The multifunctional roles of vegetated strips around and within agricultural fields. A systematic map protocol

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.

Searches

Screening

All articles identified through searching will be screened at title, abstract and then full text levels for relevance using predefined inclusion criteria (detailed below). Consistency in the application of the inclusion criteria will be tested by comparing agreement between two reviewers at abstract level screening, using a subset of 200 abstracts. Disagreements will be discussed and the inclusion criteria refined where necessary. Agreement will be tested formally using a kappa test [57], and if agreement score falls below 0.6, indicating moderate agreement, a third reviewer will be consulted and a further 200 abstracts screened following discussion of disagreements. Following abstract screening, potentially relevant studies will be retrieved in full text. Unobtainable articles will be listed in the final report. All screened full texts that are excluded from the review will be listed along with exclusion reasons in the final report.

Inclusion criteria

Critical appraisal

Critical appraisal will not be undertaken within this map, since the measurement methods will vary substantially across different outcomes. A very basic quality assessment will be undertaken in the form of a ‘free text’ meta-data variable where a brief description of the study quality will be made, flagging up clearly unreliable research that should be excluded, or serious deficiencies that should be pointed out in those studies that remain in the map.

Data coding strategy

Meta-data (i.e. descriptive data regarding the methods and setting of each study) will be extracted from included, relevant studies and entered into a searchable database. In addition, the database will be populated with a number of variables, each given a category according to a predetermined strategy (also known as coding). This database will form one of the main outputs of the review and will be supplied as a supplementary file along with a help file. Consistency of data extraction across team members will be conducted using a subset of 100 studies to ensure complex data are extracted reliably.

The following information will be entered into the systematic map database for all included studies that are available and deemed as relevant at full text. The following types of information will be recorded (see Additional file 2 for further details): author email address, study location, soil management practices, soil description, farm management practices, vegetated strip description, vegetated strip management, study design, experimental design, sampling design, measured outcome, data location, and critical appraisal comments.

Study mapping and presentation

The database will be accompanied by a report that describes the review process and the evidence base, identifying possible knowledge gaps (i.e. subtopics requiring further primary research), knowledge gluts (i.e. subtopics with enough evidence and interest to warrant a systematic review), and best practices in research methodology. Particular attention will be paid to studies that describe vegetated strips established for multiple purposes (e.g. biodiversity and nutrient retention). In addition, the database will be displayed visually in the form of a geographical information system (GIS) that maps studies by their location across a cartographical map. This GIS will be made available via the EviEM website (http://www.eviem.se).