How does roadside vegetation management affect the diversity of vascular plants and invertebrates? A systematic review protocol

Roadside management has come to be regarded as an important part of overall landscape management for biological conservation [1]. Marginal habitats like roadsides may sustain biodiversity by functioning as refugia [2, 3] or by facilitating dispersal [4, 5]. By contributing to dispersal within a landscape (e.g. as corridors) they may mitigate negative effects of fragmentation and isolation of habitats even in highly-modified agricultural or urban landscapes [6].

Roadsides have also been acknowledged as potential substitutes for open habitats like grasslands [3], which are currently declining globally [7, 8]. In particular, semi-natural grasslands (created by low-intensity, traditional management) are well known for their high biodiversity [9, 10] but have declined rapidly during the last century, especially in Europe [11]. Many species that historically were mainly associated with meadows and pastures now thrive along roads instead [12]. In Sweden, some 190,000 hectares of managed grasslands occur along infrastructure, 164,000 hectares of which constitute roadsides [13]. This equals more than a third of the total amount of meadows and pastures in Sweden (ca. 450,000 hectares) [14].

With appropriate management, many roadsides might harbour a rich flora and fauna. In general, roadsides are presented as offering opportunities for grassland species [1, 15]. Managing grassland habitats for biodiversity usually requires removing vegetation and reversing late-successional vegetation stages, e.g. by mowing, grazing, burning, harrowing or scraping [16]. Differences in mowing regimes may affect both animal and plant populations. For instance, mowing after flowering and seed production will enhance the link between pollinators and flowering plants, as many plants need assistance by pollinators to produce seeds and pollinators need the flowers for nectar resources [17]. Disturbance-tolerant species are particularly favoured, while species representing early-successional stages will eventually disappear.

The management regimes and abiotic conditions of roadsides may be quite similar to those of mown or grazed semi-natural grasslands [18]. Current recommendations for roadside management to promote conservation values are largely based on botanical research on meadows, pastures and other semi-natural grasslands [1]. The effects on biodiversity of grazing and mowing of such grasslands are well studied [19]. However, management particular to roadsides, like ditching and reinforcement activities, use of salt for de-icing, sowing of exotic plant material and other measures for infrastructure maintenance will likely impact species and communities differently to traditional management of open grasslands. Management effects on biodiversity are less well studied along roadsides than in meadows and pastures, and much of the existing evidence on roadsides is comprised of grey literature not assessed by external reviewers. For these reasons, key stakeholders in Sweden have emphasised the need for more targeted guidelines for roadside management, based on actual studies of roadsides [20].

We recently performed systematic mapping of the available evidence on how roadside management affects biodiversity and the dispersal of species [21]. In the protocol for the systematic map [20], we provided a general background on biodiversity effects of roadside management, presented the rationale for the mapping initiative, and described the methods that we intended to use.

The results of our mapping exercise show that much of the literature on ecological effects of roadside management describes attempts to revegetate recently constructed roadsides, or the use of herbicides for controlling roadside vegetation in general and invasive/nuisance plants in particular. Chemical management of roadsides is nowadays largely restricted in many countries, and the literature that we have found on such management is dominated by older American studies (many of them dating from the 1970s or earlier) of substances that are now obsolete there.

However, we found more than a hundred studies with more obvious relevance to the conservation or restoration of biodiversity in roadsides, including their role as substitutes for grasslands and other habitats under threat in intensively managed landscapes. More specifically, we identified 98 studies of how the richness or abundance of various taxonomic or functional groups of species in roadsides is affected by vegetation disturbance by managers, such as regular mowing, burning, grazing or selective mechanical removal of plants. Since all of these interventions entail removal of plant biomass, they are comparable in the sense that they all follow Grime’s [22] classic definition of disturbance as the partial or total destruction of biomass. A review of their impact on biodiversity should therefore permit some generalisable conclusions. Most of the studies have recorded management effects on vascular plants, but there are also 14 potentially relevant investigations of insects or other invertebrates.

We focus on non-chemical interventions that may aid the conservation or restoration of roadside biodiversity. These studies should be of considerable interest to roadside managers, including, e.g. transportation and conservation agencies, park authorities, municipalities, and farmers and other private landowners. This has been confirmed by our contacts with Swedish stakeholders. For instance, according to a representative of the Swedish Transport Administration, it is of central importance to clarify precisely how mowing and similar kinds of roadside management should be carried out to give the desired results under various biotic and abiotic circumstances. Effects of different timing of such management and of management regimes based on various combinations of interventions were mentioned as being of particular interest (Anders Sjölund, pers. comm.). A representative of the Swedish Biodiversity Centre also underlined the importance of considering potential effect modifiers such as soil type, nutrient status, shading, management history and presence/absence of invasive species (Tommy Lennartsson, pers. comm.).

Encouraged by the input from stakeholders, we intend to proceed with a full systematic review of how disturbance-based maintenance or restoration of roadsides affects the diversity of vascular plants and invertebrates. The review will follow the guidelines for systematic reviews in environmental management issued by the Collaboration for Environmental Evidence [23].

The primary aim of the proposed systematic review is to clarify how the diversity of vascular plants and invertebrates is affected by roadside maintenance or restoration using various forms of non-chemical vegetation removal. Since current recommendations for roadside management to promote diversity largely draw on studies of semi-natural grasslands, we will also investigate whether our findings differ from those on semi-natural grasslands. Our review will largely be based on studies selected from a systematic map of the evidence on how roadside management affects biodiversity and the dispersal of species along roadsides [21]. We will apply no geographical restrictions when collecting and analysing the evidence.

Primary question: How does roadside maintenance and restoration implementing non-chemical vegetation removal affect the diversity of vascular plants and invertebrates?