Germination characteristics and seedbank of the alien species Leucaena leucocephala (Fabaceae) in Brazilian forest: ecological implications

Plant species invasiveness is frequently associated with rapid proliferation and production of seeds that can persist in the soil for long periods of time. Leucaena leucocephala (Fabaceae) is an alien and invasive species, for example in Brazilian forest and savannah ecosystems. This study quantified the invasive potential of this species by analysing its seed rain (using seed collectors), seed longevity in the soil (stored in buried bags) and the germination capacity of the soil seedbank (by collecting soil samples in the study area). Our results showed that seed rain occurred throughout the year, although more intensely from July to September, with about 5500 seeds m^?2 year^?1 being released. The numbers of seeds in the buried bags diminished over time and intact seeds showed low germinability (approximately 15%), although their viability remained >80% of the recovered seeds after two years of in situ storage. The germinability of seeds collected directly from the soil was approximately 40%, indicating that more than half of the seeds of soil seedbank were dormant (physical dormancy). Leucaena leucocephala produces large numbers of seeds and is able to form a persistent short‐lived seedbank (viability 1–5 years). These factors may contribute significantly to its invasive potential, which makes it difficult to control this species once it becomes established. As control costs become higher over time, immediate public efforts are needed to counter this threat.     

The long term decline of a population of Avena fatua L., with different cultivations associated with spring barley cropping

The decline of a population of A. fatua established in September 1971, and not allowed to seed thereafter, was monitored in three successive barley crops Numbers of viable seeds in the son in June fell from 159 m² in 1972 to 1 m² in 1974 declining by 83° in the first, and by 96° in the second year. Seedling numbers fell from 138 m² in 1972 to 9 m² in 1974, declining by 32° in their first and by 89° in the second year. The slower decline of seedling numbers in the first year was attributed to loss of dormancy of seed reserves giving proportionally more seedlings in the second spring. Cultivations had no major influence on the pattern of seedling emergence in the spring, but they did affect population level. Where the stubble was cultivated immediately after seeding in September 1971, twice as many seedlings and three times as many seeds in the soil were present in 1972 compared with delayed autumn cultivations in 1971. From this greater reserve of seeds in the soil more seedlings arose in 1973 and in 1974. Time cultivation in the winter of 1971 resulted in slightly more seedlings in 1972 than did ploughing; this greater population declined more rapidly with line cultivation in 1973 and 1974. It is suggested that with no herbicidal control annual line cultivation will lead to a more rapid build up than ploughing: where A. fatua is controlled, the decline should be more rapid with tne cultivation. Persistence of A. fatua as a weed in arable situations seems related more to survivors shedding seeds than to the persistence of seeds on the soil. Factors which may influence the persistence of seeds in the soil are discussed.     

Seed dynamics in populations of A vena sterilis ssp. ludoviciana

Seed populations of Avena sterilis ssp. ludoviciana (Durieu) Nyman were monitored in a naturally occurring infestation throughout its life cycle. Considering the large weed population present (298panicles m^?2), total seed production was relatively low: 3838 seeds m^?2. Only 68% of these seeds were recovered from the soil surface and a further 3% were removed with wheat grain and straw during harvest operations. The numbers of seeds from the stubble between mid-July and mid-September were relatively low (10%). Ploughing the stubble in October buried most of the recently produced seed rain and resulted in a relatively uniform vertical distribution of the seedbank. Maximum seed persistence in the soil ranged from 27 to 43 months (depending on the experimental technique used to do the study). Seed decline followed an exponential pattern on a yearly basis, with the greatest decline taking place between October and April (57–90% in year 1 and 10–40% in year 2), Between May and September the buried seed populations remained practically constant. Seedbank depletion was primarily due to seedling production (25%) and ‘lethal’ germination (24%). Although the depth of burial had very little effect on seed survival, the mode of seed disappearance was closely related to their depth in the soil. Seed depletion through ‘lethal’ germination increased with increasing depth in the soil, whereas depletion through seedling emergence decreased with increasing depth.     

The survival of Alopecurus myosuroides Huds. seeds in soil

The survival of Alopecurus myosuroides seeds was studied in soil under arable cropping and short term grass leys in which seed return was prevented. At two winter wheat sites, where weed seeds were sown, the mean annual seed decline was 73–83% over a 2- or 3-year period. The rate of decline was similar with all the cultivation systems studied: ploughing, tine cultivation and direct drilling. Seeds buried initially by ploughing, and then not disturbed by cultivation, were slightly more persistent. At five arable sites with natural populations of A. myosuroides, seed numbers declined to an average of 3% of the original amount present after 3 years, and to 1% after 4 years. Initial populations of over 50 000 seeds m^?2 were recorded. Plant populations were not always proportional to the total seed content of the soil, especially on ploughed land. Seed decline in two grass fields was similar to that under arable cropping. A. myosuroides plants were recorded in a wheat crop following a 2-year grass ley. Weed plants did not persist in the vegetative state in grass used for conservation and grazing. At all sites, appreciable quantities of seeds were still present in the soil after 2–4 years. Although a relatively small proportion of seeds survived, the actual number of seeds surviving was substantial. For this reason, it was concluded that any eradication policy is unlikely to be effective in a cropping system dominated by winter cereals.     

Weed seed predation rate in cereals as a function of seed density and patch size,under high predation pressure by rodents

Weed seed predation is an ecosystem service, influencing weed population dynamics. The impact of weed seed predation on weed population dynamics depends on how predators respond to seed patches at the field scale. Seed predation will be most effective if the proportion of seeds predated increases with increasing size and seed density of patches. Density‐dependent rodent seed predation was measured by varying seed density and patch size in four irrigated conventionally managed cereal fields in north eastern Spain. Artificial weed seed patches were created by applying a range of Lolium multiflorum seed densities from 0 to 7500 seeds m^?2 in 225 m² patches (2008) or in patches that varied in size from 1 to 9 m² (2009). Seed predation was estimated using seed cards and seed frames. The granivorous rodents Mus spretus and Apodemus sylvaticus caused high seed predation rates (92%) in three fields, whereas in a fourth field, it was lower (47%). Rodents responded in an inversely density‐dependent manner, but this had little biological meaning as even in patches seeded with the highest density, the input to the soil seedbank was reduced by 88%. For the period of time this experiment lasted, hardly any new seeds would have entered the seedbank.     

Spread of proso millet (Panicum miliaceum L.) in Ontario, Canada. II. Dispersal by combines

Seeds of proso millet (Panicum miliaceum L.) are moved between and within fields on combine harvesters. The dispersal of seeds of two biotypes of P. miliaceum by combine harvesters was quantified. The golden-seeded biotype of this weedy annual grass was known to have larger seeds and to experience less seed shattering than the black-seeded biotype. An average of 3·3% of the seeds on the plants of the black-seeded biotype was carried more than 50 m by combines, while 0·9% of the golden seeds were carried the same distance. The densities of the seed rain within 50 m of the weedy patches were 3·7 seeds m⁻² for the black-seeded biotype and 9·7 seeds m⁻² for the golden-seeded biotype. This difference was proportional to the difference in the number of seeds in the respective source patches. The numbers of seeds deposited at various points within 50 m of source patches were close to uniform for both biotypes. There was, however, a significant difference (P < 0·05) in the distributions of the seeds of the two biotypes.     

Seed banks of soils under vegetable cropping in England

Estimates of viable weed seeds in soil samples taken in 1968–1975 from 89 vegetable fields in England gave a median value of 4120 m^?2 in 0–15 cm. Poa annua, Stellaria media and Matricaria matricarioides occurred most frequently, and nine species accounted for 72% of all seeds found. The numbers of seeds were lower than those from a survey made a decade earlier but the species occurring most frequently were the same; M. matricarioides, however, appeared to have increased in relative importance.     

Life cycle and potential gene flow of volunteer oilseed rape in different tillage systems

The study examined the effect of tillage (intensive vs. zero tillage) on potential gene flow during the life cycle of oilseed rape volunteers between July 2002 and August 2003. After growing oilseed rape, 4–29% of the seeds lost during harvest entered the soil seedbank when stubble tillage was performed immediately after the seed input. The seedbank was small (0–3%) when stubble tillage was delayed. Zero tillage resulted in seedbanks from 1 to 17% of the initial seed input. The seeds were distributed mainly in the upper soil layers after zero tillage or primary tillage with a rigid tine cultivator, whereas ploughing shifted most of the seeds into deeper layers. The highest number of volunteers (1 plant m⁻²) emerged and flowered in the following crop of winter wheat either when a large soil seedbank existed and/or the seedbank was located mainly in the upper soil layer. Outcrossing with other rape crops was unlikely as volunteers flowered 1 month later than rape crops sown at the normal timing. These volunteers produced a maximum of 8 viable seeds m⁻². Ploughing preserved seeds in deep soil layers transferring the risk of gene flow to the future, whereas non-inversion tillage can cause gene flow from high numbers of flowering volunteers within the first year following oilseed rape cultivation.     

Evaluation du stock en semences de la morelle jaune (Solanum elaeagnifolium Cav.) dans le sol du Tadla (Maroc)

Assessment of the stock of Solaneum elaeagnifolium Cav. seeds in the soil of the Tadla (Morocco) On the basis of soil samples from four fields on the irrigated perimeter of the Tadla (Central Morocco), selected from the area most heavily infested with Solanum elaeagnifolium, estimates of the stock of seeds of this perennial weed revealed a mean value of 375 seeds m^?2 to a depth of 60 cm (dead + viable seeds). The average for the viable seed stock, however, is 163 seeds m^?2 to a depth of 60 cm, or an average of 43·5% of the total stock of seeds counted. The average distribution profile of the seeds of S. elaeagnifolium in the soil is as follows: 71·7, 25·1 and 3·2%, respectively, in the 0–20, 20–40 and 40–60 cm horizons. The vertical distribution of viable seeds followed the same profile as the overall stock; thus we found 73 and 27% of viable seeds were located in the 0–20 and 20–40 cm horizons, respectively.     

Interference of Sinapis arvensis L. and Chenopodium album L. in spring rapeseed (Brassica napus L.)

Interference of Sinapis arvensis L. (wild mustard) and Chenopodium album L. (lamb's-quarters) in spring rapeseed (Brassica napus L.) was studied under field conditions in 1983 and 1984. Both weed species interfered with rapeseed early in the growing season, causing significant reductions in rapeseed dry weight by June of each year. Sinapis arvensis caused greater rapeseed grain yield reductions than did C. album. With weed densities of 20–80 plants m^?2, rapeseed grain yield reductions ranged from 19 to 77% with S. arvensis but only 20 to 25% with C. album. Rapeseed yield reductions caused by mixtures of both weed species ranged from being less than to being equal to the sum of reductions caused by each weed alone, depending on the weed density and year of study. Both weed species were prolific seed producers capable of returning large quantities of seed to the soil. With weed densities ranging from 10–80 plants m^?2, S. arvensis produced 5700–30 100 seeds m^?2 while C. album produced 3100–63 600 seeds m^?2.     

Uprooted buffelgrass thatch reduces buffelgrass seedling establishment

Buffelgrass (Pennisetum ciliare (L.) Link), a non-native perennial bunchgrass, invades ecologically intact areas of the Sonoran Desert. It competitively excludes native plants and increases fire frequency and intensity. Since the 1990s, whole buffelgrass plants have been manually uprooted and removed to control the invasion in southern Arizona. Uprooting plants results in bare, disturbed soil which promotes buffelgrass seed germination. This study examined whether leaving entire uprooted buffelgrass plants (thatch) on a field site reduces future buffelgrass establishment compared to removing uprooted plants from the site. A secondary goal was to determine whether light reduction and autoallelopathy were major factors in the negative effect of thatch on buffelgrass seedling density. Field plots with an average of 8,095 kg/ha thatch had 1.9 buffelgrass seedlings/m² which was significantly fewer than the 2.9 seedlings/m² in plots without thatch. Thatched portions of thatch plots (50% of their total area) had only 0.7 seedlings/m². In the greenhouse, which reduced outdoor light intensity by 35.2%, buffelgrass seeds sown in bare soil resulted in significantly higher seedling density than beneath buffelgrass thatch. Potential autoallelopathic chemicals leached from partially decomposed buffelgrass thatch and leached thatch had an intermediate but not significant (p = 0.09) effect on seedling numbers. Results suggest that leaving uprooted buffelgrass plants has the benefit of reducing seedling establishment in the area disturbed by uprooting.     

Biology of Commelina benghalensis L. in south-eastern Queensland. 1. Growth, development and seed production

Experiments conducted at Kingaroy in southeastern Queensland, investigated growth, development and seed production of Commelina benghalensis L. a weed of field crops, which produces aerial and underground flowers. Plants were grown from four seed types (large and small seeds from the two types of flowers) in pots exposed to the weather and in the field. Development was faster, and growth and seed production were greater in the field than in pots. Plants grown from aerial seeds were smaller, developed aerial flowers earlier, and produced more aerial fruits than plants grown from underground seeds. Rhizomes and underground flowers began to develop 6 weeks after emergence on plants from all seed types, prior to the development of aerial flowers. Seed production in the field was 8000 m^?2 for plants grown from underground seeds and 12000 m^?2 for plants grown from aerial seeds. Small aerial seeds represented 73–79% of the total seed production, large aerial seeds 19–22%, and underground seeds only 1–3%.     

The effect of crop,cultivation and seed addition for birds on surface weed seed densities in arable crops during winter

Weed seeds present an agronomic threat, but are also an important food resource for wildlife in winter. Weed seed densities on the soil surface in winter were examined from 1999 to 2002 in 105 fields on three different farms in UK. The effect of the preceding crop, cultivation, position within the field and the application of seed for birds (bird seed) on surface seed abundance and species composition was tested. Six or fewer species comprised c. 80% of the weed seeds. By January of each study year, the densities of seeds important for farmland birds (key seeds) were 73% or 87% lower compared with early winter on two of the farms, but were stable on the third where seeds were incorporated through cultivation. At the edge and mid‐field, seed densities only exceeded 400 m^?2 in 17%, 10% and 12% of fields for total, key and dicotyledonous seeds respectively. The preceding crop only affected seed densities at one site; stubbles of winter barley had fewer seeds compared with winter wheat or spring barley. Seed densities varied between the edge and mid‐field, but trends were inconsistent between sites. The density of the larger seeds (Atriplex patula, Viola arvensis, Polygonum aviculare and Chenopodium album) were reduced in fields receiving bird seed. The objectives of weed control and conservation may not be mutually exclusive because seed return was most reduced where the ground remained uncultivated through the winter, yet this also provided the best foraging opportunities for surface feeding seed predators.     

Invasiveness of agronomic weed species in wheat in Western Australia

Weed seeds are introduced to agronomic systems naturally or through human-mediated seed dispersal, and introduced seeds have a high chance of being resistant to selective, in-crop herbicides. However, colonisation (invasion) rates for a weed species are usually much lower than rates of seed dispersal. The current research investigated colonisation of a winter annual wheat cropping system in Western Australia by a range of winter or summer annual weed species. The weed seeds were sown (at 100 seeds/m²) directly before seeding the crop in 2016 and allowed to grow in the following 3 years of wheat. Selective herbicides were not applied, to simulate growth of weed populations if the initial seed had been resistant to herbicide. Bromus diandrus, Hordeum leporinum, Rumex hypogaeus, Sonchus oleraceus, Polygonum aviculare, Lolium rigidum, Citrullus amarus and Tribulus terrestris colonised the crop, while Dactyloctenium radulans, Chloris truncata and Salsola australis failed to establish over 3 years. The most successful weed was B. diandrus, with a plant density of 1,170/m² by the third year and seed production of 67,740/m². The high density of B. diandrus reduced wheat density by 76% in the third year and reduced average yield by 36%. Lolium rigidum reduced average yield by 11%, and the other weed species did not affect crop yield. Further research is required on the invasiveness of these species in other regions, but it is clear that the spread of B. diandrus to new areas or the introduction of resistant B. diandrus seeds via contaminated grain should be avoided.     

Seed biology of Mikania micrantha in Viti Levu,Fiji

Mikania micrantha (mile‐a‐minute) reproduces both by seed and vegetatively. A study to determine the possible pollinators, seed production rates, temperature and salinity limitations to germination and its seedbank size and persistence was conducted in Viti Levu, Fiji. Representatives of the Diptera had the greatest percentage of all floral visits (38%), followed by Hymenoptera (34%) and Lepidoptera (27%), while the honeybee was the most recurring visitor (18% of all visits). Flower heads (capitula) within the inflorescence commonly formed four viable seeds, resulting in 60 820 filled seed being produced per m². However, the seedbanks formed were not massive (600 seed m^?2) and they were moderately persistent (T₅₀: 1–3 years). Seed germination from both high and moderate rainfall regions occurred rapidly, under a wide range of temperature regimes, with no primary dormancy being observed. This study indicates that the seed reproductive success of M. micrantha in the two rainfall regions of Fiji is due to a number of factors, including the production of large numbers of flowers, successful pollination by local insects and the subsequent production of a large number of viable seeds. These seeds have high viability, no dormancy and are capable of forming small‐to‐medium seedbanks that are moderately long‐lived. In addition, seeds can germinate under a wide range of temperature and salinity conditions. This knowledge on seed production, biology, longevity and salt tolerance is vital in the development of management plans of M. micrantha in Fiji.     

Enhanced weed‐crop competition effects on growth and seed production of herbicide‐resistant and herbicide‐susceptible annual sowthistle (Sonchus oleraceus)

Enhanced crop competition could aid in the management of annual sowthistle (Sonchus oleraceus L.), a dominant weed of Australian cropping systems. A two‐year pot study was conducted to evaluate the effect of wheat (Triticum aestivum L.) planting densities (0, 82, and 164 wheat plants/m²) on growth and seed production of glyphosate‐resistant (GR) and glyphosate‐susceptible (GS) biotypes of annual sowthistle. Without competition, both biotypes produced a similar number of leaves and biomass, but the GS biotype produced 80% more seeds (46,050 per plant) than the GR biotype. In competition with 164 wheat plants/m², the number of leaves in the GR and GS biotypes was reduced by 62 and 61%, respectively, in comparison with the no‐competition treatment, and similarly, weed biomass was reduced by 78 and 77%, respectively. Compared to no‐competition treatment, the seed production of GR and GS biotypes was reduced by 33 and 69%, respectively, when grown with 82 wheat plants/m², but increasing wheat density from 82 to 164 plants/m² reduced the number of seeds only in the GS biotype (81%). Both biotypes produced greater than 6,000 seeds per plant when grown in competition with 164 plants/m², suggesting that increased crop density should be integrated with other weed management strategies for efficient control of annual sowthistle.     

Ecophysiological study on weed seed banks and weeds in Cambodian paddy fields with contrasting water availability

Weed infestations are a major cause of yield reduction in rice (Oryza sativa) cultivation, particularly with direct‐seeding methods, but the relationship between weed dynamics and water availability in Cambodian paddy fields has not been documented previously. We surveyed the weed abundance and weed seed banks in the soil of paddy fields with inferred differences in their water regime in 22 farm fields in three provinces of Cambodia in the 2005 and 2006 rainy seasons. We studied rain‐fed lowland fields in upslope and downslope topographic positions and fields at different distances from the irrigation water source inside an irrigation rehabilitation area. The weed seed banks were estimated by seedling emergence in small containers and weed abundance and vigor were estimated by a simple scoring system. The estimated weed seed bank in the top 5 cm of soil ranged from 52.1 to 167 × 10³ seeds m^?2 (overall mean of 8.5 × 10³ seeds m^?2) and contained a high proportion (86%) of sedge species, such as Fimbristylis miliacea L. and Cyperus difformis. Several fields had particularly large seed banks, including one near the reservoir. No clear difference was found in the weed seed banks between the irrigated fields that were located close to (upstream) and distant from (downstream) the water source or between the irrigated and rain‐fed lowland fields, but the weed scores were larger in the rain‐fed fields and the downstream fields within the irrigated area. A water shortage during the late growing season in 2005 led to a proliferation of weeds in some fields and an associated increase in weed seedbank size in 2006. However, the weed scores in 2006 were more strongly associated with that year's water conditions than with the weed seedbank size.     

Efficacy of UV‐C radiation to reduce seedborne anthracnose (Colletotrichum acutatum) from Andean lupin (Lupinus mutabilis)

The potential of UV‐C radiation of Andean lupin (Lupinus mutabilis) seeds to eradicate seedborne infections of anthracnose caused by Colletotrichum acutatum was investigated. UV‐C doses from 0 to 691.2 kJ m^?2 (resulting from 0 to 96 h of exposure time) on disease incidence reduction and germination on artificially and naturally infected seed were evaluated. The degree of incidence reduction and seed germination was dependent on the dose of UV‐C. The UV‐C doses of 86.4 kJ m^?2 and higher reduced incidence from 6% to 7% to undetectable levels, but these UV‐C doses also reduced seed germination. UV‐C can deleteriously affect physiological processes and overall growth. To assess its impact, L. mutabilis seeds irradiated with UV‐C doses of 57.6 and 86.4 kJ m^?2 were grown. Seedlings grown from noninfected seed and UV‐C treated seed showed an increased concentration of chlorophyll and protein contents, as well as an increase in the activation of defence enzymes peroxidase and catalase, in comparison with plants grown from infected seed. UV‐C doses resulted in seed emergence and seedling dry weight rates that were similar to the noninfected control or better than the fungicide control. Moreover, 57.6 kJ m^?2 reduced transmission of the pathogen from seed to the plantlets by 80%, while 86.4 kJ m^?2 apparently eradicated the pathogen, under greenhouse conditions. The use of UV‐C, first reported here, is advantageous for controlling anthracnose in lupin.     

Weed seed bank as affected by tillage,residue, and fertilization management under sweet corn-wheat cropping sequence in Iran

Soil weed seed bank is an important factor determining above-ground floristic composition and weed density in agricultural systems. The quantitative and qualitative measures of weed seed bank can help growers to predict the extent to which they are facing weed problems. Along with tillage, crop residues can affect the fate of weeds in the upcoming crops. To investigate such effects, we compared the effects of tillage systems [conventional tillage (CT), reduced tillage (RT), and no tillage (NT)], wheat residue retention, and nitrogen (N) rates (0, 69, 138, and 207 kg N ha⁻¹) on depth-related characteristics of the weed seed bank under a sweet corn-wheat sequence during 2014–2015 growing seasons in Shiraz, Iran. Soil bank was not affected by tillage systems but tended to be slightly higher under RT. The highest (898 seeds m⁻²) and lowest (322 seeds m⁻²) weed population at 0–10 cm depth were found when 138 kg N ha⁻¹ in 2015 and 207 kg N ha⁻¹ in 2014 were applied. Species richness and diversity were higher under NT and RT practices at the top layer, but CT system was more diversified at deeper depths. They were higher when crop residues were retained as well. Barnyard grass (Echinochloa crus-galli [L.] Beauv), common lambsquarter (Chenopodium album L.), common purslane (Portulaca oleracea L.), field bindweed (Convolvulus arvensis L.), flixweed (Descoreinia sofia [L.] Webb. & Berth.), henbit (Lamium amplexicaule L.), pigweeds (Amaranthus spp.), and stinking goosefoot (Chenopodium vulvaria L.) were the most common weeds found in all tillage systems and soil depths. Grasses were relatively lower than broadleaves regardless of treatments. Weed seed bank was mostly affected by weather conditions than treatments in this short-term experiment.     

A study on seed population dynamics of three weedy species of Eupatorium

The data on seed population dynamics of three weedy species of Eupatorium, viz. E. adenophwrum Spreng., E. riparium Regel and E. odoratum L. have been presented and discussed in the paper. The seed production and seed populations in soil seed bank were estimated, and the fate of buried seeds in soil was followed over a 2-year period. All the three species of Eupatorium produced a large number of seeds. The seed production of E. riparium per unit area was higher than that of the other two species because of its high population density. In all the three species most of the seeds were found to be present in the top 2 cm of the soil. The viable seed population of the three species of Eupatorium declined exponentially alter their burial in soil showing almost similar mortality patterns. A large proportion of seeds of all the Eupatorium spp. remained under enforced dormancy during their burial in soil with only a small fraction exhibiting induced dormancy. The longevity of the buried seeds increased on account of the dormancy imposed on them. The loss of seeds through degeneration and/or decomposition was rather low.