The ecological impacts of agricultural intensification and change in Europe since the Second World War are well documented and affect both agricultural areas and their surrounding systems [1]. Biodiversity, air and water quality, soil structure and ecology have all been affected [2]. Well-documented impacts of agricultural development include: widespread negative effects of the application of nutrients in fertilisers (mineral and organic) and agro-chemicals on soil, and surface and ground water quality [3], emission of N2O as a potent greenhouse gas [4], and negative effects of pesticides on non-target invertebrate species [5], birds [6] and biological control potential [7] together with the loss of ecological heterogeneity at multiple spatial and temporal scales [8]. The establishment and management of vegetated strips (including field margins, buffer strips and hedgerows) are key mitigation measures for these negative environmental impacts [9].
Definition of vegetated strips
Here, we define vegetative strips as any vegetated area set-aside from the main cropping regime within or around a field, and installed for the purposes of benefiting native biota, water and air quality, socio-economics, and yield. Examples of such interventions include: hedgerows, field margins, buffer strips, beetlebanks and shelterbelts (Fig. 1). For the purposes of this review, we focus on those interventions that are permanent or semi-permanent fixtures in agricultural landscapes, and the interventions must therefore be in place for longer than 12 months (see Inclusion Criteria for further details).
Vegetated strips have a multi-functionality that covers a range of processes, including protection of water quality in surface waters, habitat improvement, biodiversity, shading, carbon sequestration, flow capture, biomass production, landscape diversity, and societal services [10]. These processes are recognised to occur through a suite of pathways that impact socio-economic and environmental outcomes (Fig. 2).
Vegetated strips and water flow
Many of the ecosystem services provided by vegetated strips exist because of a reduction in water flow that occurs due to the presence of aboveground vegetation, roots and soil complexity.
As surface runoff passes across field margins, the velocity of flow tends to decrease in response to the type and density of strip vegetation as well as to any changes in slope. This reduction of flow allows suspended sediment to be deposited, which decreases the transport of sediment and sorbed nutrients and other contaminants beyond the strip. The reduction also provides potential for infiltration of water into the strip, decreasing the total volume of runoff water and the associated load of dissolved contaminants. The effectiveness of vegetated strips in reducing sediment transport off-site is known to vary with the ratio of runoff area to the area of the strip [11] as well as with other factors including soil type, topography, soil–water management (such as drainage pipes), land use, rainfall intensity and antecedent moisture conditions [12]. For instance, nutrients and pollutants may readily flow through vegetated strips from the soil surface and into drainage pipes, particularly in clay soils, through macropores, cracks and root channels. This effect may be prevalent on any soil type where heavy rain follows dry periods. Similarly, the beneficial flow reduction properties of vegetated strips can be negated either where the strips occur on steep ditch banks, or where steep channels allow flow to be diverted around the strips.
Effects on nutrients and other contaminants
Nutrients and pesticides are amongst the most important pressures on aquatic ecosystems, where excess inputs may deteriorate ecosystem integrity and/or threaten drinking water resources [13, 14]. Even strongly-sorbed compounds, including faecal pathogens from livestock or slurry fertiliser applications, can harm surface water quality through long-distance erosive runoff. Management of these pollutant losses takes place both through baseline regulations, and by introducing cross compliance or general binding rules for protection of receiving water bodies [15]: both for control at source and locally targeted regulations using incentives at high risk contaminant pathways. Buffer strips are one of the most commonly applied management measures, and are mainly designed and implemented to control sediment, phosphorus, nitrogen and pesticide losses to off-site surface waters [16, 17]. They have been shown to be highly efficient for reducing nutrient runoff from farmed fields in a wide range of climate regions across the world [18, 19]. Similarly, vegetated strips in riparian zones are also effective at removing nitrogen in proximity to watercourses, particularly subsurface nitrogen, although their efficacy appears to be variable [20]. Generally, the effectiveness of vegetated strips in controlling transport of more soluble contaminants is less than for strongly-sorbed chemicals. There is also potential that dissolved contaminants infiltrating into the margin may subsequently reach surface water via subsurface drains and/or shallow groundwater.
Where contaminants may be emitted to the air, as for pesticide spraying, vegetated strips have a dual functionality in increasing the distance between the emission source and vulnerable habitats such as surface waters or non-crop habitats, but also through the potential for interception of spray drift. Finally, it is known that pharmaceuticals used in animal husbandry may also be important contaminants of terrestrial environments adjacent to agricultural fields [e.g. 21]. In such cases, vegetated strips can provide a physical barrier where operations such as spreading of manure and biosolids are not allowed.
Effects on biodiversity
The widespread loss of spatial landscape heterogeneity, associated with the use of a few high yielding crop types across large uniform fields [8], is often viewed as a key driver of biodiversity loss on arable land across Europe [22–24]. Hence, the creation and management of various field margin habitats has the potential to restore habitat diversity for the benefit of associated farmland biodiversity [25]. Hedgerows and other field margin vegetation types have been shown to affect the richness and abundance of flora, invertebrates and birds [26–28]. For instance, grassy field margins have been shown to provide important refuge and food for invertebrates, mammals and birds [29, 30]. Yet, these effects may depend on landscape structure and regional levels of agricultural intensification [31]. As a result measures are sometimes implemented in landscapes where their effects are small or even negative [32].
As field margins comprise a variety of different vegetation types that are managed for different purposes, their effects on biodiversity and associated ecosystem services may vary. For instance, pollinator habitat enhancement in the form of hedgerows and flower-rich buffer strips may contribute to yield on adjacent fields [33] but also overall biodiversity and biological control potential in the surrounding landscape [34]. Buffer strips established using densely planted perennial grasses may primarily benefit invertebrates for pest suppression [35] but also increase the availability of suitable nesting sites for ground-foraging farmland birds on adjacent crop fields [36]. However, the access to foraging opportunities for insectivorous birds in these strips may be substantially lower compared to margins planted with wildflower mixes [37] or naturally regenerating margins on poor soils with a diverse seed bank (19). At the regional scale these benefits may be particularly valuable in resource-poor landscapes [38]. In addition, both at local and regional scales, vegetated strips provide valuable linear habitats that may promote connectivity between areas of non-agricultural land or semi-natural landscapes [39]. Finally, it is important to mention that vegetated strips around and within fields may also negatively impact on crop production and biodiversity. This is because field margins harbour weeds, pests and diseases (e.g. viruses), which could potentially create a conflict between crop production and biodiversity conservation [9, 40]. Increased habitat heterogeneity may also have negative impacts on some species that require or prefer large, homogeneous environments, such as farmland and migratory birds [41, 42]. Some of these homogeneous environments, commonly considered to be the result of agricultural development and intensification, may represent natural systems, particularly those in central and eastern Europe [43].
Other effects
Depending on the nature of their management, vegetated strips can provide various other services. For example, strips with perennial grasses or trees and/or shrubs, can counter soil erosion via filtration of larger sediment particles [44, 45], and by increasing soil stability through increased root density [46]. Some resources from vegetated strips can be harvested periodically, such as wood and fodder [16], and strips are also used to provide nesting and foraging habitat for game bird populations [e.g. 47] although elevated mortality and nest predation can occur in these habitats [48, 49]. A less well-studied aspect of vegetated strips is their potential to enhance aesthetic values and perceived “naturalness” of agricultural landscapes, especially when vegetated with trees and/or shrubs and employed in areas where such features are absent [16]. Similarly, other values may be investigated, including provision of game habitat, refugia for crop pest predators, and amenity use of agricultural land, for example by horse riders.
Multipurpose vegetated strips and conflicting objectives
One key question relating to vegetated strips as an environmental intervention on farmland is how to evaluate multifunctional effects; that is, impacts of single strips on multiple outcomes. True evaluation for areas larger than the plot-scale is difficult to undertake due to difficulties in having representative controls. One possibility to overcome large-scale evaluation problems is therefore upscaling of plot results and/or modelling, and in both cases collection of data from experimental studies conducted around the world will be invaluable as a baseline. In their review of the multifunctional role of vegetated strips on arable farms, Hackett and Lawrence [50] concluded that although different strip types can produce multiple benefits, none can wholly provide for all environmental outcomes. One way to optimise multiple benefits from field margins at the field and landscapes scale could therefore be to adjust management practices locally according to purpose.
In reality, however, many vegetated strips vary in their purpose, method of establishment and ongoing management. Common forms of field margins include those that are naturally regenerated from unused farmland, those sown with grass or wildflower mixes, those sown specifically for target organisms such as pollinators (nectar and pollen mixes) or for wild birds (seed mixes), those that are annually cultivated and those that are unmanaged [50]. The specific design and management of a vegetated strip may depend on the main reason for the intervention, and the resultant efficacy for the different outcomes described above may vary accordingly. Wildflower strips, for example, are designed to benefit pollinators such as bees [29], whereas densely vegetated strips typically established by sowing a mixture of perennial grass species adjacent to water courses, are primarily used to mitigate soil erosion [51] and reduce leaching of nutrients and agro-chemicals [52]. The access to foraging opportunities for insectivorous birds in strips designed for water protection may be substantially lower compared to strips planted with wildflower mixes [37] or naturally regenerating strips on poor soils with a diverse seed bank [30]. Accordingly, managing vegetated strips for biodiversity or for diffuse pollution purposes may entail very different management practices, since retained dissolved or particulate matter eventually accumulates within the strip, which in turn may reduce the potential for biodiversity benefits. However, removal of plant material from vegetated strips could help maintain long-term retaining capacity, avoiding their transformation into nutrient sources, and with simultaneous benefits of lower nutrient levels and/or sparser vegetation for wild flora and visual foragers such as birds [53]. An additional consideration in this context relates to pollution swapping [54], where mitigation measures for one pollutant cause an increase in another pollutant. In this way, vegetated strips for controlling nitrogen leaching could lead to simultaneous transformation of sediment-bound phosphorus into soluble reactive phosphorus.
Identification of the topic
The topic was suggested at a general stakeholder meeting arranged by MISTRA EviEM on September 24th, 2012. Suggestions for the topic were made by the Swedish Board of Agriculture, the Swedish Environmental Protection Agency, the Swedish Ministry of the Environment, Svensk Sigill, Hushållningssällskapet, WWF, and researchers from the Centre for Biodiversity and the Department of Ecology at the Swedish University of Agricultural Sciences. The focus and scope of the review was narrowed and better defined during a specific stakeholder event on September 1st, 2015.
Objective of the review
The aims of this review are to identify, collate, and describe relevant published research relating to the effectiveness of vegetated strips in and around farmland for a wide variety of purposes, including but not limited to: the enhancement of biodiversity; the reduction of pesticide and nutrient drift/runoff/leaching; the mitigation of soil loss; the reduction of pathogens and toxins; and, socioeconomic values, such as provision of game habitat and reduction of crop pests. The map will be restricted in geographical scope to boreal and temperate systems (see Inclusion Criteria below) and will consist of a report describing the review process, a searchable database describing the identified relevant studies, and an interactive, web-based geographical information system (GIS) displaying the contents of the database.
- Primary question
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What evidence exists regarding the effects of field margins on nutrients, pollutants, socioeconomics, biodiversity, and soil retention?
- Secondary question
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To what extent has this research focused on multi-use vegetated strips?
- Population
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Boreo-temperate regions as defined by the following Köppen-Geiger climate classification zones [55]: Cfa, Cfb, Cfc, Csb, Csc, Dfa, Dfb, Dfc.
- Intervention
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Vegetated strip interventions around and within fields used for arable, grazing and horticulture, orchards and vineyards, where presence of a vegetated strip or management of the strip is investigated.
- Comparator
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Before vegetated strip establishment, before a change in vegetated strip management (temporal comparisons); no vegetated strip, different vegetated strip management, including strip width (spatial comparisons); outside a vegetated strip.
- Outcome
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Outcomes will be included iteratively as they are identified within the relevant literature and will be coded accordingly