Improved physical control of glyphosate‐resistant Italian ryegrass (Lolium multiflorum) on rice paddy levees in Japan

Italian ryegrass ( Lolium multiflorum Lam.) is a non‐native annual winter grass that has seriously infested rice paddy levees and wheat fields in Japan. Recently, glyphosate‐resistant Italian ryegrass was found on paddy levees in central Japan, thereby making control of the grass by using glyphosate less effective. In this study, physical control methods were tested that combined the timing and frequency of mowing in order to more effectively control glyphosate‐resistant Italian ryegrass on rice paddy levees. A 3 year field experiment was conducted from 2012 to 2014 in a western region of Shizuoka Prefecture, where glyphosate‐resistant Italian ryegrass has become dominant. Five treatments were tested: (i) mowing once before the flowering of the grass (i.e. conventional mowing measure); (ii) mowing once during flowering; (iii) mowing twice during flowering; (iv) glyphosate application before flowering (i.e. one of the conventional mowing measures); and (v) no treatment. The above‐ground biomass, seed production, soil seed bank and seedling occurrence of Italian ryegrass were measured to determine the effectiveness of these treatments. Mowing during the flowering period resulted in reduced above‐ground biomass, seed production and soil seed bank when compared with the other treatments. Additionally, mowing twice during the flowering period resulted in a lower seedling density than mowing once. The results suggest that, in this region, physical control by mowing during the flowering period would be more effective than conventional measures for controlling glyphosate‐resistant Italian ryegrass.     

Patterns of loss of biodiversity associated with invasion by Chrysanthemoides monilifera subsp. monilifera (boneseed) across a large geographic region

To investigate the impact of invasion by Chrysanthemoides monilifera ssp. monilifera (boneseed) on plant communities, sites that were invaded and uninvaded were surveyed across Tasmania, South Australia and Victoria, Australia. Above‐ground cover abundance was estimated and samples of the seed bank from invaded and uninvaded sites in each state were collected and germinated in the glasshouse. We compared species richness and composition between invaded and uninvaded sites for both above‐ground and below‐ground samples. Across this broad geographic area, C. monilifera invaded sites had 25% fewer species than uninvaded sites and twice as many exotic species. Most growth forms showed reduced richness in invaded sites. There were no differences in the composition of native species communities between invaded and uninvaded sites in most regions, except South Australia. The composition of the seed bank differed from the above‐ground vegetation, but was similar within each state. Herbs and grasses were most abundant. The species composition of the seed bank was different in invaded sites only in Victoria, suggesting that invasion may negatively influence the capacity of the seed bank to restore vegetation in some areas. However, the underlying mechanisms for the observed patterns were not tested in this study. The capacity of all sites to regenerate effectively from the seed bank was limited and, in order to ensure the conservation of biodiversity into the future, monitoring of species richness at smaller scales, particularly those where management of C. monilifera has occurred, will be needed to ensure management can facilitate the establishment of missing species. This study suggests that C. monilifera impacts a range of functional groups and different species across its distribution and is therefore non‐selective in how it influences changes in communities.     

Impacts of soil moisture level and organic matter content on growth of two Juncus species and Poa pratensis grown under acid soil conditions

The abundance of Juncus effusus (soft rush) and Juncus conglomeratus (compact rush) has increased in coastal grasslands in Norway over recent decades, and their spread has coincided with increased precipitation in the region. Especially in water‐saturated, peaty soils, it appears from field observations that productive grasses cannot compete effectively with such rapidly growing rush plants. In autumn–winters of 2012–2013 and 2013–2014, a four‐factor, randomised block greenhouse experiment was performed to investigate the effect of different soil moisture regimes and organic matter contents on competition between these rush species and smooth meadow‐grass (Poa pratensis). The rush species were grown in monoculture and in competition with the meadow‐grass, using the equivalent of full and half the recommended seed rate for the latter. After about three months, above‐ and below‐ground dry matter was measured. J. effusus had more vigorous growth, producing on average 23–40% greater biomass in both fractions than J. conglomeratus. The competitive ability of both rush species declined with decreasing soil moisture; at the lowest levels of soil moisture, growth reductions were up to 93% in J. conglomeratus and 74% in J. effusus. Increasing water level in peat–sand mixture decreased competivitiveness of meadow‐grass, while pure peat, when moist, completely impeded its below‐ground development. These results show that control of rush plants through management may only be achieved if basic soil limitations have been resolved.     

不同稻田综合种养模式下杂草长期控制效果的调查

为明确稻田综合种养模式是否可以长期有效控制杂草危害,通过对江苏省48个样点共6种综合种养模式(稻鸭、稻蟹、稻虾、稻鱼、稻鳖和稻鳅/鳝共作)农田的杂草群落和土壤种子库进行调查,比较分析杂草群落综合草情优势度、物种多样性以及杂草群落和土壤种子库的组成和变化。结果表明,在综合种养模式实施1~3年,杂草群落综合草情优势度和土壤种子库密度均明显下降,其中稻鸭共作模式下两者均下降最多,其次是稻虾共作模式下杂草群落综合草情优势度下降较多,而稻鱼共作模式下土壤种子库密度下降较多。实施4~5年,各种养模式下杂草群落综合草情优势度和土壤种子库密度均上升,草害加剧,杂草防控效果下降;其中稻鳅/鳝共作模式下杂草群落综合草情优势度和土壤种子库密度与常规稻田相比升幅最大,分别上升28.8%和25.3%;由于稻鳖、稻鳅/鳝共作模式实施均未超过5年,在实施4~5年时整体上杂草危害最为严重,禾本科杂草、阔叶杂草以及莎草科杂草的综合草情优势度较常规稻田分别上升42.4%、12.3%、0.7%和31.5%、27.7%、38.1%。实施5年以上,稻鸭共作模式下阔叶杂草的综合草情优势度较常规稻田下降65.0%,但禾本科杂草的综合草情优势度和土壤种子库密度较常规稻田分别上升80.5%和66.6%,成为杂草群落和土壤种子库的优势种群;稻虾共作模式下莎草科杂草和阔叶杂草的综合草情优势度较常规稻田分别上升17.8%和45.0%;稻蟹共作模式下莎草科杂草、阔叶杂草和禾本科杂草的综合草情优势度较常规稻田分别上升22.7%、35.3%和29.0%。表明当长期实施同种稻田综合种养模式时,杂草群落在单一的选择压力下会加快演替,杂草危害均呈先降后升的变化趋势,不利于田间杂草的长效防控,建议实施针对耗竭土壤种子库的综合技术措施。     

Residual effects of cultivation methods on weed seed banks and weeds in Cambodia

The effects of the planting method (transplanting vs. direct seeding), rice cultivar (Sen Pidao vs. Phka Rumduol), and herbicide application (admixture of bentazone and cyhalofop‐butyl) on weeds and weed seed banks were quantified in unflooded, shallowly flooded, and deeply flooded paddy fields in Cambodia in 2005 and 2006. Broad‐leaved weeds infested more toward maturity in 2006 than in 2005, particularly in directly seeded plots. Weed dry weights at pre‐heading and/or maturity were consistently reduced by herbicide application and Phka Rumduol cultivar, while weed numbers increased under unflooded condition. The proportion of sedges was consistently larger in directly seeded and non‐herbicide plots at pre‐heading. Larger numbers and dry weights of sedges and total weeds in 2005 caused larger seed bank sizes of sedges and total weeds in 2006, which further caused their infestation in 2006. A greater weed dry weight at 62 days after sowing in 2005 resulted in larger seed banks of Cyperus iria and Fimbristylis miliacea, which were most severe under non‐herbicide, direct‐seeded treatment, while that at rice maturity resulted in larger seed banks of Cyperus difformis, Scirpus juncoides, and Lindernia antipoda. Overall, sedges shared the majority of the total weed seed bank, followed by broad‐leaved weeds and then grasses. A lower yield in 2005 led to significantly larger seed bank sizes of sedges, but not of grasses or broad‐leaved weeds. Rice yield reduction was consistently related to larger numbers of sedges by heading and those of grasses at maturity.     

Plant–herbivore asynchrony necessitates augmentative releases of the exotic beetle,Zygogramma bicolorata,to enhance the biological control of P arthenium hysterophorus

Systematic information on the quantitative impact of Z ygogramma bicolorata on the biology of P arthenium hysterophorus is crucial as the seeds of this weed continue to germinate from the accumulated soil seed bank throughout the year in the form of different germinating flushes, while the activity of the beetle ceases during winter as it enters diapause. Therefore, plant–herbivore interactions need to be explored to develop predictions of the overall impact of the introduced beetle on the weed. The findings revealed that defoliation by Z . bicolorata had a significant impact on the plant height, density and flower production in flushes F ₃, F ₄ and F ₅, but not in F ₁ and F ₂ that exhibited longer periodicity, profuse branching, a longer flowering period and maximum flower production and contributed mostly to the existing seed soil bank. Therefore, total depletion of the existing soil seed bank was not possible. Consequently, the effect of augmentative field releases of laboratory‐reared beetles was explored on F ₁ and F ₂ in February for three consecutive years (2011–2013). Before initiating the trial, random soil samples were taken from the plots that were assigned to the paired treatments (i.e. with the beetle and without the beetle [insecticide‐treated]) and it was found that the seed bank in those samples did not differ. The single release of Z . bicolorata adults at five per plant at the six‐leaf stage significantly reduced the soil seed bank, compared to without the biocontrol agent, irrespective of the flushes at the end of the season.     

New experimental hosts of Barley yellow dwarf virus among wild grasses,with implications for grassland habitats

Barley yellow dwarf virus (BYDV), an economically important virus, infects small grain cereal crops and over 150 other Poaceae species. BYDV infection plays an important role in competition among grasses in non‐managed systems, but many grasses remain unexamined as potential BYDV hosts. This study examined grass species that have not been reported as BYDV hosts but are commonly encountered in non‐managed grasslands throughout the United States and Canada. Laboratory inoculations with BYDV‐PAV using the aphid vector Rhopalosiphum padi were performed to examine the ability of 13 grass species and barley to be infected with the virus; eight of the grass species were not documented previously as virus hosts. Serological and molecular assays were used to confirm BYDV‐PAV infection. Plant height, number of leaves, number of tillers and weight were recorded to evaluate susceptibility or sensitivity to BYDV. Infection with BYDV was experimentally achieved for the first time on Achnatherum lettermanii, Achnatherum occidentale, Achnatherum thurberianum, Danthonia intermedia, Poa fendleriana, Sporobolus airoides and Sporobolus cryptandrus, but not on Alopecurus pratensis and Elymus wawawaiensis. Infection was confirmed in Bromus inermis, Elymus elymoides, Poa bulbosa, Poa secunda and Hordeum vulgare, which served as controls. BYDV infection caused reductions in plant height on P. bulbosa and P. fendleriana. BYDV‐infected P. secunda had more leaves per plant compared to healthy plants of the same species. BYDV‐infected A. lettermanii exhibited reduced dry weight in both below‐ground and above‐ground tissue. These findings have implications for the management and conservation of grassland habitats.     

Using a selectivity index to evaluate logarithmic spraying in grass seed crops

BACKGROUND: Grass seed crops are minor crops that cannot support the development of selective herbicides for grass weed control in grass seed crops. An option is to screen for selective herbicides with the use of logarithmic spraying technology. The aim of this paper is to assess selectivity of various herbicides in grass seed crops by using dose–response curves. RESULTS: Six grass species were subjected to logarithmic spraying with 11 herbicides and with Poa pratensis L. as a weed. The ratio between the doses that caused 10% of damage to the crop and 90% of damage to the weed was used as a selectivity index. Compounds with selectivity indices above 2 can be safely used in a crop. The two ACCase herbicides clodinafop‐propargyl and fenoxaprop‐P‐ethyl and a mixture of the two ALS herbicides mesosulfuron and iodosulfuron could be used selectively to control P. pratensis in Festuca rubra L., although the selectivity indices in no instances were greater than the desired 2.0. CONCLUSION: The logarithmic sprayer can be a rapid screening tool for identifying compounds with favourable selectivity indices. Good experimental design is needed to alleviate rates being systematically distributed and confounded with growth rate and soil fertility gradients. Copyright © 2009 Society of Chemical Industry     

Effects of host plant genotype and seedbank density on Striga reproduction

Prevention of seed input to the seedbank of Striga hermonthica‐infested fields is an important objective of Striga management. In three consecutive years of field experimentation in Mali, Striga reproduction was studied for 10 sorghum genotypes at infestation levels ranging from 30 000 to 200 000 seeds m^?2. Host resistance was identified as an important determinant of Striga reproduction, with the most resistant genotypes (N13, IS9830 and SRN39) reducing Striga reproduction by 70–93% compared with the most susceptible genotype (CK60‐B). Seedbank density had a significant effect on Striga seed production. Higher seedbank density resulted in more Striga plants, which led to increased intra‐specific competition and consequently a reduced level of reproduction per plant. For the most susceptible sorghum genotypes, density dependence also occurred in the earlier belowground stages. Striga reproduction continued beyond harvest. At the high infestation level just 8% of the total reproduction was realised after harvest, whereas at the low infestation level 39% was attained after harvest. Even though host‐plant genotype plays a significant role in Striga reproduction, calculations indicated that only at very low infestation levels the use of the most resistant genotype was able to lower the Striga seedbank.     

The hotter the weather,the greater the infestation of Portulaca oleracea: opportunistic life‐history traits in a serious weed

Portulaca oleracea, an r‐strategist, is one of the world's most troublesome weeds. During hot seasons, P. oleracea frequently becomes monodominant in choy sum (Brassica parachinensis) fields in Guangzhou city, southern China. Here, we studied the seasonal dynamics of P. oleracea's germinable soil seedbank, population density and above‐ground biomass in choy sum fields that had been cultivated continuously for several years. Using P. oleracea seeds collected from these fields, we tested seed dormancy, survival and germination, seedling growth and generation time. Portulaca oleracea occurred at high levels during the hot season, but its occurrence was low, and the germinable soil seedbank was much greater during the cold season. The weed's opportunistic characteristics allowed it to avoid freezing and to proliferate during optimal conditions. Portulaca oleracea's generation time was very flexible, as short as 31 days during the hot season, but longer than 100 days during the colder season. Seed dormancy tended to be shorter when the seeds were stored at a higher temperature. At a temperature of 35°C, both seed germination and seedling growth showed advantages over those of choy sum. Storage for one year at a temperature of ?20°C or burial in a paddy field did not significantly reduce P. oleracea seed germination. Nevertheless, seed storage at a temperature of 15°C and soil coverage of 0.5 cm on top of the seeds significantly constrained seed germination. ‘Stale seedbed’ and/or coverage of the surface with soil are recommended during the hot season. However, rotation of rice and upland crops is not an efficient method for managing Portulaca oleracea infestation.     

Weed seed rain interception by grass cover depends on seed traits

Weed management requires a better understanding of the dynamics of the weed seedbank, which is a primary source of weeds in a field. Seeds reaching the ground after seed rain replenish the seedbank and therefore contribute to future weed infestations. Our investigation is based on the hypothesis that a permanent vegetation cover, such as a grassland, can prevent weed seeds from reaching the ground. Therefore, we developed an innovative experimental device to simulate in controlled conditions the seed rain of 12 weed species (Capsella bursa‐pastoris, Conyza canadensis, Myosotis arvensis, Papaver rhoeas, Poa annua, Polygonum aviculare, Ranunculus sp., Rumex obtusifolius, Sonchus asper, Stellaria media, Taraxacum officinale and Veronica persicaria). We quantified the interception of weed seeds by a grass cover. Grass cover height, seed size and seed appendage (e.g. pappus, wing or awn) increased seed interception, in contrast to seed weight and shape index. From these results, we established a linear model to predict weed seed interception by a grass cover as a function of their seed trait values. The relationship between the predicted interception and weed community dynamics observed in grasslands was negative for some species, indicating that other processes may be involved depending on weed species. The weed seed interception model will be incorporated into an existing model of weed population dynamics to simulate the impact of grassland insertion into arable crop rotations.     

Parthenium hysterophorus in Nepal: a review of its weed status and possibilities for management

The rapid expansion of the alien invasive Parthenium hysterophorus is a new agricultural and environmental problem for Nepal. Although the weed was first recorded in 1967, the most significant expansion has occurred in the last 20 years. The weed is thought to have entered Nepal from India and currently is found in the Tarai, Siwalik and hill regions of Nepal. A CLIMEX modelling projection has shown these regions to be climatically suitable for the growth of P. hysterophorus, both under the present and a future projected increased temperature. From the initially invaded roadside vegetation, the weed has now spread into cropping land and forests. Vehicle movement and transportation of agriculture products are the main means by which its seed is spreading. The weed has reduced plant species richness and changed species composition in grasslands, while fodder collectors have developed contact dermatitis from the plant. To date, no sustainable management strategy has been developed for this weed. However, the fortuitous arrival of the biological control agents Zygogramma bicolorata and Puccinia abrupta var. partheniicola has had some impact upon the weed's growth and abundance. A CLIMEX modelling projection identifies many suitable locations for Z. bicolorata outside of its present range and within areas where the weed is likely to spread, both under the present and under a future projected climate of increased temperature. Mapping of the currently invaded areas, management through community involvement and further releases of new and redistribution of current biological control agents are required to manage P. hysterophorus in Nepal.     

Germination response of Phalaris paradoxa L. seed to different light qualities

The influence of different light regimes on the germination of Australian and English populations of Phalaris paradoxa L. (awned canary‐grass) seed was investigated to determine the impact of changing tillage practices on weed infestation. Seeds of all biotypes were highly viable, but differed in levels of innate dormancy (26–99%). In one experiment seed from a single Australian biotype, either enclosed in the spikelet glumes or having the spikelet glumes removed, were exposed to nine light treatments. Germination was stimulated by red and white light, but was inhibited by far‐red light. Time to 50% germination was less for seed enclosed in the spikelet glumes than for naked caryopses, although the final percentage of seed germinating when still enclosed in the spikelet glumes was significantly lower than for naked caryopses. In another experiment, six Australian and English biotypes with varying dormancy characteristics were exposed to eight light treatments. Red light did not stimulate germination in the deeply dormant biotype, however stimulated all other biotypes. Germination in darkness was below 20% in all biotypes except for one where germination was 51%. To overcome dormancy seeds were imbibed and placed in darkness at 16°C for either 7 or 14 days prior to exposure to red or white light for a single 15‐min period. Dormancy in all biotypes was overcome indicating that a period of burial may decrease the dormancy level and increase seed sensitivity to light. This increased light sensitivity suggests that exposure to light during tillage may stimulate germination in P. paradoxa seed.     

新疆野生樱桃李天然林土壤种子库特征

采取野外调查、网筛分选和温室萌发相结合的方法,对新疆霍城县大西沟山区野生樱桃李天然林的土壤种子库物种组成、密度和生活史型、垂直空间分布、物种多样性及其与地上植被间关系进行了研究。结果表明：① 试验鉴定出的幼苗共有48种,隶属于19科,且一年生植物多达20种（41.7%）,地下芽植物为15种（31.3%）;② 种子库中种子的数量自上而下呈先增加后减少的特征,有超过90%的种子分布在距地表5 cm以上的表层土壤及枯枝落叶层中,近90%的樱桃李种子分布在距地表2 cm以上的表层土壤及枯落层中;③ 土壤种子库中草本层物种的Margalef指数、Shannon-Wiener指数均为最高,其次为乔木层和灌木层,种子库物种多样性主要受草本层丰富度的影响;④ 野生樱桃李天然林土壤种子库与地上植被的相似性系数为0.315,表明地上植被对土壤种子库的贡献较大。但是,由于过度放牧,使得种子库的物种多样性在垂直空间上的分布规律受到干扰。     

The occurrence of Phialophora-like fungi related to Gaeumannomyces graminis under various grass species and some characteristics of these fungi1

Phialophora graminicola and Phialophora sp. (lobed hyphopodia) are weakly or not pathogenic to the cereal and grass roots which they colonize and are also known to restrict the infection of these plants by Gaeumannomyces graminis. The occurrence of these fungi under 15 field-grown grass species has been studied using wheat as the test plant. P. graminicola occurred under all grass species tested but at different population levels. The lowest populations were found under Bromus erectus and B. inermis, and relatively low populations under Agrostis alba, Festuca ovina and Phalaris arundinacea. Festuca pratensis, F. rubra, F. arundinacea, Dactylis glomerata and Arrhenatherum elatius harboured the highest populations of P. graminicola. Phialophora sp. (lobed hyphopodia) was found at low population levels under 7 of the 15 grasses examined. The strains of P. graminicola isolated in these studies differed markedly in growth rate on wheat roots and in restriction of G. graminis on wheat grown in perlite (preliminary results).     

Distribution of Parthenium hysterophorus and one of its biological control agents (Coleoptera: Zygogramma bicolorata) in Nepal

Parthenium hysterophorus is a noxious invasive weed of both agricultural and natural ecosystems, spreading aggressively in Nepal. Management of this weed in Nepal has been limited, mainly because of the lack of geo‐referenced data concerning the weed's distribution. We conducted a nationwide survey of P. hysterophorus and its coleopteran biological control agent Zygogramma bicolorata from 2013 to 2016 to determine their spatial distribution. Both were widespread, with the distribution of Z. bicolorata lagging behind the invasion front of P. hysterophorus. The weed was present in 21.2% of the 4838 locations examined, including several isolated satellite populations. The weed was found in the Tarai, Siwalik, Middle Mountains and High Mountains regions, reaching up to 2000 m asl. It has invaded natural and modified ecosystems including all six protected areas in the Tarai and Siwalik regions. Road access appears to be the major pathway for its long‐distance dispersal. Zygogramma bicolorata had spread from the east to the west and was present in 15.4% of the weed occurrence locations, inflicting a low amount of damage. A CLIMEX modelling projection revealed the presence of additional geographic areas in Nepal which are climatically suitable for both P. hysterophorus and Z. bicolorata. Eradication of satellite populations of the weed by physical and chemical measures, and the release of Z. bicolorata into new, but climatically suitable, locations should be prioritised for P. hysterophorus management in Nepal. In conclusion, P. hysterophorus has rapidly become widespread in Nepal and the currently available biological control agent has not been able to prevent further spread of the weed.     

Rising CO2 can alter fodder–weed interactions and suppression of Parthenium hysterophorus

Three C₄ grass (Setaria incrassata, Astrebla squarrosa and Bothriochloa decipiens) and one C₃ legume (Clitoria ternatea) suppressive fodder species, were re‐evaluated against the growth of the C₃ Parthenium hysterophorus under an ambient (390 μmol mol^?1) and an elevated atmospheric CO₂ concentration (550 μmol mol^?1). Under the elevated atmospheric CO₂, shoot dry biomass and suppression index (SI) value of the C₄ S. incrassata were both reduced by 32% and 0.7 respectively, while those for A. squarrosa were reduced by 23% and 0.3. In contrast and under the same elevated atmospheric CO₂ concentration, the shoot dry biomass and SI of the C₄ B. decipiens were increased by 8% and 0.1 respectively, while those for the C₃ C. ternatea were increased by 38% and 0.8. Our results suggest that C₃ fodder plants along with certain C₄ species could be utilised for the effective management of P. hysterophorus under the future elevated atmospheric CO₂ conditions. However, this system needs more fodder species to be investigated. Our results suggest that rising CO₂ per se may alter the efficacy of suppressive fodder management of an invasive C₃ species, P. hysterophorus.     

Predispersal seed predation of Amaranthus retroflexus and Chenopodium album growing in soyabean fields

Field experiments were made in 1998 and 1999 to determine the influence of tillage and soyabean (Glycine max) row width on predispersal weed seed predation in Amaranthus retroflexus L. (redroot pigweed) and Chenopodium album L. (common lambsquarters). Soyabean was planted in wide (76 cm) and narrow (19 cm) rows with conventional or conservation tillage. Additional control plots without soyabean were also established. The two objectives were to determine (1) whether predispersal seed predation occurs in A. retroflexus or C. album, and (2) whether disturbance (soil tillage) or microclimate (planting pattern) influence predation level. Mean rates of seed predation were 26% and 4% in A. retroflexus and C. album, respectively. Although these levels were low at the population level, individual plants of both species had predation levels ranging from 0% to 80%, however, very few individuals of C. album had levels of predation above 10%. Differences among tillage and row width treatments occurred for A. retroflexus, but not for C. album. Amaranthus retroflexus and C. album growing within the soyabean crop received less light than those in the no‐crop plots, and produced less above‐ground biomass, smaller terminal inflorescences, and fewer seeds per inflorescence. Plant height, terminal inflorescence weight, and total seeds were correlated with predation in both weed species.     

Response of riparian vegetation to the removal of the invasive forb,Solidago gigantea,and its litter layer

In order to explore the methods of recovering native grasses into exotic vegetation, the response of riparian vegetation to the removal of the above‐ground shoots and/or litter of Solidago gigantea in a flood plain in Hokkaido, northern Japan, was investigated. The four treatments were: the removal of the above‐ground shoots of S. gigantea (A); the removal of the litter of S. gigantea (L); the removal of both the above‐ground shoots and litter of S. gigantea (AL); and a control (C). The vegetation cover and S. gigantea cover decreased in the A and AL treatments and increased in the L and C treatments. The understory plant cover increased in the A and AL treatments, but did not change in the L and C treatments. The increases in the understory cover in the A and AL treatments were associated with increases in Phalaris arundinacea. The seedling emergence of P. arundinacea was promoted by AL. In a greenhouse, the S. gigantea litter tended to decrease the seed germination of P. arundinacea. The AL treatment also increased the abundance of the other exotic plant, Solidago altissima. The continuous removal of the above‐ground shoots and litter of S. gigantea long term is effective for promoting the recovery and emergence of native riparian grassland vegetation. However, this method also promotes the recovery of other exotics.     

Weed–pathogen interactions and elevated CO2: growth changes in favour of the biological control agent

In this study, we used Parthenium hysterophorus and one of its biological control agents, the winter rust (Puccinia abrupta var. partheniicola) as a model system to investigate how the weed may respond to infection under a climate change scenario involving an elevated atmospheric CO₂ (550 μmol mol^?1) concentration. Under such a scenario, P. hysterophorus plants grew significantly taller (52%) and produced more biomass (55%) than under the ambient atmospheric CO₂ concentration (380 μmol mol^?1). Following winter rust infection, biomass production was reduced by 17% under the ambient and by 30% under the elevated atmospheric CO₂ concentration. The production of branches and leaf area was significantly increased by 62% and 120%, under the elevated as compared with ambient CO₂ concentration, but unaffected by rust infection under either condition. The photosynthesis and water use efficiency (WUE) of P. hysterophorus plants were increased by 94% and 400%, under the elevated as compared with the ambient atmospheric CO₂ concentration. However, in the rust‐infected plants, the photosynthesis and WUE decreased by 18% and 28%, respectively, under the elevated CO₂ and were unaffected by the ambient atmospheric CO₂ concentration. The results suggest that although P. hysterophorus will benefit from a future climate involving an elevation of the atmospheric CO₂ concentration, it is also likely that the winter rust will perform more effectively as a biological control agent under these same conditions.