Weed seed banks in arable fields under contrasting pesticide regimes

Viable weed seed banks on a clay soil were estimated for five years from 11 arable fields receiving one of three levels of pesticide application. Low seed densities, averaging 2000 m^-2, were recorded, but dicotyledonous weed seed numbers usually increased following oilseed rape crops, reaching 26 000 m^-2 in one field. Field-to-field variability was large. Samples could be grouped using multivariate techniques into those dominated by grass weed seeds and those containing mainly dicotyledonous weed seeds. Within TWINSPAN classifications, most fields maintained similar seed banks during the study period, indicating relative stability of communities. Differences present at the outset of the experiment, including variation between fields in the same treatment area, were maintained for the five years. Patterns were not associated with straw disposal or spring herbicide application. Oilseed rape crops did not cause qualitative change in seed bank communities, but allowed certain species, notably Stellaria media and Sonchus asper, to increase seed numbers. Smaller and less diverse seed banks in the area receiving most herbicide were not statistically significant. Seed banks gave poor predictions of following weed populations.     

Seed banks and seed population dynamics of halophytes

In this review I will describe the importance of seed banks and thepopulation dynamics of seeds on the distribution of species in salinehabitats. The main questions being examined in this review include: 1.Does the seed bank represent the flora of the entire salinity gradient or isit restricted to the species in each zonal community? 2. Is the size andspecies composition of the persistent seed bank regulated by the degree ofsalt stress in habitats along an environmental gradient? 3. Does thepopulation dynamics of seeds influence the temporal and spatial distributionof plant species in saline habitats? Seed banks may be transient orpersistent depending upon the physiological responses of species and thesoil environment in which the seeds are found. The formation of zonalcommunities in salt marsh environments is affected by changes in soilsalinity and flooding along an elevational gradient. Population dynamics ofseeds have been found to determine the spatial and temporal distributionof species along salinity gradients. The flora and relative density of speciesof zonal communities are significantly dependent upon the stress toleranceof species at different stages of development and the presence of transientor persistent seed banks. The occurrence of a seed bank is related to thesalinity tolerance of species at the germination stage of development, aseeds ability to tolerate hypersaline conditions and flooding, and whetheror not species are able to maintain a persistent seed bank until hypersalineconditions are alleviated.

     

Arable weed seed bank of grassland on former arable fields in mountain regions

The changes in agricultural practice during the last century resulted in high-input farming in lowlands and the abandonment of crop fields in marginally profitable mountain regions. In Switzerland abandoned fields were converted into grassland. These fields had a rich historical flora and the few still existing fields still belong to the most species rich. As many arable weeds produce persistent seeds, abandoned fields should have a high potential to promote rare and threatened arable plants if tilled again. To test this hypothesis we collected 21 soil samples down to 20 cm depth in each the centre and the border of 38 abandoned fields. The centre and border samples of each field were each pooled, and afterwards the present seeds washed out. These seeds were then sown in pots and germination monitored in a greenhouse during six months. A total of 119 plant species were identified. Of these, 48 species were typical arable weeds and only one was red listed in Switzerland. The number of arable weeds per former field was lower at higher altitudes. Hence, the surveyed meadows have a small potential to promote threatened arable weeds if tilled again. Likely, some seeds were no longer viable because the fields were not tilled for a too long. If on newly established conservation fields no threatened species grow spontaneously after a few years, the reintroduction of desired plant species should be considered. The source of the seeds should preferably be a nearby species pool.     

Seed dispersal and seed banks in Aloe marlothii (Asphodelaceae)

Aloe marlothii flowers during dry winter months (July–September) and produces large numbers of wind dispersed seeds. Fire disturbance in a population of several thousand A. marlothii plants at Suikerbosrand Nature Reserve, Gauteng, permitted a series of seed dispersal experiments to be conducted. Germination trials indicated that seedling emergence decreased with increased distance from a well defined aloe stand and burn area margin, with seeds dispersed up to 25 m. Flowering frequency and total seed production were positively correlated with plant height, with seed production estimated to range from 26,000 to 375,000 seeds/plant. Although a large number of seeds are produced by flowering plants the survival rate of seeds did not extend beyond the following flowering season.     

Co-adaptation of seed dormancy and flowering time in the arable weed Capsella bursa-pastoris (shepherd's purse)

Background and Aims

The duration of the plant life cycle is an important attribute that determines fitness and coexistence of weeds in arable fields. It depends on the timing of two key life-history traits: time from seed dispersal to germination and time from germination to flowering. These traits are components of the time to reproduction. Dormancy results in reduced and delayed germination, thus increasing time to reproduction. Genotypes in the arable seedbank predominantly have short time to flowering. Synergy between reduced seed dormancy and reduced flowering time would create stronger contrasts between genotypes, offering greater adaptation in-field. Therefore, we studied differences in seed dormancy between in-field flowering time genotypes of shepherd''s purse.

Methods

Genotypes with early, intermediate or late flowering time were grown in a glasshouse to provide seed stock for germination tests. Secondary dormancy was assessed by comparing germination before and after dark-incubation. Dormancy was characterized separately for seed myxospermy heteromorphs, observed in each genotype. Seed carbon and nitrogen content and seed mass were determined as indicators of seed filling and resource partitioning associated with dormancy.

Key Results

Although no differences were observed in primary dormancy, secondary dormancy was weaker among the seeds of early-flowering genotypes. On average, myxospermous seeds showed stronger secondary dormancy than non-myxospermous seeds in all genotypes. Seed filling was similar between the genotypes, but nitrogen partitioning was higher in early-flowering genotypes and in non-myxospermous seeds.

Conclusions

In shepherd''s purse, early flowering and reduced seed dormancy coincide and appear to be linked. The seed heteromorphism contributes to variation in dormancy. Three functional groups of seed dormancy were identified, varying in dormancy depth and nitrate response. One of these groups (FG-III) was distinct for early-flowering genotypes. The weaker secondary dormancy of early-flowering genotypes confers a selective advantage in arable fields.     

Predicting changes in dormancy level in natural seed soil banks

The possibility of accurately predicting timing and extent of seedling emergence from natural seed soil banks has long been an objective of both ecologist and agriculturalist. However, as dormancy is a common attribute of many wild seed populations, we should first be able to predict dormancy changes if we intend to predict seedling emergence in the field. In this paper, we discuss the most relevant environmental factors affecting seed dormancy of natural seed soil banks, and present a conceptual framework as an attempt to understand how these factors affect seed-bank dormancy level. Based on this conceptual framework we show approaches that can be used to establish quantitative functional relationship between environmental factors regulating dormancy and changes in the seed-bank dormancy status. Finally, we briefly explain how we can utilize population-based threshold models as a framework to characterize and quantify changes in seed sensitivity to environmental factors as a consequence of dormancy loss and/or induction.     

Telomeres and seed banks

We have found that a progressive loss of telomeric sequences occurs from high molecular weight DNA with an increasing appearance at low molecular weight as the periods of storage in the dry state were extended in time to provide seed germination loss from 98 to 0%. Telomere distribution would appear to follow the general pattern of DNA random fragmentation that occurs in the embryos of seeds stored in the dry state, but there are also indications of an overall telomere loss from DNA as a consequence of storage. There is a need for a convenient "equality marker" for the seeds that can be monitored over time. Reviewing the implications of our results very carefully we believe that there is considerable potential in the use of telomere sequences to mark embryo ageing of seeds held in Seed Banks.     

Telomeres and seed banks

We have found that a progressive loss of telomeric sequences occurs in high molecular weight DNA with an increasing appearance at a low molecular weight as the period of storage in the dry state was extended in time to provide seed germination loss from 98 to 0%. Telomere distribution would appear to follow the general pattern of DNA random fragmentation, which occurs in the embryos of seeds stored in a dry state; however, there are also indications of an overall telomere loss from DNA as a consequence of storage. There is a need for a convenient quality marker for the seeds that can be monitored over time. Having reviewed the implications of our results very carefully, we believe that there is considerable potential for the use of telomere sequences to mark the embryo ageing of seeds held in seed banks. The text was submitted by the authors in English.     

Modelling seedling growth rates of 18 temperate arable weed species as a function of the environment and plant traits

BACKGROUND AND AIMS: The early growth rate of seedlings in the exponential phase is an important eco- physiological trait in crop/weed competition models based on assessments of relative weed green area. An understanding of the role of various plant traits in determining early growth rate may also be useful for identifying contrasting weed strategies for establishment before canopy closure. METHODS: The response of seedling relative growth rate (RGR) to the environment was measured in outdoor sand beds in the autumn and the spring for 18 temperate annual weed species and two crops. Seedling growth was modelled using thermal time and effective day-degrees (combining the effect of temperature and radiation). The contribution of various plant traits in determining variability in RGR was investigated using regression analysis. KEY RESULTS: The effective day-degree model was more effective for describing early weed growth than thermal time. Variability in RGR measured in the autumn was largely determined by differences between the species in net assimilation rate (NAR), whereas in the spring leaf area ratio (LAR) played a larger part. There were differences between the broadleaf and grass species in the relative contribution of NAR and LAR to RGR in both seasons. RGR in the spring was negatively correlated with initial seedling size. CONCLUSIONS: The parameters derived in this study can be used to calibrate empirical models of crop yield loss based on relative weed green area to different growing seasons and assessment dates. The grass weeds, which tended to have large seeds, had a higher investment in roots in the seedling stage, potentially making them more competitive later in the season when resources become limiting.     

Light-dependent changes in the relationship between seed mass and seedling traits: a meta-analysis for rain forest tree species

Seed mass is considered to be an important attribute for the establishment success of plant species being linked with their seed production, establishment, and survival. This meta-analysis shows that seed mass is also closely correlated to growth-related species attributes of the established phase of rain forest tree species, and that the strength of this relationship varies with light conditions. Seed mass is an especially good predictor of species traits under high-light conditions, when the species attain their full growth potential. At high irradiance (>20% of full light) seed mass is negatively correlated with RGR, NAR, LAR, SLA and LMF. At low irradiance (<5% of full light), seed mass is only negatively correlated with LAR and SLA. Correlations between seed mass and morphological traits are therefore strongest at low irradiance where light interception is important. Conversely, correlations between seed mass and a physiological trait are strongest at high irradiance, where maximisation of photosynthetic rates is important. The strength of the correlation between growth parameters and seed mass declines over time, and disappears after 1–4 years. Seed mass appears to be a good proxy for the shade tolerance of tropical tree species, especially at the younger stages of the life cycle.     

Predation of annual grass weed seeds in arable field margins

Seeds of three species of annual grass weeds (Alopecurus myosuroides, Avena fatua and Bromus sterilis) were placed in field margins around arable fields in a randomised block split-plot design experiment. The field margin vegetation was either sown or naturally regenerated and either cut or uncut. The seeds were either placed in cages designed to exclude small mammals and birds or were uncaged. The proportion of seeds removed was monitored on 10 occasions and mean seed loss was analysed. In general, a greater proportion of weed seeds was removed from uncaged trays in uncut swards, suggesting predation by small mammals, which inhabit tall grass. This effect was mainly due to removal of seeds of the two large-seeded species (A. fatua and B. sterilis), with A. fatua being especially favoured. It is therefore likely that small mammals play a role in the population dynamics of major crop weeds by feeding on their seeds in field margins, especially when these are dense and uncut.     

Methods of estimating seed banks with reference to long-term seed burial

We compared two standard seed-bank estimation techniques using buried seed populations that had been covered to depths of >1 m by volcanic deposits for 20 years. Some seeds were germinated in a greenhouse (germination method [GM]), and other seeds were extracted by flotation using 50% K₂CO₃ solution (floatation method [FM]). In total, FM could detect more species and seeds in the soils than GM. However, many species that were extracted by FM did not germinate by GM and smaller seeds were extracted to a lesser extent by FM. FM and GM have distinct advantages and disadvantages. We concluded that the application of a single method should be avoided in estimating seed banks, in particular for long-lived seed banks, because the seeds cannot be readily germinated and are structurally weak.     

Intraspecific variation in seed dispersal of a Neotropical tree and its relationship to fruit and tree traits

The distribution of wind‐dispersed seeds around a parent tree depends on diaspore and tree traits, as well as wind conditions and surrounding vegetation. This study of a neotropical canopy tree, Platypodium elegans, explored the extent to which parental variation in diaspore and tree traits explained (1) rate of diaspore descent in still air, (2) distributions of diaspores dispersed from a 40‐m tower in the forest, and (3) natural diaspore distributions around the parent tree. The geometric mean rate of descent in still air among 20 parents was highly correlated with geometric mean wing loading^1/2 (r = 0.84). However, diaspore traits and rate of descent predicted less variation in dispersal distance from the tower, although descent rate⁻¹ consistently correlated with dispersal distance. Measured seed shadows, particularly their distribution edges, differed significantly among six parents (DBH range 62–181 cm) and were best fit by six separate anisotropic dispersal kernels and surveyed fecundities. Measured rate of descent and tree traits, combined in a mechanistic seed dispersal model, did not significantly explain variation among parents in natural seed dispersal distances, perhaps due to the limited power to detect effects with only six trees. Seedling and sapling distributions were at a greater mean distance from the parents than seed distributions; saplings were heavily concentrated at far distances. Variation among parents in the distribution tails so critical for recruitment could not be explained by measured diaspore or tree traits with this sample size, and may be determined more by wind patterns and the timing of abscission in relation to wind conditions. Studies of wind dispersal need to devote greater field efforts at recording the “rare” dispersal events that contribute to far dispersal distances, following their consequences, and in understanding the mechanisms that generate them.     

Seed banks in Danish deciduous forests: species composition, seed influx and distribution pattern in soil

Samples of soil from six forest stands of different age and composition were collected, and the seedlings emerging during germination experiments were recorded Total seed bank densities varied between 2230 and 15,630 seeds m⁻² The seed bank size was positively correlated with the yearly seed influx, which constituted from 400 to 2720 seeds m⁻² Juncus effusus dominated the seed banks with generally 2000 to 3300 seeds m⁻², to exceptionally 10820 seeds m⁻² Seeds of Rubus idaeus, Betula pubescens and Carex spp were also abundant in the soil Similar numbers of species were present in the seed banks and vegetation, but only 7-48% were species in common Anemochorous species were more frequent in soil than above-ground, while zoochorous species were rare in soil but abundant in the vegetation A measure of the occurrence of seeds in relation to soil depth was designed Seeds of species with short-term survival in soil (e g Anemone nemorosa, Epilobium angustifolium, Oxalis acetosella and Stellaria holostea) were confined to the top soil, while many well known long-term survivors (e g Carex piluhfera, Poa annua, Rubus idaeus and Stellaria media) were found predominantly deep in the soil The horizontal distribution of seeds of herbaceous species forming short-term seed banks was largely random, while seeds of herbs capable of long-term survival in soil were significantly aggregated Except for one old and species-poor site, the dormant flora indicated generally high levels of disturbance in the past     

Differential weed seed removal in dryland cereals

Weeds persist in rain-fed cereal fields in NE Spain, despite intense herbicide use and high seed removal rates by granivorous harvester ants. Herbicide resistance is involved, but certain weed species also appear to escape seed removal by granivores. To identify the mechanisms involved, we measured seed removal rates (three fields in 2010) and the timing of seed shed (one field in 2009 and three fields in 2010) and used an existing model, which integrates short-term rates of seed shed, burial and removal, to estimate long-term seed removal rates. Averaged over years, fields and weed species, the long-term seed removal rate was estimated at 72?% (range 46?C100?%). Fifteen to 25?% of the seeds of Bromus diandrus avoided removal by being less attractive (low removal rates), and another 0?C29?% escaped through crop harvest, which made seeds inaccessible to granivores. Similarly, 20?C32?% of the Papaver rhoeas seeds escaped through crop harvest, while another 13?C17?% escaped by burial into the soil (small seed size). Other species, such as Galium spurium or Diplotaxis erucoides, had no means of avoiding seed removal by harvester ants. In particular, the more troublesome weeds, such as B. diandrus, P. rhoeas and L.?rigidum, combined herbicide resistance or tolerance with avoidance mechanisms against granivory.     

Classification of weed vegetation of arable land in the Czech Republic and Slovakia

Numerical classification of 2653 geographically stratified relevés of weed vegetation from the Czech and Slovak Republics was performed with cluster analysis. Diagnostic species were determined for each of the seven main clusters using statistical measures of fidelity. The classification reflected clear distinctions between lowland (mostly calcicole) and highland (mostly calcifuge) sites, spring and summer phenological stages, and cereals and root crops. The results of the cluster analysis were compared with traditional phytosociological units. Two clusters corresponded to calcifuge weed vegetation of theScleranthion annui alliance; one cluster represented the vegetation of root crops on moist soils of theOxalidion europaeae alliance; one cluster contained thermophilous weed vegetation of theCaucalidion lappulae alliance; two clusters included weed vegetation of root crops and of stubble fields, which can be assigned to theCaucalidion, Panico-Setarion,Veronico-Euphorbion andEragrostion alliances; one cluster included vernal weed vegetation in little disturbed habitats of theCaucalidion lappulae andScleranthion annui alliances. Our analysis did not support the concept of theSherardion andVeronico-Taraxacion alliances, which were included in earlier overviews of the vegetation units of the Czech Republic and Slovakia.     

Possible changes to arable crop yields by 2050

By 2050, the world population is likely to be 9.1 billion, the CO₂ concentration 550 ppm, the ozone concentration 60 ppb and the climate warmer by ca 2°C. In these conditions, what contribution can increased crop yield make to feeding the world?CO₂ enrichment is likely to increase yields of most crops by approximately 13 per cent but leave yields of C4 crops unchanged. It will tend to reduce water consumption by all crops, but this effect will be approximately cancelled out by the effect of the increased temperature on evaporation rates. In many places increased temperature will provide opportunities to manipulate agronomy to improve crop performance. Ozone concentration increases will decrease yields by 5 per cent or more.Plant breeders will probably be able to increase yields considerably in the CO₂-enriched environment of the future, and most weeds and airborne pests and diseases should remain controllable, so long as policy changes do not remove too many types of crop-protection chemicals. However, soil-borne pathogens are likely to be an increasing problem when warmer weather will increase their multiplication rates; control is likely to need a transgenic approach to breeding for resistance. There is a large gap between achievable yields and those delivered by farmers, even in the most efficient agricultural systems. A gap is inevitable, but there are large differences between farmers, even between those who have used the same resources. If this gap is closed and accompanied by improvements in potential yields then there is a good prospect that crop production will increase by approximately 50 per cent or more by 2050 without extra land. However, the demands for land to produce bio-energy have not been factored into these calculations.     

Seed banks of Phragmites australis-dominated brackish wetlands: Relationships to seed viability, inundation, and land cover

In tidal wetlands of the eastern United States, buried seeds of the non-native haplotype of Phragmites australis may be a source of propagules for re-establishment after eradication efforts but factors controlling the development and expression of seed banks in non-native Phragmites stands have not been examined. We sampled surface soil at four Chesapeake Bay brackish tidal wetlands dominated by the non-native (European) haplotype M of Phragmites and used the seedling emergence method to quantity species of seedlings emerging under flooded and non-flooded soil conditions. Within each subestuary, one site was dominated by Phragmites that produced viable seeds (high viability) and the other by Phragmites that did not (low viability). We also described standing vegetation in plots, measured soil salinity, analyzed soil characteristics, and described surrounding land cover. Based on number of emerging seedlings, we found that 284 and 698 Phragmites seeds m⁻² occurred at the two high-viability sites, which was significantly higher than seed densities at the low-viability sites (10 seeds m⁻²), and greater than densities reported elsewhere. We also found that emergence of Phragmites seedlings from soil samples was prevented by continuous flooding of 3.5 cm of standing water, suggesting that colonization of deep water areas is due to vegetative clonal expansion from Phragmites in adjacent higher elevations. The density of Phragmites seeds was not related to soil salinity or abundance of other species in the seed bank or vegetation, but instead was positively related to greater wave energy disturbance (much longer fetch and more open water) and lower area of wetlands nearby. The seed bank was more species-rich (15-22 species observed) than standing vegetation (3-15 species) at all sites, meaning that the dominance of Phragmites in vegetation does not prevent the development of a diverse seed bank and implying that a species-rich community may establish rapidly following control efforts. Based on these results and our findings in related studies, we postulate that wave energy disturbance generates repeated opportunities for seedling recruitment by Phragmites, which creates stands of Phragmites with higher genotypic diversity. In turn, genetically diverse stands favor greater cross-pollination and production of viable seed. These findings suggest that, in North America, targeting control efforts on non-native Phragmites patches in areas of higher exposure to wave energy may be more effective in reducing source populations than efforts in more protected locations.