Seed Bank Persistence Research Articles

Density‐Structured Models for Plant Population Dynamics

Density-structured models are structured population models in which the state variable is the proportion of populations or sites in a small number of discrete density states. Although such models have rarely been used, they have the advantage that they are straightforward to parameterize, make few assumptions about population dynamics, and permit rapid data collection using coarse density assessment. In this article, we highlight their use in relating population dynamics to environmental variation and their robustness to measurement error. We show that density-structured models are able to accurately represent population dynamics under a wide range of conditions. We look at the effects of including a persistent seedbank and describe numerical approximations for the mean and variance of population size. For simulated data, we determine the extent to which the underlying continuous process may be inferred from density-structured data. Finally, we discuss issues of parameter estimation and applications for which these types of models may be useful.

Weed seedbank responses to 12 years of applications of composts, animal slurries or mineral fertilisers

De Cauwer B, Van Den Berge K, Cougnon M, Bulcke R & Reheul D (2010). Weed seedbank responses to 12 years of applications of composts, animal slurries or mineral fertilisers. Weed Research50, 425–435.SummaryFertiliser amendments can impact weed populations in a variety of ways. This study evaluated the effects of 12‐year‐long applications of different fertilisation systems on size and composition of the weed seedbank in a conventionally managed maize monoculture field. Fertilisation systems included all factorial combinations of two dairy cattle slurry rates, three vegetable, fruit and garden waste compost rates, and three synthetic N fertiliser rates. Soil samples were taken in each subplot in May 2008 after sowing and prior to herbicide application. Residues recovered from soil samples were tested for weed seedling emergence to characterise soil seedbanks. Total weed seedbank density was affected by mineral N fertilisation, but not by compost or animal slurry application. Weed seedbank composition was related to compost amendment and mineral N fertilisation. Annual compost amendments reduced seedbank density of some persistent species (e.g., Chenopodium album and Solanum nigrum) irrespective of mineral N fertilisation. Compost is a promising tool for incorporation into integrated weed control strategies aimed at reducing weed seedbank persistence.

The demography of feral alfalfa (Medicago sativa L.) populations occurring in roadside habitats in Southern Manitoba, Canada: implications for novel trait confinement

Feral populations of cultivated crops can act as reservoirs for novel genetically engineered (GE) traits and aid in trait movement at the landscape level. However, little information is available on the potential of cultivated crops to become feral. In this study, we investigated the ferality of alfalfa populations (non-GE version) occurring in roadside habitats. Knowledge on the nature of roadside alfalfa populations would be useful for designing efficient trait confinement protocols and coexistence strategies in alfalfa. We investigated roadside alfalfa populations from 2006 to 2009 in three rural municipalities (Hanover, MacDonald, and Springfield) in Southern Manitoba, Canada. We studied the demography of these populations including seedbank, seedling recruitment, and fecundity and examined the impact of road verge mowing on key life stages of these populations. We also compared the growth and reproductive attributes of roadside and cultivated alfalfa populations. Alfalfa is reproductively successful in roadside habitats and capable of establishing self-perpetuating populations. A substantial portion of the alfalfa seeds we extracted from seedbank samples were viable but not germinable, suggesting some degree of seedbank persistence in roadside habitats. In the roadside habitat, alfalfa seedlings recruited successfully, however, seedling mortality was high when seedlings were in close proximity to well-established alfalfa plants. Mowing dramatically reduced the reproductive success of roadside alfalfa. Generally, the growth and reproduction of roadside alfalfa was comparable to cultivated alfalfa except for total fecundity. Considering the long lifespan (>10 years) of alfalfa and the levels of fecundity, seedbank, and seedling survival we observed, long-term persistence of roadside alfalfa populations seems reasonable. In the context of novel trait confinement, our results suggest that feral alfalfa populations required to be managed if there is a desire/need to confine novel traits in alfalfa.

Germination Ecology of Biennial Wormwood (Artemisia biennis) and Lanceleaf Sage (Salvia reflexa) Seeds

Biennial wormwood and lanceleaf sage have become serious weeds of several crops within the northern Great Plains of the United States and Canada. Both species are prolific seed producers but little is known about their potential for developing persistent seedbanks. Field studies were conducted to determine the influence of duration (7, 8, 11, 19, 20, and 23 mo) and depth of burial (0, 2.5, and 10 cm) on biennial wormwood and lanceleaf sage seed viability and decay. Biennial wormwood and lanceleaf sage seeds were buried in September 2003 (burial 1) and September 2004 (burial 2). In burial 1, biennial wormwood and lanceleaf sage seed viability was 65 and 66%, respectively, after 23 mo of burial. In burial 2, biennial wormwood and lanceleaf sage seed viability was 8 and 3%, respectively, after 23 mo of burial. The difference was likely because of higher soil moisture during burial 2, which promoted seed decay. Controlled-environment studies sought to determine the influence of stratification environments (freezing, chilling, and freeze–thaw) followed by exposure to diurnally fluctuating temperatures on germination of biennial wormwood and lanceleaf sage seeds. Stratified biennial wormwood seed germination was 95% or greater when incubated in fluctuating day/night temperatures of 37/20 or 37/25 C. Stratified lanceleaf sage seeds from freezing and chilling environments did not differ in germination following incubation in fluctuating temperatures and averaged 56 and 55%, respectively. Germination of stratified lanceleaf sage seeds from the freezing and thawing environment was higher than 50% during the thawing cycle, suggesting the possibility of early season emergence of this species. Our study indicates that biennial wormwood and lanceleaf sage have the potential to develop a seedbank that can persist for more than 2 yr. High moisture levels in the soil seedbank can lead to reduced seed survival.

Soil seedbank composition and dynamics across alpine summits in south-eastern Australia

Alpine soil seedbanks are generally regarded as small and unimportant to regeneration. Here, we investigate for the first time the composition of the readily germinable soil seedbank across alpine summits in south-eastern Australia. We aimed to compare the species in the seedbank with the standing vegetation, show seasonal variations in seedbank composition and identify regeneration strategies of alpine seedbank species. By using standard glasshouse and cold-stratification germination techniques, the germinable soil seedbank across the study region was found to comprise 39 species from 25 families, with species from the Asteraceae the most common. Persistent seedbanks were found across all eight alpine summits (1668–1970 m), comparable in seed density (150 ± 27 to 1330 ± 294 per m2) with those of other alpine areas in the northern and southern hemispheres. The density of germinable seeds varied widely among sites and between collection times (autumn, spring) and there were no trends in seed density with altitude. The qualitative and quantitative similarity between the seedbank species and the standing vegetation was low. Correlations between the proportions of species in regeneration categories (from obligate seeders, through to vegetative regenerators) in the standing vegetation and the seedbank were also poor. Our results indicate a divergence between the species in the current standing vegetation and those present in the readily germinable soil seed bank. The current patterns and predominance of seed-regenerating species in the seedbank indicate that these species may have an important role to play in regulating and contributing to future changes in the vegetation assemblage.

Dispersal and establishment limitation reduces the potential for successful restoration of semi‐natural grassland communities on former arable fields

Summary 1. Ex‐arable fields have been suggested as potential sites for re‐creation of semi‐natural grasslands, but information is still limited on the temporal scales needed for the natural assembly of these communities and whether colonization is related to dispersal or establishment limitation. 2. We investigated grazed ex‐arable fields of different age and adjacent semi‐natural grasslands in terms of species richness of plants, community similarity, colonization pattern and recruitment ability of 16 sown grassland species. The functional trait distribution of successful and unsuccessful colonizing species was compared using five traits related to dispersal and persistence: seed mass, seed bank persistence, specific leaf area, plant height and potential for lateral spread. 3. The youngest ex‐arable fields had the lowest species richness and contained communities with the lowest similarity to semi‐natural grassland. Species richness and similarity to semi‐natural grassland both increased with time since grazing started on ex‐arable fields, but were still significantly lower than in semi‐natural grasslands even after more than 50 years of grazing. 4. Colonization was not related to any of the investigated functional traits. The rank order of the species in terms of abundance was correlated between young and old ex‐arable fields suggesting that species performance remains the same with field age. 5. Recruitment after sowing was generally lower in ex‐arable fields than in semi‐natural grasslands, although the basic recruitment ability varied between species. Recruitment did not change with field age, suggesting that dispersal limitation rather than establishment limitation caused the temporal pattern of colonization. However, establishment limitation may act as a filter for colonization of all ex‐arable fields, regardless of their age. 6. Synthesis and applications. This study demonstrates that the temporal scale for natural assembly of semi‐natural grassland communities in ex‐arable fields extends over 50 years, even when source pools are nearby. Results suggest that a field age‐independent establishment limitation, combined with dispersal limitation, cause the delayed assembly in ex‐arable fields. Management that aims to re‐create semi‐natural grassland communities in ex‐arable fields should consider introducing seeds or improving germination conditions at the sites.

Similarities in the impact of three large invasive plant species on soil seed bank communities

Invasions by alien plant species may substantially alter soil seed bank communities. While decreases in seed bank species richness, diversity, and composition as a consequence of plant invasions have been reported, the characteristics of seed banks associated with different invasive species have not been compared in any detail. Here, we describe changes in the characteristics of soil seed banks invaded by three large herbaceous invasive plants, Fallopia japonica, Gunnera tinctoria, and Heracleum mantegazzianum. The study was carried out at the spatial scales of site and plot, to reduce variability in seed bank data. Information on seed bank persistence was inferred from seed depth (0-5, 5-10, and 10- 15 cm) and from time of sampling (May and October). Despite differences in the reproductive strategy and geographic distribution of these invad- ers, as well as in the standing vegetation and habitat types examined, the seed banks of invaded areas were similar in composition and in the relative abundance of different species. Invaded seed banks were dom- inated by seeds of a few agricultural weed species and/or rushes, suggesting that common features of the invaders, including a large standing biomass, exten- sive litter production, and the formation of mono- species stands may result in comparable selection pressures that favors traits that are largely genera or species-specific. These findings have a direct rele- vance for the development of strategies aimed at restoring previously-invaded sites while also improv- ing our understanding of the long-term implications of plant invasions.

Climate change and bet‐hedging: interactions between increased soil temperatures and seed bank persistence

AbstractIn order to predict the long‐term consequences of climate change, it is necessary to link future environmental changes to mechanisms that control plant population processes. This information can then be incorporated into strategies to more accurately model climate change impacts on species or to estimate future extinction risks. We examined the impact of increased temperatures on the longevity and dynamics of the persistent soil seed banks of eight ephemeral species from arid Australia. We found that the predicted global temperature increases under climate change will be reflected in increased soil temperatures, and that seeds in the soil seed bank will be exposed to long durations of high temperatures over the summer months. Three of the eight species studied had significantly greater levels of germination after exposure to predicted increased soil temperatures. Another species displayed a dramatic decrease in seed viability after such exposure. The capacity of such species to use the seed bank to bet hedge against rainfall events that cause germination but are insufficient to allow plant maturation, is compromised by increased germinability and subsequent loss or reduction of seed bank persistence. These predicted changes in the dynamics of soil seed banks increase the risk of local extinctions of these species, while the composition of the community may be altered by changes in species abundance. Our results show that the risk spreading mechanism provided by persistent seed banks could be compromised by the mechanistic impact of forecast temperature increases in arid habitats, and highlight the need to understand mechanisms that control population dynamics when attempting to address likely future impacts of climate change on biodiversity.

Classification of canola (Brassica napus) winter cultivars by secondary dormancy

Secondary dormancy is the major reason for seed persistence of canola (Brassica napus L.) in the soil. Volunteers emerging from the soil seed bank can lead to unwanted gene dispersal. More than 40 B. napus canola cultivars were tested for secondary dormancy under laboratory conditions. All cultivars were classified into groups of low, medium, and high dormancy by performing a cluster analysis. The results suggest that secondary dormancy is a cultivar-specific trait. Additionally, inter-year variation in dormancy indicates that it seems to be influenced by a set of environmental factors. Among years, classification of cultivars based on relative rank was more robust than classification based on absolute dormancy values. The classification of cultivars by their dormancy level would allow farmers to select and grow low-dormancy cultivars. Knowledge about the relative secondary dormancy of the currently grown cultivars could help growers and breeders lower canola seed bank persistence. Key words: Brassica napus, cluster analysis, genotype, secondary dormancy, soil seed bank

The seed bank longevity index revisited: limited reliability evident from a burial experiment and database analyses.

Seed survival in the soil contributes to population persistence and community diversity, creating a need for reliable measures of soil seed bank persistence. Several methods estimate soil seed bank persistence, most of which count seedlings emerging from soil samples. Seasonality, depth distribution and presence (or absence) in vegetation are then used to classify a species' soil seed bank into persistent or transient, often synthesized into a longevity index. This study aims to determine if counts of seedlings from soil samples yield reliable seed bank persistence estimates and if this is correlated to seed production. Seeds of 38 annual weeds taken from arable fields were buried in the field and their viability tested by germination and tetrazolium tests at 6 month intervals for 2.5 years. This direct measure of soil seed survival was compared with indirect estimates from the literature, which use seedling emergence from soil samples to determine seed bank persistence. Published databases were used to explore the generality of the influence of reproductive capacity on seed bank persistence estimates from seedling emergence data. There was no relationship between a species' soil seed survival in the burial experiment and its seed bank persistence estimate from published data using seedling emergence from soil samples. The analysis of complementary data from published databases revealed that while seed bank persistence estimates based on seedling emergence from soil samples are generally correlated with seed production, estimates of seed banks from burial experiments are not. The results can be explained in terms of the seed size-seed number trade-off, which suggests that the higher number of smaller seeds is compensated after germination. Soil seed bank persistence estimates correlated to seed production are therefore not useful for studies on population persistence or community diversity. Confusion of soil seed survival and seed production can be avoided by separate use of soil seed abundance and experimental soil seed survival.

Population Ecology and Management of Rigid Brome (Bromus rigidus) in Australian Cropping Systems

Rigid brome is a problematic weed of southern Australian cropping systems. Increased knowledge about the ecology of rigid brome and the influence of management strategies on its seedbank dynamics could facilitate development of more effective weed control programs. A field study was undertaken to investigate seedbank persistence and the influence of different management strategies on rigid brome control at Lock in South Australia during 2003 to 2005. Seeds of rigid brome were found to persist in the soil for up to 3 yr, with > 20% of the seedbank persisting from one season to the next. Therefore, a single year management program against this weed species is likely to be ineffective and could result in rapid buildup in weed infestations. However, management strategies that combined effective herbicides (ClearfieldTMtechnology) and crop competition over consecutive years provided effective control of this troublesome weed. Such cropping systems reduced rigid brome density (1 to 10 plants m−2) and seed production (8 to 160 seeds m−2) in the final crop of the 3-yr cropping sequence as compared to common grower practice of trifluralin and triasulfuron mixtures (138 plants m−2; 1,866 seeds m−2). These treatment combinations were able to deplete the initial seedbank (1,748 seeds m−2) to manageable levels (< 5 seeds m−2) within 3 yr. The results of this study should provide growers with confidence that severe rigid brome infestations can be managed effectively without compromising crop productivity.

Seed-Mediated Gene Flow in Wheat: Seed Bank Longevity in Western Canada

Development of genetically modified (GM) wheat has raised concerns about the movement and persistence of transgenes in agroecosystems and the ability of growers to segregate GM from conventional wheat. Wheat as a crop has been studied extensively but the population biology of volunteer wheat is not well characterized. Artificial seed bank studies were conducted in western Canada to provide baseline data on volunteer wheat seed persistence. Seed from two cultivars of Canadian western red spring wheat, ‘AC Splendor’ and ‘AC Superb’, were buried in mesh bags at three depths (0, 2, and 15 cm) in two different environments in the fall of 2003 and 2004. In addition, in 2004, ‘AC Superb’ seed were separated into small and large seed lots and buried with a medium seed lot to examine the influence of seed size on seed bank persistence. Seeds were withdrawn at intervals to assess seed germination and viability and regression analysis conducted on the viable seed at each sample period, after burial. Seed viability was variable within years and sites, and declined exponentially over time. In the spring, approximately 6 mo after initiation, viable seed ranged from 1 to 43%. With the exception of a single site and year, seeds on the soil surface persisted significantly longer than buried seeds and increasing burial depth accelerated loss of viability. The maximum viability of wheat seeds at 0, 2, and 15 cm depth in the spring following planting was 43, 7, and 2%, respectively. The extinction of viability for 99% (EX99) of the seed was estimated from regression analysis. The EX99values of seeds buried at 0, 2, and 15 cm ranged from 493 to 1,114, 319 to 654, and 175 to 352 d after planting (DAP), respectively, with the exception of one site in 2003 where burial depths were not different and all had an EX99value of 456 DAP. Seed size and cultivar did not significantly affect persistence, with the exception of one site in 2003 where the difference in EX99values was 20 DAP. The rapid loss of seed viability limits temporal gene flow via volunteers in years following a wheat crop. Results provide data on spring wheat biology to aid in Canadian environmental biosafety assessments of GM wheat and will be incorporated into a mechanistic model to predict wheat gene flow on the Canadian prairies.

The association between seed size and seed longevity among maternal families inAmbrosia trifidaL. populations

AbstractIn temperate plant communities, seed size is often inversely related to seed longevity at the species level. We tested if the inverse relationship between seed size and seed longevity holds among individuals within populations ofAmbrosia trifidaL. (Asteraceae), a summer annual agricultural weed in the eastern United States Corn Belt that typically emerges in two successive flushes: one before 1 May and one after 1 May. The effects of maternal phenotype on seed morphology, seed weight, late emergence (emergence after 1 May) and seed-bank persistence were determined in two 1-year experiments. All seeds were collected from individuals at weekly intervals for 3 weeks. Sixty seeds per collection were used in the analysis. Despite fluctuating environmental conditions during seed development, seed weight within maternal families remained constant for different maturation times. Differences among families constituted 69% of total variation in seed weight, whereas differences within maternal families composed 31% of the variation in seed weight. Percentage late emergence and percentage seed-bank persistence varied among maternal families. In four maternal families, either 0% or 100% of seedlings emerged after 1 May, but a majority of other maternal families produced seedlings both before and after 1 May. Seed-bank persistence rates ranged from 1 to 51% among maternal families. Percentage late emergence and percentage seed-bank persistence were inversely related to maternal-family mean seed weight in 2006, but not in 2005. These results suggest that the inverse relationship between seed size and seed longevity occurs among individuals of a population, but is affected by the environment.

Relationships between the developments of seedbank, standing vegetation and litter in a post-mined peatland

To clarify seedbank developmental pattern and its determinants, we examined relationships between seedbank, standing vegetation, and litter in a post-mined peatland on Sarobetsu Mire, northern Japan. In late fall 2006 and early summer 2007, peat was sampled from four vegetation types, and litter was also sampled from two of the four vegetations where thick litter was accumulated. Seed germination tests were performed by the samples in a glasshouse. The results were compared between the standing vegetation and litter accumulation. Seed density and species richness in seedbank gradually increased with increasing vegetation cover. Seeds in surface layer germinated more in fall than in summer. In vegetation with thick litter, small seeds, such as Drosera rotundifolia and Gentiana triflora, germinated more from peat than from litter in summer, probably due to the downward movement. Seed density in litter decreased from fall to the next summer. Moliniopsis japonica and Rhynchospora alba developed short-term persistent seedbank, whereas D. rotundifolia and G. triflora preserved long-term persistent seedbank in peat covered with thick litter. Seedbank development follows the development of the standing vegetation with litter. Litter derived from the standing vegetation is a prime determinant on the seedbank composition and development, by acting as seed trap and keeping seed dormancy.

Reducing Persistent Seed Banks of Invasive Plants by Soil Solarization—The Case of Acacia saligna

An important factor in controlling invasive plant infestations is frequently the acceleration of the deterioration of their persistent seed bank, which is often associated with physical dormancy mechanisms. We hypothesized that breaking dormancy by heat would enhance the vulnerability of the nondormant seeds to hydrothermal stresses. The aim of the present study was to examine the effect of soil solarization treatments (heating the soil by means of polyethylene mulching) on buried Australian Acacia seeds, with emphasis on Acacia saligna L. The results of three field experiments indicate that soil solarization treatments caused an almost complete eradication of buried seeds of Acacia saligna and two other Australian Acacia species, Acacia murrayana and Acacia sclerosperma. The killing mechanism of solarization was further studied in laboratory experiments. We observed two phases of the heat-induced deterioration of seed persistence: breaking the dormancy of the seeds and exposing the “weakened seeds” to lethal temperatures. From an ecological perspective of conservation, the present study shows for the first time the possible utilization of solar energy, by means of soil solarization, for reducing persistent seed banks of invasive woody plants.

Maternal and Burial Environment Effects on Seed Mortality of Velvetleaf (Abutilon theophrasti) and Giant Foxtail (Setaria faberi)

The primary defense against seed mortality, the seed coat, is maternally derived. Hence, weed seed mortality in the soil seedbank is likely to be influenced by the maternal environment and genetics. We hypothesized that seed accessions from contrasting maternal environments (seed lots) exhibit different rates of seed mortality and that the relative differences among seed lots remain consistent across burial environments. Velvetleaf and giant foxtail annual seed mortality rates were studied in field experiments in Hickory Corners, MI, and Wooster, OH, using seed lots collected from the same locations. Seeds enclosed within mesh bags and unenclosed seeds (‘‘seeded cores’’) exhibited similar levels of seedbank persistence (r 5 0.90, P , 0.001) and seed mortality (r 5 0.65, P 5 0.006). Annual seed mortality rates ranged from 16 to 56% and 27 to 91% for seed lots of velvetleaf and giant foxtail, respectively. Relative differences among velvetleaf seed lots were consistent across burial environments in both years, whereas giant foxtail differences were consistent in only 1 of 2 yr. The relative ranks among velvetleaf seed lots varied between years, indicating that maternal environment may have influenced seed persistence more than seed-lot genetics. Within years, variation in seed mortality was predicted by changes in soil moisture in the burial environment (R 2 5 0.47, P , 0.001 for velvetleaf; R 2 5 0.34, P 5 0.007 for giant foxtail). Accelerated seed mortality was associated with moist soils (soil water potential 52 6 kPa for velvetleaf, 27 kPa for giant foxtail). These results suggest that agronomic practices affecting the maternal environment and moisture levels in the soil seedbank may promote weed seed mortality in the soil seedbank. Nomenclature: Giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medik. ABUTH.

Chemical and Physical Defense of Weed Seeds in Relation to Soil Seedbank Persistence

Effective weed seedbank management requires mechanistic understanding of ecological determinants of seed persistence in the soil seedbank. Chemical and physical defense of common lambsquarters, field pennycress, giant foxtail, kochia, velvetleaf, and yellow foxtail seeds were quantified in relation to short- and long-term seedbank persistence. Seed content ofortho-dihydroxyphenols (o-DHP), a class of putative seed defense compounds, varied more than threefold between the least protected species (common lambsquarters, 9.2 µg g seed−1) and the most protected species (kochia, 34.1 µg g seed−1). Seedo-DHP was inversely related (r= −0.77, P < 0.001) to seed half-life in the soil and to short-term seed persistence in burial assays (r= −0.82, P < 0.05). The relative importance of chemical seed protection in comparison to physical seed protection, as represented by the ratio of seedo-DHP concentration to seed coat thickness, decreased linearly with increasing short-term seed persistence (r= −0.96, P < 0.01) and nonlinearly with increasing long-term seed persistence in the soil seedbank (y = 0.16 + 0.21/(0.0432 + x),R2= 0.99, P < 0.001). Mechanical damage to the seed coat, via piercing, slicing, or grinding treatments, increased short-term mortality during burial for all six species. Mortality of pierced seeds was negatively associated (r= −0.35, P < 0.05) with seed phenol concentration and positively associated with seed half-life (r= 0.42, P < 0.01) and seed coat thickness (r= 0.36, P < 0.05). Seed phenolics, as a class, supported the results foro-DHPs. Overall, these findings suggest a potential weakness, with respect to seedbank management, in the way weed seed defenses are constructed. Weed species with transient seedbanks appear to invest more in chemical defense than those species with highly persistent seedbanks. As a result, seeds in the latter category are relatively more dependent upon physical seed protection for persistence in the soil seedbank, and more vulnerable to management tactics that reduce the physical integrity of the weed seed coat.

Weed seedbank biodiversity in emmer wheat (triticum dicoccum (schrank) schübler) in a mountainous agro-ecological oasis (garfagnana, tuscany)

Phytocoenoses of conventional agroecosystems are subjected, already from several decades, to the reduction of the weed species present in the various crops. Such floristic decreasing is directly proportional to intensity of the agronomic impact. The present work is born from the hypothesis that the agro-ecological oases, managed with the ancient agrotechniques, are linked by an high degree of plant biodiversity. In this perspective it was carried out not only an analysis of the field emerged weeds, but even an evaluation of the seedbank since this one synthesizes the weed flora of a wider period. In the experimental agroecosystems, selected due to the typical Emmer wheat presence, an high degree of weed species diversity was observed, above all of terophytes, in the emerged flora as well in the seedbank. In both cases relative densities of each species were found low and without any weed dominance. Probably it occurs as a function of the high degree of competitive and allelopathic interactions. Almost scarce was the presence of exhumed seeds of graminaceae virtually due to their inability to store in the soil a persistent seedbank. Of particular importance it was the discovery of two rare species such as Agrostemma githago and Centaurea cyanus disappeared from many years by the landscape of “conventional” agricultural systems. The seedbank was found uniformly distributed in both sampled soil layers (0-15 and 15-30 cm) confirming that plowing induced an uniform burial of the annually produced seeds. The total examined soil profile (0-30 cm) showed a quantitative seedbank similar to those already found in “biological” agricultural systems (from 12.000 to 47.000 seeds m-2). However it was qualitatively formed even from several weed species of negligible agronomic impact as a function of their scarce competitivity like in the case of some caryophyllaceae (Silene noctiflora and S.alba), boraginaceae (Myosotis arvensis and Echium vulgaris) and campanulaceae (Legousia speculum-veneris). Finally both agronomic and ecological involvements of this weed complexity were discussed. This complexity was retained of crucial importance not only regards to the biodiversity conservation but even for their role in the landscape ecology. In synthesis, it has been confirmed the hypothesis that the ancient phytocoenoses are still present but only in these ecological oases in which the agronomic impact was ecologically sustainable. In conclusion, it was stressed the crucial role of these agro-ecological oases not only with the aim of the in situ biodiversity conservation, but even as ex situ germoplasma source to use in the perspective of ecological restoration of degraded areas.

Human activity facilitates altitudinal expansion of exotic plants along a road in montane grassland, South Africa

AbstractQuestion: Do anthropogenic activities facilitate the distribution of exotic plants along steep altitudinal gradients?Location: Sani Pass road, Grassland biome, South Africa.Methods: On both sides of this road, presence and abundance of exotic plants was recorded in four 25‐m long road‐verge plots and in parallel 25 m x 2 m adjacent land plots, nested at five altitudinal levels: 1500, 1800, 2100, 2400 and 2700 m a.s.l. Exotic community structure was analyzed using Canonical Correspondence Analysis while a two‐level nested Generalized Linear Model was fitted for richness and cover of exotics. We tested the upper altitudinal limits for all exotics along this road for spatial clustering around four potential propagule sources using a t‐test.Results: Community structure, richness and abundance of exotics were negatively correlated with altitude. Greatest invasion by exotics was recorded for adjacent land at the 1500 m level. Of the 45 exotics, 16 were found at higher altitudes than expected and observations were spatially clustered around potential propagule sources.Conclusions: Spatial clustering of upper altitudinal limits around human inhabited areas suggests that exotics originate from these areas, while exceeding expected altitudinal limits suggests that distribution ranges of exotics are presently underestimated. Exotics are generally characterised by a high propagule pressure and/or persistent seedbanks, thus future tarring of the Sani Pass may result in an increase of exotic species richness and abundance. This would initially result from construction‐related soil disturbance and subsequently from increased traffic, water run‐off, and altered fire frequency. We suggest examples of management actions to prevent this.

Canola (Brassica napus L.) seedbank declines rapidly in farmer-managed fields in South Australia

Canola is an important crop in southern Australia, where it is used as part of the crop rotation to manage cereal diseases, improve wheat yields, and assist in integrated weed management programs. The potential release of herbicide-tolerant transgenic cultivars into Australia has raised concerns that volunteer canola may itself become an uncontrollable weed. This study examined the persistence of the canola seedbank in farmer-managed fields in 3 geographical areas of the South Australian cropping region for up to 3.5 years after the last canola crop was grown. In total, 66 fields from minimum- and no-tillage farms were sampled for number of canola seed/m2 and the percentage of those that germinated. ANOVA analysis indicated that time since the last harvest and cultivation method were both significant factors affecting the number of seed/m2 present. Neither time since harvest nor cultivation method was significant for number of germinated canola seeds, although time since harvest approached significance at the 5% level. This demonstrates that the canola seedbank and the number of volunteers decline rapidly in managed cropping systems in southern Australia. Therefore, it is unlikely that herbicide-tolerant canola will become a major weed if volunteers are managed carefully.