Recruitment limitation of native species in invaded coastal dune communities

Recruitment limitation may limit the ability of sites to regenerate after disturbances such as weed invasion and weed management. We investigated seed bank constraints and dispersal limitation in coastal dune communities on the east coast of Australia. The ability of sites to regenerate naturally following weed removal was assessed in coastal dune communities invaded by the invasive alien, bitou bush (Chrysanthemoides monilifera subsp. rotundata). To investigate recruitment limitation, seed banks and vegetation of invaded, native, intensively managed (selective application of herbicide and some re-vegetation) and extensively managed (large-scale, non-selective herbicide application) sites were compared. We investigated the dispersal mechanisms of species in the seed bank and vegetation to determine if communities might be dispersal-limited, i.e. contain significant numbers of species with only short-distance dispersal capabilities. Species richness and composition of soil seed banks differed from the vegetation in foredunes and hinddunes. Invasion depleted seed banks further. About half of the species had short-distance dispersal mechanisms indicating the potential for dispersal limitation. Secondary weed invasion following management was evident although alien species occurred in both seed banks and vegetation. Our results indicated that coastal dune communities suffer recruitment limitation. Native, managed and invaded dune communities appear to be both seed bank and dispersal-limited although management and invasion exacerbates recruitment. Regeneration of coastal dune communities will require active reintroduction of species, particularly those with short-distance dispersal mechanisms.     

A model for the impact of herbicide tolerance on the performance of oilseed rape as a volunteer weed

The introduction of genetically modified, herbicide-tolerant, oilseed rape into the agricultural environment will have ramifications beyond weed control of the crop. Herbicide-tolerant rape will undoubtedly become part of established volunteer weed populations that occur in many cereal rotations, but its longevity in these populations and its impact as a weed and contaminant of future oilseed rape crops is uncertain. A life cycle model of volunteer oilseed rape was therefore constructed, incorporating existing information on physiological processes such as emergence pattern, longevity of buried seed, death rates of various structures and flowering and seeding as functions of density. The model was designed to allow interaction with control factors such as harvesting efficiency, herbicide treatment, ploughing and the sequence of crops in the rotation. Many of the physiological parameters (including seed decay rates, fecundity at high density) are uncertain, simply through lack of information in the appropriate context. Other parameters such as harvesting efficiency and herbicide kill rates, are inherently variable in farming. Accordingly, a Monte-Carlo approach, in which the model was run many times with different random realisations of parameter sets, was used to expose factors to which the seedbank was sensitive. Sets of 1250 realisations were compared for each of two extreme conditions: where herbicide could be used according to current intensive farming practice and where it was not an option (representing total herbicide tolerance). Modelled seedbank numbers after 5 yr ranged from 10^-3to 10⁴m^-2, realistic values found in arable soils. The use of herbicide, together with efficient harvesting of seed, clearly has an important suppressive effect on the oilseed rape seedbank, keeping it lower than 10²m^-2(a typical sowing rate) after 5 yr in more than 80% of realisations. In the absence of herbicide, seedbanks were invariably greater, but their absolute value depended strongly on harvesting efficiency and the extent to which high density of plants suppressed fecundity. Analysis of the time series from the simulations showed that the seedbank levels fluctuated by orders in magnitude from year to year in the absence of herbicide use. The sensitivity analysis of the life-cycle model led to the development of a simplified model for the seedbank dynamics. The model shows that the essential features of the dynamics result from an interaction between density-dependent fecundity and the perturbations due to management. Therefore predictions of the effect of herbicide tolerance on seedbank dynamics are highly uncertain without knowledge of the density dependence of fecundity. Furthermore, the sensitivity to management practices suggests that seedbank levels will be substantially more difficult to control if the efficacy of herbicide is compromised. It is concluded that the model and Monte-Carlo approach have many potential uses in exploring the effects of management, cultivar physiology and the nature of the transgenes.     

Weed species differ in their ability to emerge in no‐till systems that include cover crops

No‐till cropping systems that include cover crops could lead to important changes in weed communities by decreasing some annual weed populations. In this study, we predicted that seed burial depth and the presence of cover crop would affect the emergence and initial growth success of annual weed species. We tested two factors on 14 weed species in a greenhouse: the seed burial depth of weeds (buried versus soil surface) and the presence/absence of a cover crop (ryegrass). We counted the emerged seedlings and measured the height of weeds and cover crops (Hweed, Hcover), the dry matter content of weeds and cover crops (DMCweed, DMCcover) and the number of leaves of weeds (NLweed) on 1433 weed and 390 ryegrass individuals. Emergence of five weed species (AMBEL, ANGAR, BROST, CENCY and EPHHE) was affected by the seed location (?10.3% on average for unburied seeds), five other weed species (ALOMY, CAPBP, SONAS, VERPE and VLPMY) were affected by cover (on average ?9.5% for seeds emerged in the presence of cover crop), and four weed species (GERDI, LAMPU, POAAN and VIOAR) were not affected by either. Weed growth of all weed species also decreased with the presence of a cover crop (on average Hweed: ?49.9%, DMCweed: ?87.2% and NLweed: ?55.4%) and for unburied seeds (on average Hweed: ?33.7%, DMCweed: ?70.6% and NLweed: ?43.3%), with various responses according to species. This study indicates that annual weeds could be disadvantaged by no‐till systems using cover crops.     

Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m² were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m² in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m² in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low. At time of research, Robert D. Cox was graduate student.     

Soil seed banks of woodland, heathland, grassland, mire and montane communities, Cairngorm Mountains, Scotland

The size and species composition of soil seed banks were assessed at 111 altitudinally diverse sites in the Cairngorm Mountains. Mean densities of germinable seeds varied from 83 000 m^–2 in Scots pine (Pinus sylvestris L.) woodland at 230–490 m to 200 m^–2 in moss (Racomitrium lanuginosum (Hedw.) Brid.) heath at 1000–1120 m. Seed banks were dominated by Calluna vulgaris (L.) Hull, not only wherever it was prominent in the vegetation, but also at some sites with less than 5% cover of parent plants in the ground vegetation. Many species conspicuous in the vegetation were under-represented in or absent from the seed bank and surface vegetation generally was more species rich than was the underlying seed bank, especially in high montane communities. Multiple regression was used to examine the relationship between the density of buried Calluna seeds and the abundance of parent plants in the vegetation, site altitude and the organic matter content of the soil. The model fitted to woodland communities accounted for 95% of the variation in seed density. The heathland model was less predictive but still explained 52% of the variation in seed bank size. In mire communities there was no relationship, collective or individual, between buried seed density and the measured environmental variables, possibly due to variations in the duration and frequency of waterlogging at these sites. The potential role of seed banks for initiating the recolonisation of disturbed ground is discussed. Densities of buried seeds at most Calluna-dominant sites were probably sufficient to generate successful recolonisation but the prospects for recovery were poor at other sites, particularly in graminaceous communities at 800 m or higher.     

Viable soil seed banks in disturbed lowland tropical rainforest sites in North Queensland

Four 0.125 m² soil samples, 50 mm deep, from each of eight tropical lowland rainforest sites ranging from primary and secondary forests to degraded Imperata grasslands were exposed to shade-house conditions. Germinations of soil seeds were monitored over 28 weeks in April-October. The viable seed banks in the primary forest (592 m^-2) and the secondary forest sites resulting from one cycle of clearing and abandonment (1300–1400 m^-2) were composed predominantly of secondary species. The viable seed banks were small in Imperata grassland sites that had been burnt regularly (200–400 m^-2) and very large (2000–3000 m^-2) in sites that had not been burnt for five years or more. The seed banks of the burnt and unburnt Imperata grassland sites were composed mostly of agricultural weeds whilst seeds of secondary rainforest trees were rare or absent. Similarities between the species composition of soil seed banks and regrowth vegetation types suggest that soil seed composition is important in determining the initial floristic composition following disturbance and that lack of seed of secondary rainforest trees is one factor which restricts re-growth and secondary succession on degraded Imperata grasslands. Species such as Eucalyptus intermedia R. T. Bak., E. pellita F. Muell. and Imperata cylindrica (L.)Beauv. which can establish and deflect the normal rainforest secondary succession were not present in the soil seed bank, and these species appear to rely on fresh seed inputs, lignotubers or rhizomes to colonize and regenerate after disturbance.     

Cover crops and compost prevent weed seed bank buildup in herbicide‐free wheat–potato rotations under conservation tillage

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Potential for seed-mediated gene flow in agroecosystems from transgenic safflower (Carthamus tinctorius L.) intended for plant molecular farming

Safflower has been transformed for field scale molecular farming of high-value proteins including several pharmaceuticals. Viable safflower seed remaining in the soil seed bank after harvest could facilitate seed and pollen-mediated gene flow. Seeds may germinate in subsequent years and volunteer plants may flower and potentially outcross with commodity safflower and/or produce seed. Seeds from volunteers could become admixed with conventional crops at harvest, and/or replenish the seed bank. Seed in following crops could be transported locally and internationally and facilitate gene flow in locations where regulatory thresholds and public acceptance differ from Canada. Seed-mediated gene flow was examined in three studies. Safflower seed loss and viability following harvest of commercial fields of a non-transgenic cultivar were determined. We assessed seed longevity of transgenic and non-transgenic safflower, on the soil surface and buried at two depths. Finally, we surveyed commercial safflower fields at different sites and measured density and growth stage of safflower volunteers, in other crops the following year and documented volunteer survival and viable seed production. Total seed loss at harvest in commercial fields, ranged from 231 to 1,069 seeds m⁻² and the number of viable seeds ranged from 81 to 518 seeds m⁻². Safflower has a relatively short longevity in the seed bank and no viable seeds were found after 2 years. Based on the seed burial studies it is predicted that winter conditions would reduce safflower seed viability on the soil surface by >50%, leaving between 40 and 260 viable seeds m⁻². The density of safflower volunteers emerging in the early spring of the following year ranged from 3 to 11 seedlings m⁻². Safflower volunteers did not survive in fields under chemical fallow, but in some cereal fields small numbers of volunteers did survive and generate viable seed. Results will be used to make recommendations for best management practices to reduce seed-mediated gene flow from commercial production of plant molecular farming with safflower.     

SEED BANK DYNAMICS IN A SOUTHEASTERN RIVERINE SWAMP

Seed banks were examined in a Taxodium distichum L.-Nyssa aquatica L. (cypress-tupelo) swamp forest and an adjacent bottomland hardwood forest on the floodplain of the Savannah River in South Carolina. Thirty 0.01 m² soil cores were collected in each community at each of three sampling times: before seed fall, after seed fall, and after an early spring river level rise had inundated both communities. Germination and sieving techniques were used to enumerate seeds in each sample. Woody seed banks of the two communities were dissimilar in species composition with both underrepresenting the species composition of the standing vegetation. In contrast, herbaceous seed banks of both communities were similar in species composition with greater species diversity in the seed banks than in the standing vegetation. Both the herbaceous and woody seed banks of the hardwood community changed significantly in seed densities across the three collection dates. Densities increased from September to December, after seed fall for many species, but decreased in April after the early spring water level rise. Seed densities did not change significantly in the cypress-tupelo community across the three collections. Hydrologic regime appears to have a major influence on seed bank composition and dynamics in southeastern riverine swamps.     

Disturbance regimes as determinants of seed banks in coastal dune vegetation of the southeastern Cape

Soil-stored seed banks of grassland, fynbos and thicket, all growing on calcareous dunes and each subject to different disturbance regimes, were examined. Seed banks were determined from counts of germinants from 50 soil cores from each type. Aboveground estimates of plant species cover in 10 1-m² plots were used in determining vegetation/seed bank similarities. There was no evidence for seed bank densities to be markedly higher in the most frequently disturbed community (grassland -4273 seeds/m²) than the least disturbed community (thicket - 3417 seeds/m²). Highest similarity between seed bank and above-ground vegetation composition in terms of species and growth form/life-span classes was recorded for grassland (CC = 50%). Lowest similarity (CC = 13%) was found in the less frequently disturbed thicket where no seeds of climax trees were recorded in the seed bank. A fynbos community on a north-facing (warm, dry) slope had intermediate-sized seed banks (1683 seeds/m²) with intermediate vegetation/seed bank similarity (CC = 46%). However, on the south-facing slope, which has a large post-fire ephemeral herb component, seed banks were larger (4518 seeds/m²) but less similar to above-ground vegetation (CC = 39%o). Ordination (DCA) of vegetation data from the four communities was different from an ordination of their seed bank data. Fynbos shrub species were absent from seed banks of both grassland and thicket, even though secondary succession proceeds from grassland, through fynbos to thicket. Their seed banks appear less persistent than those of European heath or Californian chaparral shrubs.     

Episodic recruitment of the seedling banks in balsam fir and white spruce

• Premise of the study: In ecosystems where seed production is low and masting years are sporadic, or with species that have short-lived seeds, regeneration is assured by seedling banks rather than seed banks. Seedling establishment and survival play a critical role in determining the composition of these plant communities by supplying new individuals for their maintenance. Seedling emergence and mortality were investigated to test the hypothesis that recruitment into the seedling bank is periodic. • Materials and methods: Seed production and seedling emergence and survival was monitored during 1994-2007 in balsam fir (Abies balsamea) and white spruce (Picea glauca) in four pristine stands of the boreal forest of Quebec, Canada. Measurements were collected twice per month by sampling one permanent plot of 20 × 20 m per stand. • Key results: Seed-rain abundance reached 9 × 10³ seeds m⁻² year⁻¹, and was characterized by synchronous sequences of low and high seed production. New seedlings appeared only during the year following a seed production of at least 1 × 10³ and 1.5 × 10² seeds m⁻² year⁻¹ for balsam fir and white spruce, respectively. Seedlings emerged in July and survived 34-52 d on average, with balsam fir showing a longer lifespan and lower mortality, although 85–99% of seedlings died before completing one year of life. • Conclusions: The emergence of young seedlings was coupled with massive seed rains, which allowed synchronous replenishment of the seedling banks among stands and species, and generated different cohorts, yielding a discontinuous age structure.     

Simulating evolutionary responses of an introgressed insect resistance trait for ecological effect assessment of transgene flow: a model for supporting informed decision‐making in environmental risk assessment

Predicting outcomes of transgene flow from arable crops requires a system perspective that considers ecological and evolutionary processes within a landscape context. In Europe, the arable weed Raphanus raphanistrum is a potential hybridization partner of oilseed rape, and the two species are ecologically linked through the common herbivores Meligethes spp. Observations in Switzerland show that high densities of Meligethes beetles maintained by oilseed rape crops can lead to considerable damage on R. raphanistrum. We asked how increased insect resistance in R. raphanistrum – as might be acquired through introgression from transgenic oilseed rape – would affect seed production under natural herbivore pressure. In simulation experiments, plants protected against Meligethes beetles produced about twice as many seeds as unprotected plants. All stages in the development of reproductive structures from buds to pods were negatively affected by the herbivore, with the transition from buds to flowers being the most vulnerable. We conclude that resistance to Meligethes beetles could confer a considerable selective advantage upon R. raphanistrum in regions where oilseed rape is widely grown.     

Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. I. Effects on abundance and diversity

We compared the seedbanks, seed rains, plant densities and biomasses of weeds under two contrasting systems of management in beet, maize and spring oilseed rape. Weed seedbank and plant density were measured at the same locations in two subsequent seasons. About 60 fields were sown with each crop. Each field was split, one half being sown with a conventional variety managed according to the farmer's normal practice, the other half being sown with a genetically modified herbicide-tolerant (GMHT) variety, with weeds controlled by a broad-spectrum herbicide. In beet and rape, plant densities shortly after sowing were higher in the GMHT treatment. Following weed control in conventional beet, plant densities were approximately one-fifth of those in GMHT beet. In both beet and rape, this effect was reversed after the first application of broad-spectrum herbicide, so that late-season plant densities were lower in the GMHT treatments. Biomass and seed rain in GMHT crops were between one-third and one-sixth of those in conventional treatments. The effects of differing weed-seed returns in these two crops persisted in the seedbank: densities following the GMHT treatment were about 20% lower than those following the conventional treatment. The effect of growing maize was quite different. Weed density was higher throughout the season in the GMHT treatment. Late-season biomass was 82% higher and seed rain was 87% higher than in the conventional treatment. The difference was not subsequently detectable in the seedbank because the total seed return was low after both treatments. In all three crops, weed diversity was little affected by the treatment, except for transient effects immediately following herbicide application.     

Seed banks of a rare wetland plant community: distribution patterns and effects of human-induced disturbance

Seed banks in rare shoreline communities (Nova Scotia, Canada), were examined in order to determine their role in the persistence of rare plant species. A pristine shoreline which supports globally and nationally significant Atlantic coastal plain species averaged 8500 seeds/m². Seedsweremost abundant high on the shore and where standing crop was greatest. Rare species, which made up 22% of the standing crop and litter of the adult vegetation, comprised only 4% of the seed bank. Common rushes such as Juncus canadensis and J. filiformis, were not well represented in the adult vegetation (5% of the standing crop and litter) but were abundant in the seed bank (52% of all seeds). Seed densities were on average, much lower on an intensely disturbed shoreline (1000 seeds/m²), where heavy all-terrain vehicle traffic had destroyed the adult vegetation. These results suggest that intense disturbances will destroy both existing vegetation and the seed bank and moderate disturbances will alter community composition so that common rushes will replace already endangered Atlantic coastal plain species.     

Seed banks of Erica australis and Calluna vulgaris in a heathland subjected to experimental fire

Abstract. Soil samples were collected before and after an experimental fire on a heathland in the province of León (Spain). The seed banks were assessed by counting the numbers of seedlings appearing of Erica australis and Calluna vulgaris. A total of 2285 germinable seeds/m² before burning and 1177 germinable seeds/m² after the fire were estimated for Erica australis, and 90 and 690 germinable seeds/m², respectively, for Calluna vulgaris. After the fire the number of germinable seeds/m² of Erica australis had decreased, whilst there was a considerable increase in the number of germinable seeds/m² of Calluna vulgaris. Neither seedlings nor sprouts of Calluna vulgaris had appeared in the field plots 10 months after the fire. Erica australis did recover mainly by sprouting after fire.     

Fate of Clavibacter michiganensis ssp. sepedonicus, the Causal Organism of Bacterial Ring Rot of Potato, in Weeds and Field Crops

Crops and weeds were tested for their ability to host Clavibacter michiganensis ssp. sepedonicus (Cms), the causal agent of bacterial ring rot in potato. Ten crops grown in rotation with potato in Europe, namely maize, wheat, barley, oat, bush bean, broad bean, rape, pea and onion and five cultivars of sugar beet were tested by stem and root inoculation. About 6–8 weeks after inoculation, Cms could be detected in most crops except onion and sugar beet, in larger numbers in stems (10⁵–10⁶ cells/g of tissue) than in roots (≤10³ cells/g of tissue) in immunofluorescence cell‐staining (IF). Cms was successfully re‐isolated only from IF‐positive stem samples of maize, bush bean, broad bean, rape and pea, but not from roots. Twelve solanaceous weeds and 13 other weeds, most commonly found in potato fields in Europe, were tested in IF as hosts of Cms by stem and root inoculations. Only in Solanum rostratum, a weed present in northern America, Cms persisted in high numbers (10⁸ cells/g tissue) in stems and leaves, where it caused symptoms. In the other solanaceous weeds, Cms persisted at low numbers (approximately 10⁵ cells/g of tissue) in stems but less so in roots. The bacteria could be frequently re‐isolated from stem but not from root tissues. In 2 consecutive years, plants from 14 different weed species were collected from Cms‐contaminated potato field plots and tested for the presence of Cms by dilution plating or immunofluorescence colony‐staining (IFC), and by AmpliDet RNA, a nucleic acid‐based amplification method. Cms was detected in roots but not in stems of Elymus repens plants growing through rotten potato tubers, and in some Viola arvensis and Stellaria media plants, where they were detected both in stems and roots, but more frequently by AmpliDet RNA than by IFC.     

Secondary seed dispersal in Montrichardia arborescens (L.) schott dominated wetlands in Laguna Grande, Venezuela

Laguna Grande, Monagas State, Venezuela, is a shallow, V-shaped lake created by the confluence of two rivers. Montrichardia arborescens (L.) Schott. dominated wetlands cover most of the north and south arms and the littoral zone of the main body of the lake. The vegetation and seed banks of Montrichardia wetland sites were sampled in the north arm, south arm and main body five times from the end of the dry season in 1991 to beginning of the rainy season in 1992. The composition of the vegetation was similar and changed very little at all three sites during the course of the study. These wetlands had 53 species. Besides M. arborescens, other common species were Hamelia patens Jacq., Mikania cordifolia (L.) Wild., Sarcostemma clausum (Jacq.) Roem. & Schult., and Vitis caribaea L. In both the vegetation and seed banks, species richness was highest during the dry season. Altogether, the seed banks contained the seeds of 61 species of which 35 were also found in the vegetation. Seeds of three tree species were found in the seed banks that did not grow anywhere in the lake. In the seed bank, seeds of Cyperus odoratus L., Eleocharis interstincta (Vahl.) R&S, Ludwigia hyssopifolia (G. Don.) Excell, L. lithospermifolia (Mich.) Hara, Polygonum acuminatum H.B.K., and Sacciolepis striata (L.) Nash were the most abundant. Mean total seed density over the entire study was 6,500, 3,800, and 6,000 seeds/m² in the north arm, south arm, and main basin, respectively. Seed production and dispersal occur primarily during the dry season, and the highest seed densities at all sites were found in the dry season when there was no or little standing water. The lowest seed densities at all sites were found during the rainy season during which seed densities declined over 80% at the north and south arm sites. In the main body of the lake, however, seed densities during the rainy season, although lower than during the dry season, actually increased significantly from 3,600 seeds/m² in August 1991 to 6,000 seeds/m² in October 1991. A significant decrease in seed density in either the north or south arms or both and a significant increase in the main body site during the rainy season occurred for 5 of the 8 species whose seeds were the most abundant, for all life-form guilds, except hydrophytes and for the entire seed bank. Secondary dispersal by water currents during the rainy season appears to be transporting seeds from the north and south arms into the seed bank of the main body of the lake.     

灵山湾国家森林公园垂序商陆土壤种子库时空动态及其对防除措施的响应

土壤种子库是植物潜在的种群,对生态系统稳定和未来植被的结构、功能有着举足轻重的影响。垂序商陆(Phytolacca americana)的入侵已对山东省胶南市灵山湾国家森林公园的森林生态系统构成严重危害。为调查该物种在公园内的潜在扩散能力,于2009—2014年调查了刺槐(Robinia pseudoacacia)林、黑松(Pinus thunbergii)林、刺槐黑松混交林和紫穗槐(Amorpha fruticosa)林(灌丛)4种林分类型,以及2009年采用物理和化学控制入侵危害严重的刺槐林内的土壤种子库。结果表明:垂序商陆种子在4种林分中均有发现,在刺槐、黑松林内表现为由林缘至林内逐步降低的趋势。垂序商陆种子只存在于枯落物层和5 cm以上的土壤中,随土层深度的增加而减少,枯落物层土壤种子库由林缘到林内呈递减趋势。与2009年相比,2014年黑松林和灌丛中平均种子库密度分别降低46%和44%,刺槐林和混交林分别增加299%和355%。垂序商陆种子发芽率在10%以下,随土层深度的增加而依次降低。采用物理、化学控制措施的刺槐林内垂序商陆的更新量、结实量、种子库含量均明显高于紫穗槐林的。调查结果表明垂序商陆土壤种子库具有明显的空间异质性,并且保持了较强的繁殖潜力,营造紫穗槐灌丛可以有效控制垂序商陆的扩散,可为经营沿海防护林提供理论指导。     

Weed seed resources for birds in fields with contrasting conventional and genetically modified herbicide-tolerant crops

The UK Farm Scale Evaluations (FSEs) have shown that the use of broad spectrum herbicides on genetically modified herbicide-tolerant (GMHT) crops can have dramatic effects on weed seed production compared to management of conventional varieties. Here, we use FSE data and information on bird diets to determine how GMHT cropping might change the food resources available to farmland birds. More than 60 fields of each of four crops, spring- and winter-sown oilseed rape, beet and maize, were split, one half being sown with a conventional variety, the other with a GMHT variety. Seed rain from weeds known to be important in the diets of 17 granivorous farmland bird species was measured under the two treatments. In beet and spring oilseed rape, rain of weed seeds important in the diets of 16 bird species was significantly reduced in GMHT compared to conventional halves; for no species did it increase. In winter oilseed rape, rain of weed seeds important in the diets of 10 species was significantly reduced in GMHT halves; for only one species did it increase significantly. By contrast, in maize, rain of weed seeds important in the diets of seven species was significantly greater in GMHT halves; for no species was it reduced. Treatment effects for the total weed seed energy available to each bird species were very similar to those for seed rain alone. Measuring the effects on individual bird species was outside the scope of this study. Despite this, these results suggest that should beet, spring and winter rape crops in the UK be largely replaced by GMHT varieties and managed as in the FSEs, this would markedly reduce important food resources for farmland birds, many of which declined during the last quarter of the twentieth century. By contrast, GMHT maize would be beneficial to farmland birds.     

The competitive effects of volunteer barley (Hordeum vulgare) on the growth of oilseed rape (Brassica napus)

The effects of competition from volunteer barley (Hordeum vulgare) on the growth and yield of oilseed rape (Brassica napus) were investigated in four experiments over three seasons. The growth of rape in the autumn was reduced by 50 - 91 % by competition from 400 barley plants m^-2. A lower barley density of 200 plants m^-2 had less effect but still reduced growth of rape by 65 - 81% in two of the experiments and 25 - 40% in the other two. During winter and spring the barley decreased in vigour and in the spring the rape started to recover, especially on the early drilled (23 - 30 August) plots. The rape sown in mid-September recovered less quickly. In Experiment 3, herbicides applied in November to control barley did not result in increased growth of rape in winter but led to greater recovery in spring. The barley died during the winter in Experiments 2 and 4, even in the absence of herbicides. Despite the marked effects of barley on the growth of rape in the autumn, yields on plots that had previously contained 200 barley plants m^-2 were reduced by a maximum of only 16% in three of the experiments. In Experiment 3, where the barley was most competitive, this density and 400 plants m^-2 lowered yields by 39% and 78%, respectively. Where a herbicide was used in November to control the barley these yield losses were reduced to 5%. In many rape crops the cost of herbicide treatment would be greater than the financial returns from the expected increase in yield resulting from the control of weeds. Possible reasons for the small loss in yield of rape from barley densities that had substantial effects on the growth of rape in the autumn are discussed.