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.     

Ecological relationships between seedling emergence and litter cover in the earliest stage of plant succession on sandy soil

The relationships between seedling emergence and litter cover were studied in the earliest successional stage of a plant community. During a period of 3 years, changes in vegetation cover and species composition were assessed on three permanent quadrats with virgin sandy soil substrates A successional pattern from summer annual grasses to winter annual herbacoeus plants to biennial and perennial plants was confirmed, and invasion and replacement of the component species were conspicuous. The spatial distributions of seedlings and litter were heterogeneous in winter, and some patches consisting of both seedlings and litter were observed. There was a strong positive correlation between distribution patterns of seedlings and litter; seedling density increased with increasing litter cover. The results suggest that the litter cover of the previous stage plays an imporrant role in promoting successional changes in the early stage of plant succession.     

Seedling emergence patterns and dormancy/germination physiology of <Emphasis Type="Italic">Primula modesta</Emphasis> in a subalpine region

On the basis of the germination/dormancy responses of seeds to temperature and light, and local seed rain, we attempted to interpret the seedling emergence patterns of Primula modesta Bisset et Moore (Primulaceae) in two different types of habitats in a subalpine zone of Mt. Asama: an oligotrophic flat moor and a grassland with relatively dense herbaceous vegetation cover. The seasonal pattern of seedling emergence was well explained by the dormancy/germination physiology revealed in laboratory germination tests. The seeds were demonstrated to have a strict light requirement even after experiencing moist chilling, which might facilitate the incorporation of the seeds into the soil seed bank. Despite sufficient seed production, the seedlings emerging were far less at the grassy site than the moor site, but the number of seedlings was significantly dependent on the seed rain within previous season and on the litter cover of the microsite in both sites. Therefore, the spatiotemporal patterns of seedling emergence in the habitats could be well explained by the spatiotemporal patterns of seed rain and safe-sites for germination.     

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

Field experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface‐sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment.     

Fire,canopy cover and seedling dynamics in Mediterranean shrubland of northeastern Spain

Abstract. Seedling establishment and survival of the main woody species of early-successional shrubland in northeastern Spain were studied from 1992 to 1995 with emphasis on the importance of vegetation cover (existence of open areas and the situation beneath the plant canopy) and microhabitat (occurrence of stones, litter and bare soil). In the absence of fire, vegetation cover (measured at a scale of 30cm × 30 cm) was not correlated with seedling emergence of most species, nor with growth and survival of seedlings up to one year old. Seedlings older than one year showed a similar pattern: their density was not significantly different in both kinds of habitats. The emergence of seedlings was mainly associated with the presence of mature plants – which can provide seeds – and with the absence of unsuitable microhabitats (large stones, deep litter and bare soil). In April 1994 a wildfire burned the study area. This enabled a study of the pattern of post-fire establishment. After the wildfire, seedling emergence of several species increased and most species produced seedlings with higher survival and growth rates than in the period before the fire. Pre-fire cover, however, did not show significant effects on post-fire seedling dynamics in most cases.     

Sensitivity of seedling recruitment to moss, litter and dominant removal in an oligotrophic wet meadow

The effect of a dominant species, the litter layer, and the moss layer on seedlings and established vegetation was evaluated in two manipulative experiments in an oligotrophic wet meadow (Molinion with some features ofViolion caninae according to phytosociological classification). In the first experiment, the dominant species (Nardus stricta), litter layer, and litter layer together with the moss layer were removed and the response of the established vegetation and seedlings were compared with a control. Results revealed that after one season there was no significant effect of the treatments on established vegetation, but seedling recruitment was significantly affected. The greatest increase of seedling recruitment for many species was observed in the plots with the moss layer removed. In the second experiment, seeds ofSelinum carvifolia were sown into plots subjected to the following treatments: complete vegetation removal, mowing, mowing combined with removal of the moss layer, and an untreated control. The highest number of seedlings was found in plots with the vegetation removed, but the seedling recruitment was nearly as high in plots with the moss layer removed. Low numbers were found in mown plots and the lowest in the untreated control. The results show that seedling recruitment is more sensitive to competition than the established vegetation, at least over one season. This supports the idea of the importance of the regeneration niche for the maintenance of diversity in grassland communities.     

The role of seed depth,litter, and fire in the seedling establishment of amphicarpic peanutgrass (Amphicarpum purshii)

Summary Amphicarpum purshii is an annual grass which mostly grows in disturbed areas of the New Jersey Pine Barrens, USA. It is amphicarpic, producing spikelets (and seeds) both above and below the soil surface. Previous research has shown that subterranean seed production ensures reproduction in the event of a major disturbance such as fire and results in rapid post-burn colonization of these sandy habitats. The effects of fire, litter, and seed depth were further examined by planting subterranean seeds at four depths in 16 litter-covered flats buried at ground level and comparing plants arising from burned flats with those in undisturbed litter-covered flats. At 0 and 1 cm depth, rates of seedling emergence were lowest in burned flats. Surface-sown seeds produced seedlings more likely to desiccate. Sowing depth had a greater influence on most measured characters than burning treatments. The mean depth of subterranean seed placement by Amphicarpum is 3.5 cm and this coincides with the seed depth from which plants showed the greatest height growth, shoot biomass, and reproductive output. In a second experiment, subterranean seeds on the bare soil surface in clay pots were more likely to lose viability and less likely to germinate than seeds protected by litter or burial in soil. In addition to providing protection from fire, placement of seeds below ground in the sandy habitat of peanutgrass provides conditions more suitable for seed survival and subsequent seedling establishment.     

Establishment and growth of seedlings of Eucalyptus obliqua: Interactive effects of litter,water, and pathogens

We studied how leaf litter, water and fungal pathogens affect the establishment, survivorship, and growth of Eucalyptus obliqua using glasshouse and field experiments. In a glasshouse experiment, the presence of leaf litter of E. obliqua increased the establishment of seedlings when the seeds were placed beneath the litter, but dense litter layers reduced seedling emergence when seeds were placed on top. Results from a complementary experiment suggest that the effect of litter may be mediated by the high humidity microenvironment it creates, which may enhance imbibition of unburied seeds. In another glasshouse experiment, litter in combination with excessive water applications reduced seedling emergence and increased seedling mortality, but the addition of fungicide reverted these effects. Litter with lower water application or fungicide added increased seedling emergence. In all cases higher biomass in seedlings growing in litter treatments was associated with earlier emergence. In two recently burnt sites, the density of seedlings of E. obliqua was positively correlated to the presence of litter. Experimental addition of litter in the field reduced soil temperature during the summer, and together with watering, increased soil water content, and the water potential of established seedlings. However, neither treatment had any positive effect on their growth or survivorship. On the contrary, litter addition reduced the height and number of leaves, probably as a result of increased herbivory. We conclude that in Mediterranean-type systems litter may produce various effects on seedling establishment depending on their developmental stage, and the season.     

Effects of bryophytes and lichens on seedling emergence of alvar plants: evidence from greenhouse experiments

Emergence of seedlings of four alvar grassland species ( Arenaria serpyllifolia , Festuca ovina , Filipendula vulgaris and Veronica spicata ) in bryophyte and lichen carpets was analysed in a series of greenhouse experiments. The aspects investigated were: the influence of thickness of moss mats, both in dry and moist conditions, the effects of thick Cladonia spp. clumps, and of living vs dead moss shoots and lichen podetia. Overall, Festuca seedlings emerged best whereas the small-seeded species, Arenaria and Veronica , had the lowest emergence. Moisture had a significant effect only on the emergence of Festuca seedlings, which emerged better in the dry treatment than in the moist. A thick moss cover negatively affected seedling emergence of Arenaria and Veronica but did not affect the emergence of Festuca . Filipendula showed lower seedling emergence in both thick and thin moss than on bare soil only in the dry treatment, whereas in the moist treatment emergence did not differ among the three substrates. Arenaria seedlings emerged less in thick and thin moss than on bare soil in the dry treatment, whereas in the moist treatment emergence in the thin moss was not different from bare soil. Thus, in relatively dry environments even a thin moss cover may inhibit rather than facilitate seedling emergence. The lichen clumps inhibited only the emergence of the forbs. Both living moss shoots and lichen podetia inhibited emergence of Veronica seedlings but did not affect Festuca . In contrast, emergence in the presence of dead mosses and lichens did not differ from emergence in their absence for both species. Hence, inhibition of seedling emergence by bryophytes and lichens of at least some vascular plant species may be mediated by some biotic factor. However, the effect of differences in substrate properties on germination cannot be excluded     

Effect of fire on hard-coated Cistaceae seed banks and its influence on techniques for quantifying seed banks

The impact of fire on hard-coated Cistaceae (Halimium ocymoides, Cistus ladanifer, and C. salvifolius) soil seed banks in a Mediterranean 'maquis' shrubland, and its effect on seed germinability were studied. The study also contrasts the effectiveness of two widely used techniques for quantifying seed banks, the seedling emergence and the physical separation methods, in relation to fire. The null hypothesis that a massive enhancement of physically-dormant Cistaceae seed germination by fire would make use of the time-consuming physical separation technique unnecessary was tested. Fire reduced Cistaceae seed banks in the 0–2 cm deep soil layer by both seed fire-consumption and lethal temperatures, revealed by the significant decreasing of the seed bank density and by the increase of apparently-intact but soft-unviable seeds, respectively. In contrast, no damage was recorded in the 2–5 cm soil layer. A dramatic seed bank depletion (> 90%) in both soil layers was recorded one year after fire in the burnt area, coinciding with a significant increase of seedling density confined to the first post-fire year. The ecological consequences of this massive post-fire seed bank input are discussed. A germinability test revealed that germination of surviving Cistaceae seeds was significantly enhanced in all cases except for the C. salvifolious seed bank in the deeper soil layer. However, final germination levels (60–75%) did not correspond to the magnitude of seed bank depletion, especially for C. salvifolious, which suggests that other environmental factors not exclusively associated with fire may also be important in softening Cistaceae seeds. Germination enhancement by fire soil-heating was not high enough to reject the physical separation technique, at least in the deeper soil layer. The simultaneous use of both seedling emergence and physical separation is recommended for reliable seed bank estimates when a physically-dormant hard-seeded component can be expected in the soil, as in many Mediterranean ecosystems, regardless of fire occurrence.     

Microenvironmental conditions for Japanese alder seedling establishment in a hummocky fen

In this article, we focused on hummocky microtopography as a prominent feature of mires and explored the microenvironmental conditions suitable for alder seedling establishment. Japanese alder (Alnus japonica) forest is widely distributed in wetlands in northern Japan. However, because alder seedlings are rare in mires, alder population dynamics and conditions that favor the establishment of alder seedlings are still unknown. The study was conducted in northern Japan at a site in mesotrophic mire. We surveyed the seedling density, the microenvironmental conditions (light, litter cover, and soil quality), and the density of dispersed seeds in alder forest and in the adjacent herbaceous fen. In addition, we performed a laboratory experiment to examine the germination characteristics of alder. Seedlings grew only on hummocks in alder forest. The percentage of litter cover on hummocks was lower than in hollows, and the density of dispersed seeds in alder forest was much higher than in herbaceous fen. Seeds of Japanese alder germinated under both light and dark conditions, and the germination rate were high under light and high-temperature conditions. Our results suggest that litter cover may inhibit seedling establishment and hummocks that characterized by less litter cover are suitable place for the establishment of seedlings. We conclude that hummocky microtopography and abundant seed rain in the mire enable the establishment of Japanese alder seedlings.     

Effect of grass litter on seedling recruitment of the critically endangered Cistus heterophyllus in Spain

Cistus heterophyllus Desf. is an Iberian-north African endemic plant species with only one wild population left in Spain. A sowing experiment under controlled conditions was designed to assess both the seedling emergence rate and the growth fitness of C. heterophyllus on a litter layer of Brachypodium retusum (Pers.) Beau., which forms dense carpets around the remnant population of C. heterophyllus. In order to test the physical effects of B. retusum litter on Cistus seedling recruitment, six sowing treatments in pots were compared that combined three doses of litter (0, 0.6 and 1.1 kg m⁻²) and different physical locations of the seeds (above, below the litter layer or within a litter–peat mixture). B. retusum litter interfered with the seedling recruitment of C. heterophyllus, negatively affecting both seedling emergence and their early growth. Such an effect was more severe when the seeds were located above a 0.6 kg m⁻² litter layer, although seedlings from seeds sown under the litter showed a growth reduction ranging from 58% to 100% compared to the no-litter treatment. A significant negative effect of the litter–peat mixture on the number of leaves, leaf size and number of branches of seedlings was also inferred. These data provide experimental evidence on the serious threat that B. retusum litter represents for the conservation and regeneration of the last population of C. heterophyllus in Spain.     

Crown development in tropical rain forest trees in gaps and understorey

Emergence and establishment of Tsuga sieboldii seedlings were monitored for six years in an old-growth mixed forest in Japan, to clarify the reason why adult T. sieboldii trees are distributed only on convex slopes. T. sieboldii seedlings germinated on exposed soils without broad-leaved litter cover on both convex and concave slopes, but they died out from concave slope during the first two years after germination. Along a gradient from convex to concave slope, litter cover became higher and surface soil erosion became severer, and T. sieboldii seedlings died at the plots with severe surface soil erosion. Other environmental variables, such as relative illuminance, vegetation cover of seedling layer and soil water potential were not significantly different between convex and concave slopes. We concluded that the restricted seedling establishment of T. sieboldii to convex slope is determined by stability of surface soils and broad-leaved litter cover.     

死、活地被物对红松伴生树种天然更新影响的实验研究

1963年在小兴安岭丰林自然保护区红松林及有关的皆伐迹地上设置了61组,325个实验样方,定期观察到1965年,以研究死、活地被物对人工播种的红松林伴生树种种子发芽和幼苗初期存活、生长的影响。实验表明死、活地被物,特别是死地被物的干湿变化剧烈,影响幼苗的成活,甚至使种子或幼根无法接触土壤,因而种粒越小,或轻而带翅,受影响越大。此外本文还指出了桦树种子的强迫休眠和延迟发芽现象。讨论了实验结果在红松林演替和人工促进天然更新中的意义。     

Recruitment in a Mediterranean annual plant community: seed bank, emergence, litter, and intra- and inter-specific interactions

A small‐scale field study was conducted to explain seedling emergence and recruitment of reproductive individuals in a four‐year‐old Mediterranean annual plant community. The analyzed levels were populations, functional types, and total number of individuals in the community. We hypothesized that the number of germinable seeds positively affects the number of emerged seedlings, which positively affects the number of reproductive individuals. We hypothesized as well that litter mass, biomass of established adults, and number of non‐conspecific emerged seedlings negatively affect the numbers of emerged seedlings and reproductive individuals. The results supported our multivariate causal explanation of plant recruitment, although concordances with the initial hypotheses were more frequent at the community level. The effect of the number of germinable seeds on the number of emerged seedlings was found to be robust only at the community level. At the population level, this relationship did not have a consistent tendency and depended on species identity and range of seed density experienced. Litter had negative effects on total number of individuals in the community, and usually non‐significant effects at the population level. Litter effects were found to be inversely related to seed mass, and sharper cotyledons did not improve seedling emergence likely by favoring litter mat penetration. Interactions among plant species appeared not to have any effect on seedling emergence and survival, and had positive effects on certain populations. Biomass of established adult plants exerted no influence on seedling emergence and survival, probably due to their low abundance in this community during the germination period. Results permitted the inference of the main stages in the recruitment process and causal factors. They provided evidence for the greater importance of germination and emergence in comparison with seedling survival to insure reproductive recruitment in this community.     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

Soil seed bank and the effect of needle litter layer on seedling emergence in a tropical pine plantation

The soil seed bank is the basis for community establishment and permanence and plays a primary role in natural restoration of degraded or altered ecosystems. As part of a restoration project, this study aimed to quantify the soil seed bank and to evaluate the effect of the needle litter layer on seedling emergence. Soil samples from a pine plantation were collected at random in the field and set to germinate in a greenhouse. Half of them were covered by a 6cm layer of dead pine needles simulating field conditions. In the field, 20 x 20cm plots were established, half were left intact and half were cleaned from the litter needles. All four treatments had 15 replicates and seedling emergence was recorded during six months. Soil seed bank density was 1 222/m2 from 17 morphotypes. In the field, the number of morphotypes and seedlings was only 9% and 6% respectively, of those emerged in the greenhouse, possibly due to watering and lack of predation in the latter. In both cases, herbs and graminoids were the dominant emerging seedlings, making up to 70-90% of the total. The needle layer didn't prevent seeds from reaching the soil but strongly reduced (> 50%) seedling emergence, although high variability within treatments resulted in no statistically significant differences. These results show that the needle layer hinders germination and/or emergence of seedlings from the seed bank. Its removal may be a recommended technique to accelerate natural restoration in pine plantations.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

Emergence and survival of herbaceous seedlings in temperate grassy woodlands: Recruitment limitations and regeneration niche

Abstract Field experiments examined herbaceous seedling emergence and survival in temperate grassy woodlands on the New England Tablelands of New South Wales. Effects of intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds, grazing and seed theft by ants on seedling emergence and survival were studied in two field experiments. Thirteen species with a range of traits were used in the experiments and their cumulative emergence was compared with laboratory germination studies. Field emergence correlated to laboratory germination but all species had lower emergence in the field. Little natural emergence of native species was observed in the field in unsown treatments. Short‐lived forbs had the highest emergence, followed by perennial grasses; rhizomatous graminoids and perennial forbs had the lowest emergence. Soil surface and cover treatments did not markedly enhance emergence suggesting that intertussock spaces were not prerequisites for forb emergence. No consistent pattern of enhanced emergence was found for any treatment combination across all species. Seedling survival varied among species, with perennial grasses and short‐lived forbs having the highest seedling mortality. Low mortality rates in the graminoids and rhizomatous forbs appeared partially to compensate for lower seedling emergence. All perennial grasses and some short‐lived forbs showed increased risk of mortality with grazing. Differences in emergence and survival of species were related to ground cover heterogeneity, soil surfaces and, to some extent, herbivory. The complexity of these patterns when superimposed on temporal variability suggests that no generalizations can be made about the regeneration niche of herbaceous species groups. Strong recruitment limitation and partitioning of resources in the regeneration niche may reduce competition among native species and explain the high species richness of the herbaceous layer in the temperate grassy communities of eastern Australia.     

Safe Site and Seed Limitation in Cytisus Scoparius (Scotch Broom): Invasibility, Disturbance, and the Role of Cryptogams in a Glacial Outwash Prairie

In all plant populations, establishment is controlled by two factors: the supply of propagules and their access to ‘safe sites’ for growth. An infestation of invading pest plants results in a seed-production gradient, from the edge where seeds are limiting, to the center where seeds may be in excess. Do invaded sites become ‘saturated’ with seeds? How rapidly does this occur, and how does the process depend on the availability of safe sites? Are safe sites, and consequently invasion, promoted by disturbance? I quantified the response of seedling establishment to seed input and disturbance in Cytisus scoparius (Scotch broom), an exotic shrub invading glacial outwash prairie remnants in western Washington, USA. I used disturbance treatments to investigate the role of the thick cryptogamic layer in these prairies, disturbing cryptogams by scraping or by fire. The effect of fire was partitioned into two factors: burning of the background vegetation/substrate versus breaking C. scoparius seed dormancy, by adding seeds either before or after the burn. Seed treatments ranged from 20 to 1000 seeds per m². Both seed number and surface treatment showed significant effects on seedling density, along with a significant interaction between the two factors. Disturbance did not promote C. scoparius establishment; undisturbed plots produced more seedlings than burned or scraped plots. Within the burned plots, fire scarification appeared to increase germination but this effect was not significant. For germinated seedlings, mortality through the dry season (June–August) was not significantly different among surface treatments, nor did survivorship depend on density, with the result that initial differences in germination among the treatments persisted. The message that the undisturbed cryptogam layer facilitates C. scoparius establishment suggests that ‘ecosystem management’ strategies promoting healthy, undisturbed sites will not always be effective against invasive pest species.