Seedbank persistence and seedling emergence of seven weed species in autumn-sown crops following a single year's seeding

Seedbanks and seedling emergence, resulting from seeds shed in wheat crops and ploughed in, were monitored in two experiments for 3–4 years in successive autumn sown crops, in which herbicides were used to prevent re-seeding. Populations of Galium aparine, Papaver rhoeas, Lamium purpureum, Myosotis arvensis, Veronica persica, Viola arvensis and Veronica hederifolia were studied in separate plots. The numbers of viable seeds in the soil declined exponentially; G. aparine and V. hederifolia declined most, and P. rhoeas and V. arvensis least rapidly. Total weeds (all species) over the 3–4 years, (obtained by adding successive numbers of seedlings recorded in each autumn or winter), represented 3–4% of the first year seedbank for the combined species. Few seedlings emerged in the first year after seeding due to effective burial by the initial ploughing. The species showing the greatest emergence in the first year, relative to seedbank size, were G. aparine and V. hederifolia. Other, smaller seeded, species produced their main seedling flushes in the second and third years after seeding. It is concluded that poor weed control in one year is likely to result in large weed infestations for up to four years and probably longer for some species.     

Seed survival and periodicity of seedling emergence in twelve weedy species of Compositae

Achenes of 12 species of Compositae were collected in each of 3 years and mixed with the top 7.5 cm of sterilised soil which was confined in cylinders sunk in the ground outdoors and cultivated three times yearly. Seedling emergence was recorded for 5 yr and the numbers of viable seeds remaining then determined. Emergence of Arctium lappa, A. minus, Picris hieracioides, Taraxacum officinale, Sonchus arvensis and S. asper was mainly in spring. Seedlings of Chrysanthemum segetum, Lapsana communis, Anthemis cotula and Sonchus oleraceus were often most numerous in spring but also emerged at other times, while Matricaria matricarioides and M. recutita showed no consistent seasonal pattern of emergence. Except for C. segetum and A. cotula, in which there was evidence of innate dormancy imposed by the seed coat, the numbers of seedlings were greatest in the first year and thereafter decreased approximately exponentially from year to year. Few viable seeds of A. lappa or A. minus remained after 5 yr and those of T. officinale, P. hieracioides and S. arvensis accounted for less than 1.5% of the seeds sown. Seed survival was greatest in the annual weeds and ranged from 2.1% (S. asper) to 8.6% (M. matricarioides) after 5 yr.     

Seed survival and periodicity of seedling emergence in eight species of Cruciferae

Seeds of eight species of Cruciferae were collected, usually in each of 3 years, and mixed with the top 7-5 cm of sterilised soil confined in cylinders sunk in the ground outdoors and cultivated three times yearly. The seedlings emerging were recorded for 5 yr and the numbers of viable seeds remaining then determined. Emergence of Alliaria petiolata was almost entirely restricted to February and March. That of Erysimum cheiranthoides, Lepidium campestre, Sinapis arvensis and Raphanus raphanistrum took place mainly in March and April, but seedlings continued to appear until late autumn. Some seedlings of Arabidopsis thaliana, Cardamine hirsuta and Sisymbrium officinale appeared in spring, but most did so in summer (C. hirsuta and S. officinale) or early autumn (A. thaliana). Seed survival in A. petiolata was of short duration and there were few seedlings after the second year, whereas 18% of the seeds of R. raphanistrum were still present and viable after 5 yr. Seed survival at this time for the other six species ranged from 1 -6% (C. hirsuta) to 6-1% (S. arvensis).     

Seed survival and periodicity of seedling emergence in some species of Atriplex, Chenopodium, Polygonum and Rumex

Seeds or fruits of 10 weed species were collected in each of 3 years and mixed with the top 7·5 cm of sterilised soil which was confined in cylinders sunk in the ground outdoors and cultivated three times yearly. The numbers of seedlings emerging were recorded for 5 yr and the numbers of viable seeds remaining then determined. Few, if any, seedlings of Atriplex, Chenopodium or Polygonum spp. emerged in the autumn of sowing; the main emergence was consistently in March–May (A. hastata, A. patula), April–May (P. lapathifolium, P. persicaria), April–June (C. polyspermum), May–June (C. rubrum) or June–August (C. hybridum). When sown before November, some seedlings of Rumex crispus, R. obtusifolius and R. sanguineus appeared immediately. Most emerged in April in subsequent years. Seedling emergence was usually greatest in the first year; thereafter, with all species, seedling numbers decreased approximately exponentially from year to year. The mean numbers of seeds found to be viable after 5 yr ranged from 2·5% (R. sanguineus) to 16·6% (C. rubrum) of those sown.     

Seed survival and seasonal pattern of seedling emergence in some Leguminosae

Seeds of 11 species of Leguminosae were collected, usually in each of 3 years, and mixed with the top 7·5 cm of sterilised soil confined in cylinders sunk in the ground outdoors and cultivated three times yearly. Emergence was recorded for at least 3 yr. Some seedlings of all species emerged soon after sowing but their numbers varied both within and between species, a probable reflection of the percentage of ‘hard’ seeds in the samples. Appreciable numbers of seedlings appeared in the following 3 yr but few seeds remained viable and dormant after 5 yr. The annual weed Vicia hirsuta was an exception, with an average of 11% of the seeds sown still viable at this time. Most seedlings of Lotus corniculatus, Medicago lupulina, Melilotus altissima, Trifolium repens and Vicia cracca emerged in spring, V. cracca rather later than the others. In contrast, maximum emergence of Trifolium arvense, T. campestre and T. dubium took place from June to September. Limited data indicated a similar pattern for T. striatum and Lathyrus pratensis. Seedlings of Vicia hirsuta emerged from October to May but scarcely at all in summer. Although variation in the percentage of hard seeds influenced the extent of immediate germination and seed persistence, the seasonal patterns in seedling emergence of most species were found to be very consistent.     

Seed survival and seasonal emergence in some species of Geranium, Ranunculus and Rumex

Seeds of 14 species were collected, usually in each of 2 or 3 yr, and mixed with the top 7.5 cm of sterilised soil confined in cylinders sunk in the ground outdoors and cultivated three times yearly. Seedling emergence was recorded for at least 3 and usually 5 yr. Most seedlings of Ranunculus bulbosus and Rumex acetosa appeared during the autumn of sowing, while the main emergence of Geranium pratense. Ranunculus acris, Rumex conglomeratus and R. maritimus was in the following spring. Emergence of G. pyrenaicum and G. robertianum took place throughout much of the year after sowing. Seeds of all these species were relatively short-lived in cultivated soil, and few seedlings appeared after the second year. Seed survival was greater in G. dissectum. G. molle, Rumex acetosella. Ranunculus sceleratus and especially R. repens, of which emergence in the fifth year after sowing amounted to 4% of the seeds sown. Emergence from the persistent seed bank was mainly from May to September for G. dissectum and G. molle and during autumn for Ranunculus flammula and R. sceleratus, while seedlings of Ranunculus repens and Rumex acetosella appeared sporadically over most of the year.     

Breeding System Studies with Three Species of Anagallis(Primulaceae): Self-incompatibility and Reduced Female Fertility in A. monelli L.

The pollination biology, fruit set and breeding system of Anagallisarvensis, A. parviflora andA. monelli were studied in the fieldand glasshouse. No pollinators were observed at flowers of A.arvensis andA. parviflora , and hand pollinations indicatedthat these species are self-compatible taxa which automaticallyself-pollinate as the petals close at the end of the first day.Flowers of A. monelli were visited by several small halictidbee pollinators and a field diallel with seven plants indicatedthat this species is self-incompatible. Fruit set inA. monelliis correlated with distance to the nearest conspecific neighboursuch that beyond 2 m fruit set is relatively low. The diallelcross, and also an analysis of the number of seeds per capsulefor all plants located within 2 m of a conspecific, indicatedthat about 18% of the population of A. monelli showed low femalefecundity and were thus ‘male dominant’ individuals.Copyright 2001 Annals of Botany Company Primulaceae, Anagallis arvensis, Anagallis parviflora, Anagallis monelli, Mediterranean, breeding system, self-incompatibility, fruit set, female sterility     

DORMANCY AND GERMINATION IN SEEDS OF COMMON RAGWEED WITH REFERENCE TO BEAL'S BURIED SEED EXPERIMENT

Common ragweed (Ambrosia artemisiifolia L.) was one of 19 herbaceous weedy species used by Beal in his buried viable seed experiment started in 1879. No seeds germinated during the first 35 years of the experiment when germination tests were performed in late spring, summer or early autumn. Germination did occur in seeds buried for 40 years when seeds were exhumed and tested for germination in early spring. Data obtained in more recent research provide the probable explanation for these results. Seeds of common ragweed that do not germinate in spring enter secondary dormancy by mid to late spring and will not germinate until dormancy is broken the following late autumn and winter. Thus, during the first 35 years of the experiment seeds were dormant when tested for germination, whereas seeds buried for 40 years were nondormant. Seeds buried 50 years or longer did not germinate when tested in spring, probably because they had lost viability and/or seeds germinated during burial and seedlings died.     

Migration of alate morphs of the bird cherry aphid (Rhopalosiphum padi) and implications for the epidemiology of barley yellow dwarf virus

Suction trapping data indicate three periods of migration of Rhopalosiphum padi in spring, summer and autumn. Four alate morphs are present at different times during the year. A comparison of data from suction traps operating at 12·2 and 1·5 m suggests a different behaviour of females in autumn with more being recorded at 12·2 than 1·5 m. Males, which are only present in autumn, were also more numerous at 12·2 m. During tests to measure barley yellow dwarf virus (BYDV) infectivity, only 9% of female R. padi reproduced on oat seedlings in autumn compared with 74% in summer. Tests on alate female R. padi trapped alive showed that in summer all were exules, but during the first half of September these were largely replaced by gynoparae so that in autumn only 5% of all R. padi trapped at 12·2 m were alate exules. The aerial densities of gynoparae and males were 10 times greater at 12·2 than 1·5 m while densities of alate exules were similar at both heights. It is suggested that gynoparae and males fly higher to increase the chance of finding a taller dispersed host plant. The implications for BYDV epidemiology of the behaviour and presence of the various R. padi alate morphs indicate that autumn-sown cereals emerging before mid-September are particularly at risk from colonisation by alate exules before the transition to a mainly sexual migrant population is complete. Alate exules introduce BYDV from comparatively local sources. The ratio of total R. padi to Sitobion avenae in suction trap samples in autumn usually exceeds 100: 1, but on crops it was only 10: 1. The ratio of alate exule R. padi to S. avenae in suction traps in autumn was only 12: 1, similar to that observed on crops.     

Mechanisms of structural change derived from patterns of seedling emergence and mortality in a semi-natural meadow

Questions: Is seedling emergence limited by the set of viable seeds, by incompatibility between the phenology of seed shedding and timing of mowing, or by dry weather in germination periods? Does seedling mortality fluctuate with season and weather? Location: Negrentino, southern Alps, Switzerland. Methods: Fecundity estimates of the dominant grass Bromus erectus; highly frequent counts of spontaneous seedlings by species and calculation of a community-level average mortality rate across 5 years; species-level records of seed shedding date and measurements of seed mass; measurement of soil moisture. Results: B. erectus produced 143.9 viable seeds/m²/year while the density of its seedlings was a 55 times smaller fraction. Grasses had fewer seedlings than forbs and their phenology of seed shedding was less compatible with mowing date. Soil moisture was a strong determinant of seedling emergence in spring and less so in autumn. Average seedling mortality declined with age of the populations and reached a maximum in an extremely dry summer. In relatively wet summers establishment success was positively related to seed mass. Conclusion: Community structure is susceptible to drought through mechanisms that selectively reduce recruits of coexisting plant functional groups. We propose that (1) more frequent intense droughts tend to reduce species that depend on frequent recruitment from seed, hence favour long-lived clonally spreading species, (2) drought timing selects between species with different germination phenology and drought resistance, and (3) drought impacts can be mitigated by changing management regimes that affect seed shedding.     

Spatial patterns in seed bank and vegetation of semi-natural mountain meadows

Soil seed bank composition and vegetative spatial patterns were studied in four mountain meadow communities in the Broto Valley (N. Spain), in order to analyse the differences that might exists between the two life forms. Soil and vegetation samples were taken at 1 m intervals from 10 m × 10 m quadrats in each meadow in one-year study. The spatial distribution of species was analysed along with the calculation of an autocorrelation coefficient which takes account of the relative position of samples: (Moran’s I). The results indicate that the abundance of the majority of the species in the seed bank and in the vegetation are randomly distributed, the percentage of species with a clumped distribution only exceeds 35% in the vegetation of one meadow and none of the taxa identified showed a uniform spatial organisation. The species that were distributed in the seed bank in a clumped pattern in more than one meadow were those of the pioneer species (Anagallis arvensis, Centaurium erythraea, Lamium purpureum and Stellaria media). All of these formed long-term persistent seed banks but were absent in the established vegetation in these meadows. According to the results, there exist not only differences between the spatial distribution of the species present in the same community, but also, that some species change their pattern of distribution according to the life form and to the grassland type in which they are found.     

Seed germination of exotic and native winter annuals differentially responds to temperature and moisture,especially with climate change scenarios

Seeds of winter annuals require a summer after-ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co-occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After-ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after-ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear-cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.     

Experimental demography of the rare Gentianella germanica: seed bank formation and microsite effects on seedling establishment

The persistence of populations of short-lived species requires regular reproduction and seedling establishment. A persistent seed bank can buffer populations against extinction in unfavourable years. We experimentally investigated seed fate in Gentianella germanica, an endangered biennial species characteristic for species-rich nutrient-poor calcareous grasslands in central Europe. We studied the effect of experimental gaps on seedling establishment from sown seeds and the fate of seeds buried in bags over two years. In December 1993 experiments were established at seven calcareous grassland sites in the Swiss Jura mountains. In spring 1994 seedlings emerged in all plots where seeds had been sown, including previously unoccupied patches. This suggests that limited dispersal within sites contributes to small population sizes. Significantly more seedlings emerged at sites with current populations of G. germanica than at unoccupied sites (5.95% vs 3.40%). Because this difference was not explained by germinations from the natural seed bank it indicates differences in habitat quality. Clipping of the vegetation and disturbance of the soil reduced vegetation cover in the following spring and enhanced seedling emergence. In undisturbed plots 4.5% of seeds sown produced a seedling in spring 1994, whereas in plots with clipped vegetation 9.9% and in disturbed plots 12.7% produced seedlings (p>0.01). This suggests that management measures which create gaps in the vegetation (e.g. grazing) could positively influence population size and persistence of G. geymanica. On average, we recovered 7.55% viable seeds after one year of burial in bags, and 4.05% after two years, indicating that G. geymanica has a persistent seed bank. The demographic data indicate that the number of viable seeds in the seed bank exceeds the number of established plants in a population at least by a factor of 20. Restoration of extinct populations of the species from the seed bank may thus be possible if appropriate management measures are taken within a few years.     

Relationships of Anagallis foemina and A. arvensis (Myrsinaceae): new insights inferred from DNA sequence data

We investigated the relationship between Anagallis arvensis and A. foemina using nuclear and plastid molecular data. Information from the nuclear rDNA internal transcribed spacer (ITS) region and four different chloroplast loci; ndhF, trnL-F, rpl16, and rps16 was analysed using both parsimony and Bayesian inference. Anagallis foemina was found to be most closely related to the perennial A. monelli, and not to A. arvensis. The existence of two different cpDNA haplotypes was revealed; one shared by Anagallis foemina, A. monelli, A. platyphylla, and one A. arvensis individual, while all other investigated A. arvensis individuals shared the second haplotype. Ancestral cpDNA polymorphism within Anagallis arvensis or hybridization are possible explanations, however, information in ITS data is too scarce to falsify any of these hypotheses.     

Ectomycorrhiza formation ofTricholoma matsutake isolates on seedlings ofPinus densiflora in vitro

Tricholoma matsutake forms ectomycorrhizas withPinus densiflora under field conditions. The present study aimed to test the ability ofT. matsutake isolates to form mycorrhizas with aseptic seedlings ofP. densiflora in vitro. Pine seeds were germinated aseptically on a nutrient agar medium, and pairs of 1-wk-old seedlings were transplanted into polymethylpentene bottles containing autoclaved sphagnum moss/vermiculite substrate. The substrate was saturated with nutrient medium containing glucose. At the same time, the bottles were inoculated with aT. matsutake isolate. Three mo after inoculation, the fungus formed a sheath and Hartig net on the pine lateral roots. Ectomycorrhizas were also confirmed on 4-6-mo-old seedlings which showed the same or slinghtly better growth than the control plants. These results indicate that culturedT. matsutake mycelium can form true ectomycorrhizas withP. densiflora seedlings in vitro.     

VIABILITY OF SEEDS UP TO 211 YEARS OLD EXTRACTED FROM ADOBE BRICK BUILDINGS OF CALIFORNIA AND NORTHERN MEXICO

Seeds recovered from adobe walls of historic buildings in California and Northern Mexico were assayed for viability and germinability. Seven of the approximately 40 species tested had one or more viable seeds. These were (age of seeds in parentheses) Medicago polymorpha (200), Malva parviflora (200, 195, 183), Hordeum leporinum (200), Trifolium sp. (193), Melilotus indicus (183), Chenopodium murale (183), and C. album (143). Additionally, 41-yr-old seeds of Lotus corniculatus collected from living plants were found to be viable. Seeds of Medicago polymorpha and Lotus corniculatus germinated and produced normal seedlings. The importance of seed longevity in weedy species is briefly discussed.     

The reproductive cycle of female Ballan wrasse Labrus bergylta in high latitude,temperate waters

This 2 year study examined the reproductive cycle of wild female Ballan wrasse Labrus bergylta in western Norway as a precursor to captive breeding trials. Light microscopy of ovarian histology was used to stage gonad maturity and enzyme‐linked immuno‐absorbent assay (ELISA) to measure plasma concentrations of the sex steroids testosterone (T) and 17β‐oestradiol (E₂). Ovarian recrudescence began in late autumn to early winter with the growth of previtellogenic oocytes and the formation of cortical alveoli. Vitellogenic oocytes developed from January to June and ovaries containing postovulatory follicles (POF) were present between May and June. These POF occurred simultaneously among other late maturity stage oocytes. Plasma steroid concentration and organo‐somatic indices increased over winter and spring. Maximal (mean ±s.e .) values of plasma T (0·95 ± 0·26 ng ml^?1), E₂ (1·75 ± 0·43 ng ml^?1) and gonado‐somatic index (I_G; 10·71 ± 0·81) occurred in April and May and decreased greatly in July when only postspawned fish with atretic ovaries occurred. Evidence indicates that L. bergylta are group‐synchronous multiple spawners with spawning occurring in spring and peaking in May. A short resting period may occur between late summer and autumn when previtellogenic oocytes predominate and steroid levels are minimal.     

Rodent seed predation and seedling recruitment in mesic grassland

Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m⁻²) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m⁻² but had no impact at 50,000 seeds m⁻², presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation. Received: 2 March 1998 / Accepted: 28 September 1998     

Optimal time of seedling emergence in a dune-population ofOenothera glazioviana

Oenothera glazioviana (= O. Erythrosepala) behaves as a facultative biennial in an infertile sand-dune system in Japan. In the field, seedlings emerge mainly in autumn following seed ripening in late summer with an additional small peak in spring. The effects of emergence time on subsequent survival and fecundity, and the ultimate numbers of seeds produced per emerging seedling (F _s) were examined by a demographic model. The model showed that seedlings emerging either in March or in September–October will attain higherF _s than those emerging in other months. If mortality in the seed bank is taken into account, autumn seems to be the best season for emergence to maximize the number of seedlings in the next generation.     

Factors Limiting Post‐logging Seedling Regeneration by Big‐leaf Mahogany (Swietenia macrophylla) in Southeastern Amazonia,Brazil, and Implications for Sustainable Management1

Post‐logging seedling regeneration density by big‐leaf mahogany (Swietenia macrophylla), a nonpioneer light‐demanding timber species, is generally reported to be low to nonexistent. To investigate factors limiting seedling density following logging within the study region, we quantified seed production rates, germinability, dispersal patterns, and seed fates on the forest floor through germination and the first seedling growing season in southeastern Amazonia, Brazil. Fruit production rates were low by three logged and one unlogged populations compared to reports from other regions. Commercial‐sized trees (>60 cm diameter) were more fecund than noncommercial trees (30–60 cm diameter) at two sites, averaging 14.5 vs. 3.9 fruits/tree/year, respectively, at Marajoara, a logged site, over 8 yr. Fruit capsules contained an average of 60.3 seeds/fruit, 70 percent of which appeared viable by visual inspection. Sixty‐seven to 72 percent of apparently viable seeds germinated in nursery beds 2.5 mo after the dispersal period, when wet season rains began. Dry season winds blew most seeds west‐northwest of parent trees, with median dispersal distances of 28 and 9 m on west and east sides of parent trees, respectively. Nearly 100 percent of seeds fell within an area of 0.91 ha. On the forest floor beneath closed canopies, mammals, invertebrates, and fungal pathogens killed 40 percent of apparently viable seeds, while 36 percent germinated. Nine months after seedling establishment—midway through the first logging season following seed dispersal—14 percent of outplanted seeds survived as seedlings, representing 5.8 seeds/fruit. We conclude that seedlings are likely to survive in logging gaps at appreciable densities only in rare cases where previous year fruit production rates by logged trees were high (4–12.5% of commercial‐sized trees/year at Marajoara) and where tree crowns were felled in west or northwest directions.