Not All Weed Communities Reduce Crop Yield

Weed communities can both cause severe crop yield losses (Oerke Reference Oerke2006) and provide benefits to crops and the broader environment (Blaix et al. Reference Blaix, Moonen, Dostatny, Izquierdo, Le Corff, Morrison, Von Redwitz, Schumacher and Westerman2018; Gaba et al. Reference Gaba, Cheviron, Perrot, Piutti, Gautier and Bretagnolle2020; Kleiman et al. Reference Kleiman, Koptur and Jayachandran2021; Smith et al. Reference Smith, Aebischer, Ewald, Moreby, Potter and Holland2020). A growing body of research suggests that the competitive effects of weeds on crops depends on a multitude of factors, including the functional composition of weed communities, crop traits, and environmental conditions (Bàrberi et al. Reference Bàrberi, Bocci, Carlesi, Armengot, Blanco-Moreno and Sans2018; Cirillo et al. Reference Cirillo, Masin, Maggio and Zanin2018; Gaba et al. Reference Gaba, Perronne, Fried, Gardarin, Bretagnolle, Biju-Duval, Colbach, Cordeau, Fernández-Aparicio, Gauvrit, Gibot-Leclerc, Guillemin, Moreau, Munier-Jolain, Strbik and Reboud2017; Gunton et al. Reference Gunton, Petit and Gaba2011). Under some circumstances, weed communities may provide ecosystem services without affecting crop yield and quality. In this review, we define such weed communities as neutral weed communities. In France, Adeux et al. (Reference Adeux, Vieren, Carlesi, Bàrberi, Munier-Jolain and Cordeau2019) identified six weed communities over 3 yr of observations that included weed biomass, weed density, and winter cereal crop biomass. Two of the six communities identified did not significantly reduce grain yield compared with weed-free treatments. These two communities did not consistently have lower weed density or biomass than the communities that did reduce grain yield. The two neutral weed communities were mostly composed of Persian speedwell (Veronica persica Poir.), common chickweed [Stellaria media (L.) Vill], cutleaf geranium (Geranium dissectum L.), ivyleaf speedwell (Veronica hederifolia L.), and catchweed bedstraw (Galium aparine L.).

In Italy, Esposito et al. (Reference Esposito, Cirillo, Cozzolino, De Vita and Maggioin press) obtained similar results, identifying neutral weed communities in a field experiment with winter wheat (Triticum aestivum L.) under three different soil nutrient levels (low, optimal, and surplus). Under surplus nutrition, one detrimental community was identified. This finding suggests that high soil fertility may promote the growth of dominant weed species that capitalize on high rates of fertilization to compete aggressively with crops (Little et al. Reference Little, DiTommaso, Westbrook, Ketterings and Mohler2021). Under both low and optimal nutrient levels, one neutral weed community and one detrimental community were identified. The neutral communities did not negatively affect grain yield or quality. Corn chamomile (Anthemis arvensis L.) was the most abundant weed in neutral communities, accounting for 28% and 46% of total weed density in optimal and low nutrient treatments, respectively. The relative density of this species was higher in neutral communities than in detrimental communities. Under optimal nutrition, S. media was present only in the neutral community, while V. persica was mostly present in the neutral weed community. The annual legume California burclover (Medicago polymorpha L.) apparently contributed to the detrimental communities by accumulating a large amount of aboveground biomass and causing wheat lodging. Medicago polymorpha, which is native to the Mediterranean region and adapted to semiarid conditions (Yousfi et al. Reference Yousfi, Saïdi, Slama and Abdelly2015), may also compete with crops for water. Under optimal and low nutrition, M. polymorpha was mostly present in the detrimental communities compared with the neutral communities. Finally, Esposito et al. (Reference Esposito, Cirillo, Cozzolino, De Vita and Maggioin press) noted that the density of the neutral community was higher than the density of the detrimental weed community under optimal nutrition, suggesting that density is not always a good predictor of a weed community’s deleterious effect on crop yield.

Boström et al. (Reference Boström, Milberg and Fogelfors2003) studied the relationship between weed community composition and yield losses in spring wheat and barley (Hordeum vulgare L.; syn.: Hordeum distichum L.) in 33 field trials over 3 yr. Weeds such as scarlet pimpernel (Anagallis arvensis L.), nightflowering catchfly (Silene noctiflora L.), Euphorbia spp., field violet (Viola arvensis Murray), common lambsquarters (Chenopodium album L.), and Polygonum spp. were not associated with yield losses. In contrast, wild radish (Raphanus raphanistrum L.) and hempnettle (Galeopsis spp.) were among the most detrimental species, causing large crop yield losses.

In a 26-yr experiment in Sweden, Milberg and Hallgren (Reference Milberg and Hallgren2004) ranked weed species from the most benign to the most detrimental. Using 1,691 samples from on-farm trials, they identified benign weeds as those consistently occurring in situations with small cereal yield losses and detrimental weeds as those associated with larger yield losses. In autumn-sown cereals, benign weed species included wild buckwheat [Polygonum convolvulus L. var. convolvulus; syn.: Fallopia convolvulus (L.) Á. Löve], prostrate knotweed (Polygonum aviculare L.), false cleavers (Galium spurium L.), field forget-me-not [Myosotis arvensis (L.) Hill], S. media, and Veronica spp. Benign weed species in spring-sown cereals were P. convolvulus, Lamium spp., and wallflower mustard (Erysimum cheiranthoides L.). Detrimental weed species were scentless mayweed [Tripleurospermum inodorum (L.) Sch. Bip.] and shepherd’s purse [Capsella bursa-pastoris (L.) Medik.] in autumn-sown cereals and Galeopsis spp. and curlytop knotweed [Polygonum lapathifolium L.; syn. Persicaria lapathifolium (L.) Gray] in spring-sown cereal systems.

Similarly, in winter wheat, Jones and Smith (Reference Jones and Smith2007) grouped weeds as very desirable, desirable, and undesirable based on agronomic issues and biodiversity benefits. Out of the 18 very desirable or desirable weeds identified in this study, 6 species were also defined as benign by Milberg and Hallgren (Reference Milberg and Hallgren2004). Analogously, S. media was classified as very desirable by Jones and Smith (Reference Jones and Smith2007). This species was representative of neutral weed communities in winter wheat according to Esposito et al. (Reference Esposito, Cirillo, Cozzolino, De Vita and Maggioin press). These points of agreement between different studies suggest that certain weed species are relatively benign in several cereal-cropping systems, although any species can be harmful given the right conditions.

Much research on the relative competitiveness of different weed species and communities has been carried out in Europe. However, evidence from other world regions also exists. In an arid region of India, Bhandari and Sen (Reference Bhandari and Sen1979) showed that sowing an annual leguminous weed, Indigofera cordifolia B. Heyne ex Roth, improved growth parameters and yield in millet [Pennisetum glaucum (L.) R. Br.] and sesame (Sesamum indicum L.). Millet yield was 19.8% higher and sesame yield was 22.4% higher in plots sown with I. cordifolia, compared with weeded plots. In the same area, weeds like Arabian-primrose [Arnebia hispidissima (Lehm.) A. DC.], Spermacoce articularis L.f., and feather cockscomb (Celosia argentea L.) increased the growth parameters and yield of millet but not sesame (Sen [1978] as cited by Bhandari and Sen [Reference Bhandari and Sen1979]).

This survey reveals that much research on weed community competitiveness has focused on annual cropping systems. One reason is that annual crops account for much more harvested cropland area than perennial crops (FAOSTAT 2020). However, evidence for neutral weed communities has also emerged from perennial cropping systems. Liang and Huang (Reference Liang and Huang1994) highlighted the need to distinguish beneficial from detrimental weeds in citrus orchards. They noted that some weeds do not compete substantially with the citrus trees, instead providing economic and ecological benefits. Beneficial weeds, such as tropical ageratum (Ageratum conyzoides L.), sessile joyweed [Alternanthera sessilis (L.) R. Br. ex DC.], and many dicotyledonous weeds, often had soft tissues, shallow roots, and broad leaves. In contrast, detrimental weeds, such as goosegrass [Eleusine indica (L.) Gaertn.], cogongrass [Imperata cylindrica (L.) P. Beauv.], bermudagrass [Cynodon dactylon (L.) Pers.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and many monocotyledonous weeds, had opposite characteristics. Liang and Huang (Reference Liang and Huang1994) considered A. conyzoides one of the most beneficial weeds, because it can support natural enemies of citrus pests and is suitable as a green manure. Other weeds reported to be beneficial in citrus orchards include C. argentea, alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.], Chinese giant-hyssop [Agastache rugosa (Fisch. & C.A. Mey.) Kuntze], beefsteakplant [Perilla frutescens (L.) Britton], and Exallage auricularia (L.) Bremek. (Liang and Huang Reference Liang and Huang1994 and references therein).

Neutral weed communities have also been identified in other orchard types. In a 3-yr experiment in organic and conventional apple (Malus domestica Borkh.) orchards in China, Meng et al. (Reference Meng, Li, Liu, Li, Li, Wu, Yu, Guo, Cheng, Muminov, Liang and Jiang2016) demonstrated the possibility of managing pernicious weeds in the organic orchard by propagating Indian mock strawberry [Duchesnea indica (Andrews) Teschem.; syn. Potentilla indica (Andrews) Th. Wolf]. This native species was able to suppress undesirable weeds through competition, enabling good weed suppression without the use of herbicides. Despite the fast growth of P. indica and its increasing dominance in the understory ground cover of the organic orchard, apple yield was not different between the organic and conventional orchards in the last 2 yr of the study. In a comparison between organic and conventional olive (Olea europaea L.) groves in Greece, organic and conventional groves did not differ in edible olive yield or olive oil yield (Solomou and Sfougaris Reference Solomou and Sfougaris2011). Wild carrot (Daucus carota L.) and ovate goatgrass (Aegilops geniculata Roth) were the most frequently occurring herbaceous plants in 10-yr and 6-yr certified organic groves, respectively.

Caveats

Data on which weeds are most detrimental or beneficial can be used to focus weed management efforts on maximizing crop productivity and ecosystem services, rather than removing all weeds. Such data should be collected in diverse climates and cropping systems, as the identities of “detrimental” and “beneficial” weed species will vary from place to place. Several species characterized as benign by the studies discussed in this section are highly problematic in other contexts. Notably, different crops may display different degrees of competitiveness against weeds (Andrew et al. Reference Andrew, Storkey and Sparkes2015; Corre-Hellou et al. Reference Corre-Hellou, Dibet, Hauggaard-Nielsen, Crozat, Gooding, Ambus and Jensen2011; Lemerle et al. Reference Lemerle, Luckett, Lockley, Koetz and Wu2014). Consequently, neutral weed communities are more likely to occur in more competitive crops, such as wheat, barley, and corn. In addition, it is important to note that the effect of a particular weed species depends not only on climate and cropping system but also on the presence of other weed species. Species-level data cannot always substitute for community-level analysis.

Researchers and stakeholders should also adopt a multiyear perspective when considering neutral weed communities and how they should be managed. It is not advisable to modify weed management without considering how this decision will change plant community composition over time. Even if a weed community is unlikely to cause yield loss in the current year, eliminating weed control operations might allow high levels of weed seed production, increasing the soil seedbank and potentially contributing to yield loss in future years. This consideration becomes especially important when a farmer makes weed control decisions in a competitive crop, but then grows a less competitive crop in a later phase of the rotation cycle. We advocate holistic analysis of the long-term costs and benefits of weeds and management tactics.

Coexistence between Weeds and Crops

To understand why weed communities are not always detrimental, it is useful to draw on broader theories of plant–plant coexistence. A foundational idea in ecology is the concept of competitive exclusion, that is, the idea that two or more species that occupy the same niche cannot coexist (Gause Reference Gause1934). According to this paradigm, stable coexistence of multiple species is best explained by differences in their functional traits (i.e., differences in how they respond to and affect their biotic and abiotic environments). For example, species might require different resources at different times. Even among plants, all of which require similar resources, there is substantial potential for resource partitioning to promote coexistence (Silvertown Reference Silvertown2004). Resource partitioning is one of several proposed stabilizing mechanisms of coexistence, defined as mechanisms that increase the magnitude of negative intraspecific interactions relative to negative interspecific interactions (Chesson Reference Chesson2000). In general, intraspecific interactions do tend to be more negative than interspecific interactions in plant communities, consistent with stabilizing mechanisms of coexistence (Adler et al. Reference Adler, Smull, Beard, Choi, Furniss, Kulmatiski, Meiners, Tredennick and Veblen2018). In the presence of stabilizing mechanisms of coexistence, equalizing mechanisms (i.e., mechanisms that reduce fitness differences between species) may further promote stable coexistence (Chesson Reference Chesson2000). Alternatively, equalizing mechanisms could help support models of unstable coexistence. Notably, Hubbell (Reference Hubbell2001) proposed a neutral theory in which drift, migration, and speciation are more important than stabilizing mechanisms and fitness differences. Niche-based and neutral theories are not mutually exclusive: stabilizing mechanisms and fitness differences between species do exist, but their magnitudes and effects on community assembly can vary (Adler et al. Reference Adler, HilleRisLambers and Levine2007). The uncovering of mechanisms promoting coexistence is ongoing and crucial research, especially given that such mechanisms may maintain ecosystem function as well as biodiversity (Godoy et al. Reference Godoy, Gómez-Aparicio, Matías, Pérez-Ramos and Allan2020; Turnbull et al. Reference Turnbull, Levine, Loreau and Hector2013).

In applying ecological perspectives to agricultural situations, one should remember that (1) most cropping systems are frequently disturbed and therefore best characterized as early successional habitats, and (2) the effects of weeds on crops are more frequently measured than the effects of crops on weeds. For both reasons, a report of weeds that do not appear to impact crop yield does not constitute a report of stable coexistence. Nevertheless, arguments developed to explain coexistence in natural systems can provide helpful insights into agroecological dynamics. For instance, niche complementarity (a lack of niche overlap, often reflecting different spatiotemporal resource use patterns) is a primary explanation for higher yields in some intercropping systems relative to monocultures of the component species (Brooker et al. Reference Brooker, Bennett, Cong, Daniell, George, Hallett, Hawes, Iannetta, Jones, Karley, Li, McKenzie, Pakeman, Paterson and Schöb2015). This overyielding in polyculture indicates that interspecific interference is less detrimental than intraspecific interference, implying that stabilizing mechanisms such as resource partitioning are at work. Other mechanisms, including facilitation, may also contribute to the success of intercropping systems (Brooker et al. Reference Brooker, Bennett, Cong, Daniell, George, Hallett, Hawes, Iannetta, Jones, Karley, Li, McKenzie, Pakeman, Paterson and Schöb2015).

In the context of agricultural weeds, greater niche overlap between weed species and crop species may increase weed–crop competition (Zimdahl Reference Zimdahl2004). Weeds that consume the same resources as the crop during the same time periods are typically more problematic than weeds with different requirements. More broadly, functional similarity between weeds and crops may involve shared morphological, physiological, or phenological traits (Navas Reference Navas2012). Shared traits sometimes reflect homology (common descent); therefore, understanding weed phylogeny could contribute to a better understanding of weed–crop competition (Gibson et al. Reference Gibson, Young and Wood2017). The effects of functional traits on ecological interactions between weeds and crops can be observed in the absence of weed control. However, it is also true that similarities between weeds and crops complicate efforts to develop selective weed control tactics and that weed management programs often inadvertently favor weeds that are similar to crops. This unintentional selection promotes crop mimicry in weeds. For example, early water grass [Echinochloa oryzoides (Ard.) Fritsch] closely resembles rice (Oryza sativa L.) (Barrett Reference Barrett1983).

One of the principal drivers of weed–crop competition and weed community assembly is the diversity of available resource pools. According to the resource pool diversity hypothesis, an increase in the spatiotemporal diversity of soil resource pools leads to decreased crop yield loss per unit of weed density (Smith et al. Reference Smith, Mortesen and Ryan2010). This trend occurs because diverse soil resource pools enable more resource partitioning among species. The negative relationship between resource pool diversity and crop yield loss is particularly strong when weeds and crops have different resource acquisition traits (i.e., greater capacity for niche differentiation). At the same time, resource pool diversity shapes weed community structure and may support the persistence of species that are functionally different from the crop. Overall, the resource pool diversity hypothesis predicts that practices such as crop rotation, cover cropping, use of diverse fertility amendments, and integrated weed management will reduce the dependence of crop yield on weed abundance (Smith et al. Reference Smith, Mortesen and Ryan2010). More generally, the relationship between weed abundance and yield loss is affected by environmental factors, management factors, and weed community composition (Ryan et al. Reference Ryan, Mortensen, Bastiaans, Teasdale, Mirsky, Curran, Seidel, Wilson and Hepperly2010; Swinton et al. Reference Swinton, Buhler, Forcella, Gunsolus and King1994; Wilson and Wright Reference Wilson and Wright1990).

Agricultural activities represent filters that shape the species and functional composition of weed communities (Armengot et al. Reference Armengot, Blanco-Moreno, Bàrberi, Bocci, Carlesi, Aendekerk and Sans2016; Cordeau et al. Reference Cordeau, Wayman, Ketterings, Pelzer, Sadeghpour and Ryan2021; Mhlanga et al. Reference Mhlanga, Cheesman, Maasdorp, Muoni, Mabasa, Mangosho and Thierfelder2015). Higher-strength values of these filters (intensive agriculture) are associated with reduced weed diversity and increased abundance of a few dominant weed species (Adeux et al. Reference Adeux, Vieren, Carlesi, Bàrberi, Munier-Jolain and Cordeau2019; Storkey and Neve Reference Storkey and Neve2018). Conversely, a greater diversity of weak filters can select for more diverse and less damaging weed communities. Examples of practices to promote these more-neutral weed communities include integrated weed management, crop management diversity in space and time, and organic fertilizer application (MacLaren et al. Reference MacLaren, Storkey, Menegat, Metcalfe and Dehnen-Schmutz2020).

Recent research suggests that increased weed diversity (i.e., coexistence among many species) is associated with reduced crop yield loss. Understanding the role of weed functional diversity in maintaining ecosystem function and preventing yield loss is among the top five research priorities in weed science, according to a group of experts (Neve et al. Reference Neve, Barney, Buckley, Cousens, Graham, Jordan, Lawton-Rauh, Liebman, Mesgaran, Schut, Shaw, Storkey, Baraibar, Baucom and Chalak2018). The next subsection summarizes existing knowledge about the relationship between diversity and yield.

Increased Weed Diversity Is Associated with Reduced Crop Yield Loss

Negative relationships between weed species richness and crop yield loss have been demonstrated by several authors. Storkey and Neve (Reference Storkey and Neve2018) identified such a relationship using data from the long-term Broadbalk winter wheat experiment, which was initiated in 1843. In this experiment, weedy and weed-free plots were maintained under different fertilization regimes. Using weed species richness data from 19 yr of the experiment, the authors found a strong negative correlation between weed species richness and percentage yield loss due to weeds. This finding was used to illustrate the hypothesis that increased weed diversity is associated with reduced crop yield loss when the weed diversity reflects habitat heterogeneity. Weed species diversity in heterogenous habitats generally implies functional diversity and the presence of species that do not compete strongly against crops. This hypothesis by Storkey and Neve (Reference Storkey and Neve2018) expands on the resource pool diversity hypothesis (Smith et al. Reference Smith, Mortesen and Ryan2010) described in the previous subsection. Separately, Storkey and Neve (Reference Storkey and Neve2018) also hypothesized that weed seedbank diversity is a useful indicator of agronomic and environmental sustainability. In a different study, Brooker et al. (Reference Brooker, Karley, Mitchell, Newton and Pakeman2020) reported a positive relationship between total weed species richness and barley biomass.

Crop yield loss may also be negatively associated with weed evenness or diversity indices combining richness with evenness. Yield was positively correlated with evenness and the Shannon and Simpson indices in coconut (Cocos nucifera L.) and banana (Musa × paradisiaca L.) (Cierjacks et al. Reference Cierjacks, Pommeranz, Schulz and Almeida-Cortez2016). In a long-term study (1996 to 2011), greater weed diversity was associated with a greater capacity for soybean yield increase (Ferrero et al. Reference Ferrero, Lima, Davis and Gonzalez-Andujar2017). However, greater weed diversity also interacted with cold temperatures to reduce corn yield; this result could not be fully explained (Ferrero et al. Reference Ferrero, Lima, Davis and Gonzalez-Andujar2017). Adeux et al. (Reference Adeux, Vieren, Carlesi, Bàrberi, Munier-Jolain and Cordeau2019) reported that more diverse weed communities generally produced less weed biomass and caused lower crop yield losses. Over the gradient of weed community evenness, weed biomass decreased by 83% and crop productivity increased by 23%. It was not possible to separate the effect of reduced weed biomass from any direct effect of weed diversity on crops. However, this study did support the hypothesis that higher weed diversity is associated with lower dominance of competitive weed species that are likely to cause substantial yield loss.

Future research is needed to understand why greater weed diversity is associated with reduced dominance of highly competitive weed species. As noted earlier, this association does not necessarily reflect a causal relationship. However, it is also true that processes of weed–weed interference become more significant in diverse weed communities, sometimes limiting the growth of competitive weed species (Adeux et al. Reference Adeux, Vieren, Carlesi, Bàrberi, Munier-Jolain and Cordeau2019; Clements et al. Reference Clements, Weise and Swanton1994; Pollnac et al. Reference Pollnac, Maxwell and Menalled2009). Real-world weed communities are multispecies assemblages in which complex interspecific interactions occur with variation in space and time. Continued study of these interactions may provide insight into the negative association between weed community diversity and weed–crop competition (Adeux et al. Reference Adeux, Vieren, Carlesi, Bàrberi, Munier-Jolain and Cordeau2019).

Thus far, this section has focused on the competitive effects of weeds on crops. It is worth reiterating that these effects, and therefore the prevalence and composition of neutral weed communities, should be expected to vary between cropping systems and environments. In the following subsection, we turn to the topic of positive weed–crop interactions, which are similarly context specific.

Facilitative Weed–Crop Interactions

Facilitation occurs when the presence of one organism improves the survival, growth, or reproduction of another organism. Facilitative interactions among plants have been well recognized in plant ecology (Brooker et al. Reference Brooker, Maestre, Callaway, Lortie, Cavieres, Kunstler, Liancourt, Tielbörger, Travis, Anthelme, Armas, Coll, Corcket, Delzon and Forey2008; Callaway Reference Callaway2007; Hunter and Aarssen Reference Hunter and Aarssen1988). In agriculture, facilitative interactions have frequently been reported between crops grown in mixture (Brooker et al. Reference Brooker, George, Homulle, Karley, Newton, Pakeman and Schöb2021; Li et al. Reference Li, Li, Sun, Zhou, Bao, Zhang and Zhang2007; Ren et al. Reference Ren, Hu, Zhang, Sun, Tang, Yuan and Chen2014). Facilitative interactions among crops may involve numerous mechanisms, including the production of root exudates to enhance biological nitrogen fixation (Li et al. Reference Li, Li, Wu, Zhang, Li, Li, Lambers and Li2016) or mobilize nutrients (Li et al. Reference Li, Tilman, Lambers and Zhang2014) and hydraulic lift (Sekiya and Yano Reference Sekiya and Yano2004; Sekiya et al. Reference Sekiya, Araki and Yano2011). Given that these facilitative interactions can occur between different crop species, it is reasonable to assume that they might also occur between weeds and crops. Facilitative interactions between weeds and crops may also be indirect and mediated by beneficial insects or soil microbes.

Facilitative effects of weeds mediated by beneficial insects involve ecosystem services such as pollination or biological control of crop pests. The potential of weeds to promote crop pollination is an active area of research, as demonstrated by recent studies in oilseed rape (Brassica napus L.; Crochard et al. Reference Crochard, Julliard, Gaba, Bretagnolle, Baude and Fontaine2022), mango (Mangifera indica L.; Kleiman et al. Reference Kleiman, Koptur and Jayachandran2021), and sweet cherry [Prunus avium (L.) L.; Gilpin et al. Reference Gilpin, O’Brien, Kobel, Brettell, Cook and Power2022]. This topic merits further study. In contrast, the potential of weeds to promote biological control has been established for decades (Altieri and Whitcomb Reference Altieri and Whitcomb1979). For example, a study in corn showed that pests such as fall armyworm (Spodoptera frugiperda Smith & Abbot), corn leaf aphid (Rhopalosiphum maidis Fitch), and sap beetles (Colopterus spp.) were less abundant in weedy plots compared with weeded plots (Penagos et al. Reference Penagos, Magallanes, Valle, Cisneros, Martínez, Goulson, Chapman, Caballero, Cave and Williams2003). Predators of corn pests, such as the carabid Calosoma calidum Fabricius, were more abundant in weedy plots. Corn yield did not differ significantly between weedy and weed-free plots, suggesting that conservation of some weeds may be a reasonable method of enhancing biological control (Penagos et al. Reference Penagos, Magallanes, Valle, Cisneros, Martínez, Goulson, Chapman, Caballero, Cave and Williams2003). In that study, the most abundant weed species were purple nutsedge (Cyperus rotundus L.), niruri (Phyllanthus niruri L.), E. indica, and garden spurge (Euphorbia hirta L.). Similarly, Brust (Reference Brust1991) found that the activity of beneficial nematodes was higher in weedy corn plots relative to weed-free plots, whereas Patriquin et al. (Reference Patriquin, Baines, Lewis and Macdougall1988) found that black bean aphids (Aphis fabae Scopoli) were less numerous in weedy faba bean (Vicia faba L.) plots relative to weed-free plots. Moreover, no significant corn and faba bean yield reduction was reported in weedy plots compared with weed-free plots (Brust et al. Reference Brust1991; Patriquin et al. Reference Patriquin, Baines, Lewis and Macdougall1988). DiTommaso et al. (Reference DiTommaso, Averill, Hoffmann, Fuchsberg and Losey2016) reported that common milkweed (Asclepias syriaca L.) harbored aphids, which provided food for parasitoid wasps (Trichogramma spp.) and other beneficial insects that attack the eggs of insect pests such as the European corn borer (Ostrinia nubilalis Hübner). This positive effect of A. syriaca offset its negative (competitive) effect on corn. Consequently, the role of A. syriaca in promoting biological control increased the economic injury level of this weed (i.e., the minimum weed density at which weed control is worth the cost).

Soil microorganisms can play a crucial role in modulating positive plant–plant interactions (Rodríguez-Echeverría et al. Reference Rodríguez-Echeverría, Armas, Pistón, Hortal and Pugnaire2013). Weeds can be a source of beneficial soil microorganisms such as bacteria that promote crop growth (Sarathambal et al. Reference Sarathambal, Ilamurugu, Priya and Barman2014; Sorty et al. Reference Sorty, Meena, Choudhary, Bitla, Minhas and Krishnani2016; Sturz et al. Reference Sturz, Matheson, Arsenault, Kimpinski and Christie2001). In addition, some agricultural weeds are strong hosts of arbuscular mycorrhizal fungi (AMF), although many weeds are weak AMF hosts (Vatovec et al. Reference Vatovec, Jordan and Huerd2005). A meta-analysis showed that weak host weeds tend to show negative responses to AMF (Li et al. Reference Li, Li, Wu, Zhang, Li, Li, Lambers and Li2016). Even strong host weeds tend to exhibit lower plant growth responses to AMF than strong host crops under fertilized conditions (Li et al. Reference Li, Li, Wu, Zhang, Li, Li, Lambers and Li2016). These findings support the view that AMF can contribute to both crop yield and weed suppression. Additional research has suggested that the presence and appropriate management of AMF host weeds may be harnessed to promote AMF colonization of annual crops (Brito et al. Reference Brito, Carvalho and Goss2013; Feldmann and Boyle Reference Feldmann and Boyle1999). The potential for plant facilitation through mycorrhizal symbiosis might be increased when the plants involved are phylogenetically distant (Montesinos-Navarro et al. Reference Montesinos-Navarro, Valiente-Banuet and Verdú2019) and differ in their AMF assemblages (Montesinos-Navarro et al. Reference Montesinos-Navarro, Segarra-Moragues, Valiente-Banuet and Verdú2012). Therefore, a diverse weed community may be more likely to enhance AMF colonization of a crop, relative to a weed community dominated by a few aggressive species that are weak AMF hosts.

Other forms of plant facilitation may involve plant responses to signals emitted by neighbors, such as volatile organic compounds (VOCs). Signals including VOCs may trigger biochemical responses that positively or negatively affect plant performance, depending on the plant species and signal (Baluška and Mancuso Reference Baluška and Mancuso2009; Brosset and Blande Reference Brosset and Blande2022; Gagliano and Renton Reference Gagliano and Renton2013; Vivaldo et al. Reference Vivaldo, Masi, Taiti, Caldarelli and Mancuso2017). A better understanding of signaling between weeds and crops could provide further insight into why different weed communities have different effects on crop production.

This section has explored ecological mechanisms that may help explain the presence of neutral weed communities in agroecosystems. In the following section, we consider how this ecological knowledge can be applied to promote the establishment of neutral weed communities.