their seeds well locally to quickly colonize and fill the new patch. We test this ability in a 7-year field experiment with Carduus acanthoides, an invasive weed in several North American ecosystems. Founder plants were planted in the center of 64 m² plots and we monitored the recruitment, distribution pattern, mortality, and seed production of the seedlings that originated from these founding plants. Competing vegetation was clipped not at all, once, or twice each year to evaluate the importance of interspecific competition. More seedlings recruited in the intermediate once-clipped plots, and these seedlings also survived better. The control plots had fewer microsites for seedling recruitment; clipping a second time in September stimulated grasses to fill up the gaps. The number of C. acanthoides recruits and their median distances from the founder plants were also explained by the initial seed production of the founding plants. Overall, the experiment shows that the success of founder plants can fluctuate strongly, as 55% of the plots were empty by the sixth year. Our study suggests that the local invasion speed following initial establishment depends strongly on both the propagule pressure and availability of suitable microsites for seedling recruitment and growth.     

Effects of plant competition,seed predation,and nutrient limitation on seedling survivorship of spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis>)

We measured seed germination and seedling survivorship of spotted knapweed, Centaurea stoebe, in a series of laboratory and field experiments to evaluate the efficacy of seed limitation as a management focus. This work was initiated 6 years after introduction of several biological control agents. The soil seed bank of the site used in this study contained a mean density of 5,848 seeds/m² (ranging from 0 to 16,364 seeds/m²), and 92% of the seeds isolated from soils were shriveled, discolored, and/or partially decayed. Additionally, none of the intact seeds germinated, suggesting that the viable seed bank at our field study site has been exhausted. Centaurea stoebe seeds were planted into pots under a range of soil nitrogen (N) availability, with half of the pots containing a single density of previously established seedlings of a native cool-season grass, slender wheatgrass (Elymus trachycaulus). A watering regime mimicking local precipitation was applied. Spotted knapweed exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Surprisingly, low soil N conditions improved knapweed survivorship in the presence of grasses. Nevertheless, recruitment and biomass were still far below the levels reached in the absence of competition. To evaluate the effect of density on successful recruitment, Centaurea stoebe seed was introduced into a meadow at three densities matching reduced levels of seed production under the constraints of seed predators. These densities were sown with or without a seed mixture of native species, into an existing plant community lacking C. stoebe, and seedling recruitment was recorded over 2.5 years. Across all plots and densities sown (568–2,272 seeds m⁻² year⁻¹), seedling recruitment was less than 1%. The invasion potential of spotted knapweed was greatly diminished when realistic levels of plant competition and biological control limit seed production. We therefore conclude that a combination of seed limitation and shortage of ‘safe sites’ within undisturbed vegetation can limit densities of C. stoebe.     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Disturbance,drought and dynamics of desert dune grassland,South Africa

A seven-year study of marked plants and plots in Stipagrostis ciliata (Desf.) de Winter dune grassland, in the arid (<100 mm yr^–1) Bushmanland area of the Northern Cape province of South Africa, was designed to test the hypothesis that establishment of ephemeral plants, and recruitment of perennial grasses was dependent upon disturbances that reduced the density of living perennial grass tussocks. In 1989, eight 4 m² plots were cleared of perennial vegetation by uprooting and removing all plants so as to resemble small-scale disturbances made by burrowing mammals or territorial antelope. The vegetation on the cleared plots and surroundings was monitored until 1996. Initial results supported our hypothesis. In wet years, when ephemeral plants were abundant, their average fresh mass was 2–3 times greater per unit area on the cleared plots than in control plots in adjacent, undisturbed grassland. Many Stipagrostis seedlings established in the cleared plots over the two years following clearing but were rare in adjacent areas among established conspecifics. However, a drought in 1992 (11 mm of rain over 12 months) lead to widespread mortality of the perennial grass, killing 56% (range 22–79%) of established tufts. High densities of Stipagrostis seedlings appeared following the drought-breaking rains in January 1993, both in the disturbed plots and in the surrounding `undisturbed' dune grassland. Ephemeral plants established in large numbers throughout the area during the high rainfall year of 1996 and were generally more numerous in the old disturbances than in control plots. Seven years after clearing the biomass of grass on the cleared plots was approximately 34% of the mass removed from the plots in 1989 whereas in the undisturbed grassland biomass was 66% of 1989 levels. Drought had little long-term effect on community composition, and Stipagrostis ciliata constituted 94–98% of plant community before and after drought. Cleared plots were recolonised by S. ciliata, but the contribution of other grass species increased by 6–9%. Synchronous recruitment following occasional drought-induced mortality can generate even-aged populations of the dominant desert dune grasses.     

Abundance and diaspore weight in rare and common prairie grasses

Summary Abundance (g/m²) and diaspore weight are positively correlated in seven species of perennial grasses that occur in prairies. The rare grasses (<10.0 g/m²) have light dispersal units (0.06 to 1.76 mg); the common grasses (>10.0 g/m²) have heavy dispersal units (2.23 to 2.80 mg). This result is the first reported differentiating trait between related rare and common organisms occurring in same habitat.Three hypotheses that explain this phenomenon are compared; the third most likely holds. First, rare grasses may be rare because their small seeds are less successful in establishment than those of common grasses. Second, if the persistence of small populations is marginal, rare grasses may devote less energy (or other currency) to seed production. Third, rare grasses may be colonizers of spatially and temporally rare microsites appropriate for growth and thus have seeds adapted for longer distance dispersal than those of common grasses. This last hypothesis suggests a new pathway for the evolution of weeds.     

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.     

Effects of soil disturbance,fire and litter accumulation on the establishment of Cistus clusii seedlings

Cistus clusii is an early successional shrub, the most drought-resistant species of Cistus which colonises perturbed areas in the southern Iberian Peninsula and regenerates entirely from seeds after a fire. Some of the factors controlling germination and seedling establishment were examined in a field experiment in which we tested the effect of soil disturbance, fire, and litter accumulation on seedling emergence. In a first experiment, soil disturbance and fire were applied to plots in the field with a factorial design in April 1992. In a second experiment, we measured the effect of adding C. clusii litter. The number of seedlings, annual plants and grasses, and the coverage of mosses were recorded in 1993 and 1994.Soil disturbance increased the number of C. clusii seedlings almost seven-fold but did not change the number of grasses, annual plants nor moss cover. Fire significantly increased the number of C. clusii seedlings only in combination with soil disturbance, and it did not affect the presence of annuals, grasses nor moss cover. Litter increased the number of C. clusii seedlings in the following two years.Overall, soil disturbance as a single factor had the most significant effect on seedling emergence, though plots disturbed and treated with fire had the highest number of seedlings, implying that germination of C. clusii seeds was enhanced by processes that alter the hard seed coat and break the physical dormancy imposed by the testa. In addition, recruitment of C. clusii was dependent on rainfall, as drought significantly reduced the number of seedlings appearing in winter.     

Carbon isotope composition of C4 grasses is influenced by light and water supply

The carbon isotope composition of C⁴ grasses has the potential to be used as an indicator of changes in the isotopic composition and concentration of atmospheric CO2, especially for climate reconstruction. The usefulness of C₄ grasses for this purpose hinges on the assumption that their photosynthetic discrimination against ¹³C remains constant in a wide range of environmental conditions. We tested this assumption by examining the effects of light and water stress on the carbon isotope composition of C₄ grasses using different biochemical subtypes (NADP-ME, NAD-ME, PCK) in glasshouse experiments. We grew 14 different C₄ grass species in four treatments: sun-watered, sun-drought, shade-watered and shade-drought. Carbon isotope discrimination (Δ) rarely remained constant. In general, Δ values were lowest in sun-watered grasses, greater for sun-drought plants and even higher for plants of the shade-watered treatment. The highest Δ values were generally found in the most stressed grasses, the shade-drought plants. Grasses of the NADP-ME subtype were the least influenced by a change in environmental variables, followed by PCK and NAD-ME subtypes. Water availability affected the carbon isotope discrimination less than light limitation in PCK and NAD-ME subtypes, but similarly in NADP-ME subtypes. In another experiment, we studied the effect of increasing light levels (150 to 1500 μmol photons m^?2 s^?1) on the Δ values of 18 well-watered C₄ grass species. Carbon isotope discrimination remained constant until photon flux density (PFD) was less than 700 μmol photons m^?2 s^?1. Below this light level, Δ values increased with decreasing irradiance for all biochemical subtypes. The change in A was less pronounced in NADP-ME and PCK than in NAD-ME grasses. Grasses grown in the field and in the glasshouse showed a similar pattern. Thus, caution should be exercised when using C4 plants under varying environmental conditions to monitor the concentration or carbon isotopic composition of atmospheric CO2 in field/glasshouse studies or climate reconstruction.     

Roles of cottony hairs in directed seed dispersal in riparian willows

Willows usually establish on wet substrates with fine sediments at sites that are created by large disturbances, but suitable microsites are spatially and temporally limited. Thus, we hypothesized that willow seeds are selectively dispersed to suitable microsites, such as those with a wet substrate, rather than unsuitable microsites, such as those with a dry substrate, with seedling establishment mediated by the cottony hairs attached to seeds (directed dispersal). To test our hypothesis, we compared several recruitment-related traits, including buoyancy, germination, and trapping at favorable microsites, in seeds of the riparian willows Salix sachalinensis and S. integra with and without cottony hairs in laboratory and field experiments. In both field and laboratory experiments, more seeds with cottony hairs were trapped in water and wet sand than in dry sand, in which no seeds of either species germinated. These results indicate that cottony hairs facilitate the recruitment of seeds to microsites favorable for seed germination and help seeds avoid unfavorable microsites. On the water surface, 17.6% of S. sachalinensis seeds and 68.0% S. integra seeds with cottony hairs floated for more than 6 days, whereas all seeds without cottony hairs sank immediately after being placed on the water surface. These results suggest that cottony hairs facilitate long-distance dispersal via flowing water and also help avoid germination under water, where willow seedlings fail to establish. Seeds of the two willow species were released from the cottony hairs and germinated immediately after the seeds were placed on wet sand, but not after placement on water or dry sand. These results suggest that the seeds are released from the cottony hairs when the hairs become wet and the seeds are striking to a suitable microsite for seedling establishment, such as wet sand. In riparian willows, the cottony hairs promote directed dispersal by moving seeds to discrete and predictable microsites where the seedling establishment is disproportionately high.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical disturbance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m^–2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.     

Effects of experimental and real land use on seedling recruitment of six fen species

Seedling emergence and early establishment of six fen species differing in seed mass and growth form were investigated under experimental land use with changed vegetation structure and under real land use in a calcareous fen. Seeds of all six species were sown in plots with different experimental land-use treatments: summer and autumn mowing with or without litter removal, trampling and abandonment. Additionally, emergence and survival of experimentally sown seeds was investigated under real land use on adjacent sites managed by mowing, grazing, intense trampling or abandonment.On abandoned plots and on plots without litter removal of the land-use experiment, emergence rates of all species were negatively affected either by high litter and moss cover or by tall canopy. No differences were found between autumn and summer mowing. Gap creation by experimental trampling did not increase germination rate. Under real land use, establishment of seedlings of most species was positively affected by litter cover and tall canopy. Trampling, in contrast, had a severe negative effect on seedling survival.The investigated species differed in their germination ability which was tested in the germination chamber and in their response to land use. Succisa pratensis with the highest seed mass germinated well in the chamber and in the field more or less regardless of land use. The low germination rate of Parnassia palustris in the germination chamber indicated a limitation of viable seeds. In the field, however, seedling emergence was additionally limited by microsite availability. Seeds of Serratula tinctoria and Primula farinosa germinated well in the germination chamber, but seedling recruitment in the field was hampered in the presence of a high litter or moss cover. Seeds of Tofieldia calyculata and Pinguicula vulgaris were strongly dependent on the availability of suitable microsites in the field. They hardly germinated under natural conditions, in spite of a high number of germinable seeds in the germination chamber.     

Are plant populations seed limited? A critique and meta-analysis of seed addition experiments

We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (establishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta-analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degree are populations seed and establishment limited? (2) Under what conditions (e.g., habitats and life-history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. However, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large-seeded species, species in disturbed microsites, and species with relatively short-lived seed banks. Most seed addition experiments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be overcome by increasing the number and range of seed addition treatments.     

Effects of rain, nitrogen, fire and grazing on tree recruitment and early survival in bush-encroached savanna, South Africa

Moisture, nutrients, fire and herbivory are the principal factors governing tree–grass cover ratios of savannas. We investigated tree (Acacia mellifera) recruitment after fire and under conditions of maximum-recorded rainfall, nitrogen addition and grazing in a completely-crossed field experiment. We employed a similar garden experiment with the exception of the fire treatment. Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that only received natural rain, and insufficient watering frequency in irrigated plots. Due to low germination in the field experiment, no treatment significantly affected tree recruitment. In the garden experiment, frequent watering, nutrient control (i.e. no nitrogen addition) and grazing enhanced tree recruitment with significant interactions between rain, nitrogen and grazing. We infer that above-average rainfall years with frequent rainfall events are required for mass tree recruitment. Grass defoliation makes space and resources available for tree seedlings. Nitrogen enrichment increases the competitive ability of fast-growing grasses more than that of the N₂-fixing tree component. In contrast to conventional wisdom that grazing alone causes encroachment, we suggest that there are complex interactions between the above-mentioned factors and ‘triggering’ events such as unusually high rainfall.     

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.     

Revegetation Methods for High-Elevation Roadsides at Bryce Canyon National Park, Utah

Establishment of native plant populations on disturbed roadsides was investigated at Bryce Canyon National Park (BCNP) in relation to several revegetation and seedbed preparation techniques. In 1994, the BCNP Rim Road (2,683–2,770 m elevation) was reconstructed resulting in a 23.8‐ha roadside disturbance. Revegetation comparisons included the influence of fertilizer on plant establishment and development, the success of indigenous versus commercial seed, seedling response to microsites, methods of erosion control, and shrub transplant growth and survival. Plant density, cover, and biomass were measured 1, 2, and 4 years after revegetation implementation (1995–1998). Seeded native grass cover and density were the highest on plots fertilized with nitrogen and phosphorus, but by the fourth growing season, differences between fertilized and unfertilized plots were minimal. Fertilizers may facilitate more rapid establishment of seeded grasses following disturbance, increasing soil cover and soil stability on steep and unstable slopes. However the benefit of increased soil nutrients favored few of the desired species resulting in lower species richness over time compared to unfertilized sites. Elymus trachycaulus (slender wheatgrass) plants raised from indigenous seed had higher density and cover than those from a commercial seed source 2 and 4 years after sowing. Indigenous materials may exhibit slow establishment immediately following seeding, but they will likely persist during extreme climatic conditions such as cold temperatures and relatively short growing seasons. Seeded grasses established better near stones and logs than on adjacent open microsites, suggesting that a roughened seedbed created before seeding can significantly enhance plant establishment. After two growing seasons, total grass cover between various erosion‐control treatments was similar indicating that a variety of erosion reduction techniques can be utilized to reduce erosion. Finally shrub transplants showed minimal differential response to fertilizers, water‐absorbing gels, and soil type. Simply planting and watering transplants was sufficient for the greatest plant survival and growth.     

Establishing Tallgrass Prairie on Grazed Permanent Pasture in the Upper Midwest

The goal of the study was to learn whether native prairie grasses and, eventually, a diverse mixture of native forbs could be incorporated in permanent pastures by means of rotational grazing by cattle. An experiment was established on a farm in northeastern Iowa on a pasture that had never been plowed but had been grazed since the 1880s. One treatment was protected from grazing to test for the presence of remnant vegetation. Andropogon gerardii, Sorghastrum nutans, Panicum virgatum, and Desmanthus illinoensis were introduced in plots first treated with glyphosate; seeds were either drilled (DR) or hand-broadcast and incorporated by controlled cattle trampling (BT). Seedling establishment and aboveground biomass were followed over 3 years. There was no evidence for remnant native plants on uplands, but seven species of native forbs and four native graminoids flowered in exclosures erected within waterways. D. illinoensis initially established up to 12 seedlings/m² but had disappeared from all but one plot by the third year. Variation in native grass establishment among replicate plots within treatments was very high, ranging initially from 0.2 to 9.9 plants/m². In August of the second year, native grasses made up only 8% of the available forage in DR plots and 1% of BT plots. One year later, however, native grasses made up 56% of the available forage in DR plots and 37% of BT plots, and these differences were significant (p = 0.05). A pilot study seeded in late winter (frost seeding) suggested that seeds spread after cattle trampling produced five times more seedlings (2.5/m²) than seeds spread before cattle trampling (0.5/m²). Frost seeding had advantages because it did not require herbicide for sod suppression or tractor access to the site. New plantings could be safely grazed in early spring and late fall, before and after most native grass growth, to offset the negative economic impact of protecting new plantings from burning during the growing season. But this practice precluded subsequent prescribed burning. I propose a strategy for incorporating native wildflowers into the pasture over time with minimum cost.     

Agronomic factors in the establishment of tetraploid seeded Miscanthus × giganteus

To meet US renewable fuel mandates, perennial grasses have been identified as important potential feedstocks for processing into biofuels. Triploid Miscanthus × giganteus, a sterile, rhizomatous grass, has proven to be a high‐yielding biomass crop over the past few decades in the European Union and, more recently, in the United States. However, high establishment costs from rhizomes are a limitation to more widespread plantings without government subsidies. A recently developed tetraploid cultivar of M. × giganteus producing viable seeds (seeded miscanthus) shows promise in producing high yields with reduced establishment costs. Field experiments were conducted in Urbana, Illinois from 2011 to 2013 to optimize seeded miscanthus establishment by comparing seeding rates (10, 20, and 40 seeds m^?2) and planting methods (drilling seeds at 38 and 76 cm row spacing vs. hydroseeding with and without premoistened seeds) under irrigated and rainfed conditions. Drought conditions in 2011 and 2012 coincided with stand establishment failure under rainfed conditions, suggesting that seeded miscanthus may not establish well in water‐stressed environments. In irrigated plots, hydroseeding without premoistening was significantly better than hydroseeding with premoistening, drilling at 38 cm and drilling at 76 cm with respect to plant number (18%, 54%, and 59% higher, respectively), plant frequency (13%, 30%, and 40% better, respectively), and the rate of canopy closure (18%, 33%, and 43% faster, respectively) when averaged across seeding rates. However, differences in second‐year biomass yields among treatments were less pronounced, as plant size partially compensated for plant density. Both hydroseeding and drilling at rates of 20 or 40 seeds m^?2 appear to be viable planting options for establishing seeded miscanthus provided sufficient soil moisture, but additional strategies are required for this new biomass production system under rainfed conditions.     

Requirement of vegetation gaps for seedling establishment of two unpalatable grasses in a native grassland of central Argentina

Abstract The objective was to determine the effects of root and shoot competition on seedling establishment of the unpalatable grasses Stipa gynerioides and S. tenuissima in a native grassland of central Argentina dominated by the palatable grass S. clarazii. Seeds of the two unpalatable species were sown in natural occurring microsites with shoot and root competition from the palatable species, and in artificially created microsites without either shoot competition or shoot and root competition. In addition, fresh seeds of the unpalatable species were subjected to daily alternating temperatures under laboratory and field conditions to determine the effect on seed dormancy and germination. Seedling establishment of S. gynerioides and S. tenuissima occurred only in microsites without shoot and root competition. Also, the fluctuation of temperature near the soil surface in these microsites reduced dormancy and promoted rapid germination in both species. Our results support the hypothesis that, in swards dominated by palatable grasses, vegetation gaps of low competitive pressure favour seedling establishment of unpalatable grasses. It is suggested that the creation of these gaps by overgrazing can be an important mechanism in the process of species replacement in native grasslands.