Linking species traits to agroecosystem services: a functional analysis of weed communities

There is a growing interest in the use of functional approaches for the study of weed assemblages, to disentangle underlying processes determining their composition and dynamics. Functional approaches are based on the assumption that weed community composition and dynamics can be best explained by a set of species traits expressing their response to agricultural disturbance. This knowledge should help develop more sustainable, ecologically based weed management systems. Trait‐based data required for this kind of analysis are available from various sources, but most of them either cover mainly non‐weedy species or, in the case of weed‐focussed trait databases, they cover a limited number of species. In this work, we present a trait database for 240 weed species common throughout Europe, including not only response traits but also effect traits, that is linked to selected agroecosystem services and disservices. A case study is presented where our weed trait database is used in conjunction with appropriate statistical analysis to highlight the distribution of weed functional groups in soyabean crop communities from an experiment including different tillage and weed management systems. Finally, we discuss the strengths, weaknesses, opportunities and threats of this functional approach. By highlighting the links between weed species and agroecosystem (dis)services, this approach could be a useful resource for scientists, farm managers and policymakers.     

Incorporated cover crop residue suppresses weed seed germination

Cover crops have been shown to be important integrated weed management tools. In addition to directly competing with weeds, cover crops can provide weed suppressive effects following incorporation through release of allelopathic compounds and/or changes to nutrient availability. Incorporation of a cover crop mixture may provide a synergistic or antagonistic effect on weed suppression by further altering nutrient dynamics. To investigate this phenomenon, we evaluated the suppressive effects following incorporation of annual ryegrass, buckwheat, brown mustard, and phacelia sown with and without field pea on germination and growth of several pernicious weed species. Further, we used the additive partitioning model to determine if pea synergistically improved biomass production and weed suppression of cover crops. Our results demonstrate that following incorporation, cover crop residues suppress weed germination and weed biomass production. According to the additive partitioning model, the addition of pea had an antagonistic effect on buckwheat and brown mustard biomass production and decreased buckwheat weed suppression by 8%. In contrast, the addition of field pea greatly enhanced biomass production of phacelia at a reduced seeding rate suggesting a positive biodiversity effect. Limited evidence was found for changes to nutrient availability following cover crop incorporation, however, a dose-dependent effect of cover crop residue on weed suppression suggests allelopathy and/or nutrient availability may have a role on weed seed germination success. Together, our results support the use of incorporated cover crop residues as an integrated weed management tool.     

Size and composition of the weed seedbank after 7 years of different cover-crop-maize management systems

The objective of this study was to obtain detailed information on the long‐term weed suppression potential of four winter soil cover types included in an arable crop system managed at various input levels. We used weed seedbank size and composition to assess weed suppression potential. A field experiment was established in 1993 as a split‐split‐plot design with four replications, including two tillage systems [a conventional system (CS) including ploughing in the cover crops and a low‐input system (LIS) including no tillage with surface mulching of the cover crops] in the main plots, three mineral nitrogen fertilization rates for the main crop in the sub‐plots and four soil cover types (main crop residue, rye, crimson clover and subterranean clover) in the sub‐sub‐plots. Seedbank sampling took place in winter 2000/01. The weed seedbank was analysed with the seedling emergence method. Data were analysed using anova and multivariate techniques. Results indicated that the seedbank density in the LIS was about five times higher than in the conventional input system. In the CS, use of a rye cover crop resulted in a lower seedbank density with respect to the crop residue treatment (?25%), whereas in the LIS the subterranean clover cover crop decreased weed seedbank density as compared with the other cover crops and the crop residue treatment (?22% on average). Differences in species composition were mainly related to tillage system. Implications for cover crop management and the development of sustainable cropping systems are discussed.     

Effects of weed harrowing and undersown clover on weed growth and spring cereal yield

Weed competition and nutrient scarcity often restrict organic cereal production, especially where the availability of livestock manure is limited. While harrowing of annual weeds and legume cover crops can be used, these methods are both executed in early spring and may hinder each other. Two cycles of a 2‐year crop rotation were carried out in south‐east Norway (60°42′N, 10°51′E, altitude 250 m) with weed harrowing and undersown cover crops (WHCC) at two fertiliser rates (40 and 100 kg nitrogen ha^?1). The effect of the WHCC treatments was measured by weed density and species, weed biomass, changes in weed seedbank and grain yield. The weed density depended on the interaction between WHCC, fertiliser and year. On average, pre‐emergence weed harrowing reduced weed density by 32% and weed biomass by 49%, while pre‐ and post‐emergence weed harrowing reduced weed density by 59% and weed biomass by 67% compared with the untreated control. Spergula arvensis became more abundant at low rather than at high fertiliser rates. On average, white clover cover crop sown after pre‐emergence weed harrowing resulted in the highest yields for both oat (+12.1%) and wheat (+16.4%) compared with the untreated control. Despite differences in weed population density and biomass among WHCC treatments within years, the weed biomass, weed density and seedbank increased for all WHCC treatments over the 4‐year period. More research is required into improving the efficacy of mechanical and cultural weed suppression methods that organic systems rely on.     

Mulched cover crops as an alternative to conventional weed management systems in vineyards

Conventional methods of weed management in vineyards rely primarily on herbicides and tillage. The desire to adopt alternatives to these methods is driven by environmental and economic reasons. Weed suppression and grape yield under mulched cover crop systems at two rainfed northern California vineyards were similar to, and at times exceeded, those under conventional tillage or herbicide management. Cover crop productivity was positively correlated with weed suppression and mulch decomposition rates and seemed to be determined primarily by location and then by cover crop type. The mulch from mowed cover crops averaged 603(± 94) gm⁻² at the two sites. Weed suppression was linked to light interception by the mulch cover for most weed species. Subterranean clover planted directly in the vine row significantly reduced weed cover where it established. The increased dominance of the perennial Convolvulus arvensis and reduction of certain annual species was indicative of species compositional changes in all treatments. Profits under the cover cropping systems exceeded those under conventional tillage and herbicide systems by €  794 ha⁻¹ averaged over the duration of the experiment at both locations.     

Diversity in weed seed production and the soil seed bank: Contrasting responses between two agroecosystems

This study was carried out to compare the diversity in seed production and the soil seed bank in a dryland and an irrigated agroecosystem in the dry tropics. Both agroecosystems showed a comparable number of species, but only 25% and 38% similarity during the winter and rainy cropping seasons, respectively. In the irrigated agroecosystem, the amount of seed production diversity was almost double in the winter season, compared to the rainy season. The weed seedbank diversity was low but was sensitive to cropping practices and seasons in both agroecosystems. A considerably smaller soil seedbank size in the irrigated agroecosystem (cf. dryland) was related to lowered weed seed production. The dryland agroecosystem showed a greater accumulation of the seeds of broad‐leaved weeds, whereas the irrigated agroecosystem accumulated more seeds of the grasses or sedges. About three‐fourths of the seeds during the winter season were accounted for by Anagallis arvensis and Chenopodium album in the dryland agroecosystem and by C. album and Melilotus indica in the irrigated agroecosystem. However, during the rainy season, Ammannia baccifera, Echinochloa colona and Cyperus rotundus dominated in both agroecosystems. The changes in the weed seed bank and its diversity are mainly attributed to differences in water management, which tends to reduce species diversity, especially at a lower depth, but leads to the dominance of some potentially noxious weeds (e.g. Phalaris minor and M. indica). Approximately double the soil seedbank size and a greater diversity at a lower depth might indicate an adaptive mechanism in the storage of weed seeds in the dryland agroecosystem.     

Effects of alternative winter cover cropping systems on weed suppression in organically grown tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis>)

Weed control is a major concern for organic farmers around the world and non-chemical weed control methods are now the subject of many investigations. Field studies were conducted in tomato (Solanum lycopersicum L.) from 2004 to 2006 at the Black Sea Agricultural Research Institute experiment field to determine the weed suppressive effects of winter cover crops. Treatments consisted of ryegrass (Lolium multiflorum L.), oat (Avena sativa L.), rye (Secale cereale L.), wheat (Triticum aestivum L.), gelemen clover (Trifolium meneghinianum Clem.), Egyptian clover (Trifolium alexandrinum L.), common vetch (Vicia sativa L.), hairy vetch (Vicia villosa Roth.) and a control with no cover crop. Treatments were arranged in a randomized complete block design with four replications. To determine the weed suppressive effects of the cover crops, weed density and total weed dry biomass were assessed at 14, 28, and 56 days after termination (DAT) of the cover crops from all plots using a 50 × 50 cm quadrat placed randomly in each plot. After cover crop kill and incorporation into soil, tomato seedlings variety ‘H2274’ were transplanted. Broadleaved weed species were the most prominent species in both years. Total weed biomass measured just prior to cover crop incorporation into the soil was significantly lower in S. cereale plots than in the others. The number of weed species was lowest at 14 DAT and later increased at 28 and 56 DAT, and subsequently remained constant during harvest. This research indicates that cover crops such as L. multiflorum, S. cereale, V. sativa and V. villosa could be used in integrated weed management programs to manage some weeds in the early growth stages of organic tomato.     

An on-farm approach to investigate the impact of diversified crop rotations on weed species richness and composition in winter wheat

Weed species diversity may benefit from organic farming due to enhanced temporal diversification of crop species in a rotation and omission of herbicide applications. However, in intensively managed conventional systems, little evidence exists as to what extent diversified crop rotations contribute to higher weed species richness. Using an on-farm approach, the effect of crop rotation (organic, conventional diverse (CD) and conventional simple (CS) crop rotations) and weed control (with vs. without) on weed species richness, cover, community composition and crop biomass, was analysed in 24 winter wheat fields. Weed species with beneficial functions for invertebrates and birds were analysed separately. Weed species richness was higher in the organic crop rotation, but did not differ between CD and CS crop rotations. Weed control treatment reduced species richness in both conventional rotations, but not in the organic one. Redundancy analyses revealed that crop rotation intensity accounted for the largest part of the explained variation in weed species composition. Results from the study indicate that the maintenance of weed species richness and conservation of species with important ecological functions requires not only temporal diversification of crop species in the rotation, but also an adjustment of weed control strategies.     

Impact of Indian mustard growth and incorporation on annual weed population dynamics and communities

Biofumigation from Brassica cover crops may be used to control soilborne pests and weeds. A study was conducted to understand the influence of biofumigation on key processes of annual weed population dynamics. Five combinations of Indian mustard (M) and oat (O) cover crop treatments were assessed in a 3 year field study at two locations in Québec, Canada. Treatments included four spring/fall cover crop combinations (M/M, M/O, O/M, O/O) and a weedy check control with no cover crop. Prior to mowing and incorporation of cover crops, weed identification, count and biomass measurements were recorded to evaluate the total weed density, to calculate the relative neighbour effect (RNE) and weed diversity metrics and to perform principal co‐ordinates analyses. Indian mustard cover crops had no impact on weed establishment in 2014 due to low biofumigant potential compared to the oat cover crop. In 2015 and 2016, Indian mustard isothiocyanate (ITC) production increased and weed establishment within the Indian mustard cover crop decreased. Moreover, post‐cover crop incorporation decreased the next year spring weed emergence. Allelopathic interference of Indian mustard was significant when plant tissues produced more than 600 μg of allyl‐ITC g^?1. It is now possible to rationalise the use of Brassica cover crops and biofumigation for weed control with an enhanced understanding of the impact of biofumigation on key processes of weed population dynamics.     

Cover crops and mulches influence weed management and weed flora composition in strip‐tilled tomato (Solanum lycopersicum)

This study was conducted in the Mediterranean environment of Central Italy from 2011 to 2013 with the aim of evaluating the effects of winter cover crops and their residues on weed composition in a cover crop‐tomato sequence. Treatments consisted of five soil managements (three cover crop species ‐ hairy vetch, phacelia, white mustard, winter fallow mulched with barley straw before tomato transplanting and conventionally tilled soil), two nitrogen fertilisation levels (0 and 100 kg N ha^?1) and two weed management levels (weed free and weedy) on tomato. Cover crop residues were arranged in strips on the soil surface and then used as beds for transplanting the tomato seedlings in paired rows. Rotary hoeing was performed in the bare strips between paired tomato rows. At tomato harvesting, the weed aboveground biomass and density was higher in nitrogen‐fertilised tomato than unfertilised tomato, except in hairy vetch and barley straw that showed similar values. Hairy vetch used as a cover crop and dead mulch was the most suppressive species with the highest production of residues, while phacelia and mustard were not suitable for controlling weeds. The tomato yield was high in nitrogen fertilised and weed‐free treatments, except in barley straw mulch, which showed similar values among the weed management treatments. The mulch strips caused variations in weed species composition that was mainly composed of perennial ruderal weeds, while in tilled soil, the weed flora was dominated by annual photoblastic weeds.     

Predicting the competitive effects of weed and crop density on weed biomass, weed seed production and crop yield in wheat

Competition between winter-sown wheat and Viola arvensis Murray or Papaver rhoeas L. was studied in two experiments in two successive years. The effects of varying crop and weed density were modelled in terms of weed biomass over time, weed seed production and crop yield. Biomass model parameters, representing maximum weed biomass and intra- and interspecific competition, were obtained for different assessment dates, enabling biomass levels to be predicted during the two growing seasons. Weed biomass declined, and its maximum level was reached earlier, with increasing crop density. Intraspecific competition was higher in the absence than in the presence of crop, increasing with time and with weed density. Halving the wheat population increased June biomass of V. arvensis by 74% and of P. rhoeas by 63%. Crop yield losses with increasing weed density were greater with low than with medium and high crop populations. P. rhoeas was significantly more competitive than V. arvensis in both years. Weed biomass in 1989 responded more to reductions in crop density following the milder winter of 1988/89 than in the previous year; however crop yields were less affected in 1989 due to summer drought, restricting late weed growth and competition. Weed seed production was related to weed biomass; the progressive lowering of crop density increased seed production, and both species were very prolific in the absence of crop. By combining models, seed production could be derived for a given competitive effect on the crop. Threshold weed populations, based on low weed levels that are not economic to control, could then be equated with the accompanying weed seed production.     

Weed suppression by cover crops: comparative on‐farm experiments under integrated and organic conservation tillage

Cover crops are increasingly being used for weed suppression and to enhance the sustainability of agro‐ecosystems. However, the suitability of cover crops for weed suppression in integrated and organic conservation tillage systems is still poorly investigated. Therefore, a 2‐year field study at eight sites was conducted to test the weed suppressive potential of six legume‐based cover crops, with the aim to reduce herbicide input or mechanical weed management interventions. In all experiments, cover crops were directly sown after cereals before next year's main crop (grain maize or sunflower). The presence of cover crops caused a 96% to 100% reduction of weed dry matter at the four sites managed under integrated production, while effects were lower at the four sited managed under organic production, ranging from 19% to 87%. Cover crops that covered soil quickly and which produced much dry matter had the best weed suppressive potential. However, their weed suppressing effect was difficult to predict, as it depended on the year of the investigation, experimental site, cover crop species, the speed of soil cover in autumn and the density of the resulting mulch layer in spring. The study demonstrated that cover crops are a useful tool to suppress weeds under integrated and organic conservation tillage practices. Our recommendation for supporting weed management in conservation tillage systems is to use locally adapted cover crops that have rapid establishment, good soil coverage and high dry matter production. However, additional weed management measures are required for reliable weed control under on‐farm conditions.     

Contribution of allelopathic effects to the overall weed suppression by different cover crops

Cover crops can suppress weeds within agricultural fields due to competitive and allelopathic effects. Glasshouse experiments were conducted to evaluate the relative proportions of allelopathic effects to the total weed inhibition. Six different cover crop species were combined with three weed species in the presence or absence of active carbon over a period of four weeks. Active carbon was used as an adsorbent for allelopathic substances in the soil. Our study revealed that the competition between cover crops and weeds shifted, possibly due to the minimisation of allelopathic effects by active carbon in the soil. We assume that the degree of cover crops allelopathic effects on weeds is species‐specific, both on the side of cover crops and on the weed side. The cover crops Raphanus sativus, Fagopyrum esculentum and Avena strigosa showed the highest allelopathic weed suppression with up to 28%. Additionally, Stellaria media turned out to be the most sensitive weed against allelopathic effects induced by all cover crops, except for Linum usitatissimum and Guizotia abyssinica. The knowledge about the contribution of competitive and allelopathic effects by cover crops would help to create cover crop mixtures with high weed suppressive ability.     

Reduced crop sowing density improves performance of rare arable weed species more effectively than reduced fertilisation

Characteristic arable weed species of dryland cereal fields have undergone significant declines, due to agricultural intensification, to the point that some of them are considered rare. Crop edges host higher abundances of arable weed species, therefore they may act as a refuge for the conservation of these rare arable species. Using mesocosms, we experimentally tested how conditions at field edges (i.e. lower sowing densities and less intensive fertiliser applications) operate on the growth (biomass and height) and reproduction (reproductive biomass and flower onset) of six rare arable species. We found the rare arable species achieved lower biomass when growing with wheat compared with growing alone, and biomass of most of species was lower under high wheat sowing density than under low sowing density. In contrast, fertiliser application affected only two of the six arable species tested, especially when they were growing alone. Although the time to flowering was not affected by the conditions tested, reproductive biomass showed the same trends as overall biomass. These results indicate that conservation of rare arable species must primarily consider reductions in crop competition to increase their biomass and reproductive ability.     

Changes in functional diversity and intraspecific trait variability of weeds in response to crop sequences and climate

How weed communities assemble represents one the key issues of weed science. For a decade, functional approaches have been applied to investigate the processes that govern weed community assembly. In most previous studies, trait values have been generally averaged over multiple populations and habitats. Consequently, conspecifics display similar trait values while neglecting the considerable influence of intraspecific variability to detect changes in functional diversity in response to environmental drivers. However, this influence has been shown to be critical, especially, at local scales. Here, we studied changes in weed functional diversity at the field scale in four crop sequences. We focused on intra‐ and interspecific variability of four key functional traits involved in response to resource acquisition processes, the latter being modified by climate, management and competition. The relative influence of intra‐ and interspecific variability among the crop sequence types was highlighted using a diversity partitioning approach. It provides evidence for substantial amount of intraspecific variability in the weed community and underlines its essential role in response to fine‐scale environmental drivers. In addition, we investigated the response of the three most abundant species to competition with the crop, the growing season and the crop sequence type. We highlighted that these species showed a wide range of combinations of trait values, suggesting the co‐existence of several successful strategies. Based on these results, we emphasise that neglecting intraspecific variability can lead to substantial underestimations of the functional weed response to management and crop‐weed competition at the field scale.     

The impact of landscape structure and sown grass margin strips on weed assemblages in arable crops and their boundaries

A paired-field study was made in southern England in arable fields, each with or without sown 6-m wide grass margin strips. Seven field pairs were located in each of small, intermediate and open landscapes, which were based on mean field size. Ground cover, plant species diversity and assemblages were assessed in crop centres, crop edges and non-crop field boundaries. The data were used to test for effects of sown grass margins, differences in field location, impacts on rare weed species and for landscape effects on weed assemblages. Significantly higher plant species diversity was found in boundaries protected by buffer strips. Annual weeds associated with field edges, notably Anisantha sterilis , were found at lower cover where perennial grass strips were present. Sown grass strips enhanced boundary plant diversity, particularly by increasing polycarpic species. Margin strips had a small influence on the weed flora of the crop edge, possibly reducing weed cover, but had no influence on floras of field centres. Field size and landscape context did not affect weed assemblages, which show marked field-to-field variability, though crop type was an important influence. Grass margins did not enhance rare arable weed species and may be a threat to them, if margins are sited where such species are known to occur in the seedbank. With this exception, grass strips are a positive influence on boundary flora diversity and reduce margin weeds in arable landscapes.     

Response of weed flora to combinations of reduced tillage, biocide application and fertilization practices in a 3-year crop rotation

Weed flora in a crop rotation of soybean, sugar beet, and spring wheat were studied under combinations of conventional and reduced tillage, biocide application, and chemical fertilization to investigate whether any combination of conservational practices offers the potential to increase weed community diversity while maintaining weed biomass at an acceptable level. Weed density increased under reduced biocide application. Weed density and size increased under manure compost application (with reduced chemical fertilization) because of weed seed introduction. Weed emergence from seeds dispersed in the previous year was greater under reduced tillage. Two-year individuals increased under a combination of reduced tillage, reduced biocide application and manure compost application. These increases in weed population density and size under conservational practices were yet not consistent; rather, they showed an annually fluctuating trend. In terms of weed diversity, species richness and Shannon's diversity index were higher under manure application because of the introduction of new species and probably as a result of suppression of weed growth and reproduction, which can reduce the chance of dominance by certain species. When combined with manure application, reduced tillage may offer the potential for increasing diversity by reducing the competitiveness of dominant species and facilitating establishment of new or minor species.     

Cropping systems modify soil biota effects on wheat (Triticum aestivum) growth and competitive ability

Plants alter soil biota which subsequently modifies plant growth, plant–plant interactions and plant community dynamics. While much research has been conducted on the magnitude and importance of soil biota effects (SBEs) in natural systems, little is known in agro‐ecosystems. We investigated whether agricultural management systems could affect SBEs impacts on crop growth and crop–weed competition. Utilising soil collected from eight paired farms, we evaluated the extent to which SBEs differed between conventional and organic farming systems. Soils were conditioned by growing two common annual weeds: Amaranthus retroflexus (redroot pigweed) or Avena fatua (wild oat). Soil biota effects were measured in wheat (Triticum aestivum) growth and crop–weed competition, with SBEs calculated as the natural log of plant biomass in pots inoculated with living soil divided by the plant biomass in pots inoculated with sterilised soil. SBEs were generally more positive when soil inoculum was collected from organic farms compared with conventional farms, suggesting that cropping systems modify the relative abundance of mutualistic and pathogenic organisms responsible for the observed SBEs. Also, as feedbacks became more positive, crop–weed competition decreased and facilitation increased. In annual cropping systems, SBEs can alter plant growth and crop–weed competition. By identifying the management practices that promote positive SBEs, producers can minimise the impacts of crop–weed competition and decrease their reliance on off‐farm chemical and mechanical inputs to control weeds, enhancing agroecosystem sustainability.     

Modelling the effects of weeds on crop production

In most quantitative studies on interplant competition, static regression models are used to describe experimental data. However, the generality of these models is limited. More mechanistic models for interplant competition, which simulate growth and production of species in mixtures on the basis of the underlying physiological processes, have been developed in the past decade. Recently, simulation models for competition between species for light and water were improved and a detailed version was developed for sugarbeet and fat hen (Chenopodium album L.). The model was validated with data sets of five field experiments, in which the effect of fat hen on sugarbeet production was analysed. About 98% of the variation in yield loss between the experiments (which ranged from –6 to 96%) could be explained with the model. Further analysis with the model showed that the period between crop and weed emergence was the main factor causing differences in yield loss between the experiments. Sensitivity analysis showed a strong interaction between the effect of the variables weed density and the period between crop and weed emergence on yield reduction. Different quantitative approaches to crop-weed competition are discussed in view of their practical applicability. Simulations of experiments, where both the weed density and the period between crop and weed emergence were varied over a wide range, showed a close relation between relative leaf cover of the weeds shortly after crop emergence and yield loss. This relation indicates that relative leaf cover of the weeds accounts for both the effect of weed density and the period between crop and weed emergence. This relation has the potential to be developed into a powerful tool for weed-control advisory systems.     

Ecological weed management by cover cropping: effects on weed growth in autumn and weed establishment in spring

Cover crops grown in the period between two main crops have potential as an important component of a system‐oriented ecological weed management strategy. In late summer and autumn, the cover crop can suppress growth and seed production of weeds, whereas the incorporation of cover crop residues in spring may reduce or retard weed emergence. Based on these two criteria, six cover crop species were evaluated for their weed suppressive potential in 2 years of experimentation in the Netherlands. Fodder radish, winter oilseed rape and winter rye had the strongest competitive ability in autumn; the competitive strength of Italian ryegrass was intermediate and white lupin and lucerne were poor competitors. Competitiveness was strongly correlated to early light interception. Surprisingly, doubling the recommended sowing density did not increase weed suppressive ability. Although a poor competitor in the fall, after incorporation in spring, lucerne had the strongest inhibitory effect on seedling establishment, followed by winter oilseed rape and white lupin. Winter rye and fodder radish did not affect seedling establishment, whereas Italian ryegrass was not evaluated because of re‐growth after incorporation. Competition in autumn and subsequent residue‐mediated suppression of weed establishment in spring varied among the cover crop species, with winter oilseed rape offering relatively strong effects during both periods.