SR17 How does vegetation management affect roadside plants and invertebrates?
Many species that previously associated with meadows and pastures can now be found on roadsides instead. For traffic-safety reasons, vegetation is regularly managed along most roadsides. But how should such management be carried out in order to also benefit biodiversity?
Roadsides a potential substitute for declining grasslands
As agriculture has been modernised, the area of meadows and pastures has fallen sharply, in Sweden and elsewhere. Today only fragments of such semi-natural grasslands remain. In contrast, roadside habitats have increased in area.
Roadsides are usually mowed every year for traffic safety reasons, and sometimes plant litter is removed to favour more demanding plant species. In some cases, vegetation along roadsides is controlled by grazing or burning instead. This means that there are similarities between roadside maintenance and management of semi-natural grasslands, and many species that were previously mainly associated with such grasslands can now largely be found along the roads instead.
Recommendations for roadside management to promote conservation values are mainly based on botanical studies in meadows, pastures and similar grasslands. However, key stakeholders have emphasised the need for more targeted management recommendations based on actual roadside studies. Due to the use of road salt for de-icing, ditching and reinforcement activities, and other measures particular to the maintenance of roads and roadsides, plants are not likely to respond to management in the same way there as in other grasslands.
Therefore, we conducted a systematic review of how maintenance or restoration of roadsides using various forms of non-chemical vegetation removal affects the diversity of vascular plants and invertebrates. The ultimate aim of the review was to facilitate evidence-based management of roadsides to conserve or restore biodiversity.
Results from 54 studies synthesised
The review included a total of 54 studies. Nearly all of these were conducted in Europe (29 studies) or North America (24). One study was made in South Africa, but we found no relevant studies from Asia, Australia and South America and none from tropical regions.
The vast majority of included studies (48) examined effects of mowing, whereas effects of burning were reported by seven studies. One study reported on effects of grazing and one on effects of shrub removal.
Most studies in the review were relatively short-term – 36 of them were made over a period of three years or less. The longest-term study had run for 23 years.
Nearly all of the included studies (51) investigated effects of roadside management on vascular plants, whereas effects on invertebrates were reported in eight studies.
Quantitative analysis of plant species richness and diversity showed that mowing effects were dependent on the interplay between mowing frequency and hay removal. There were no statistically significant overall effects of mowing vs. no mowing, frequency of mowing, timing of mowing or hay removal on plant species richness and diversity (measured as Shannon diversity index).
Nevertheless, species richness of plants was higher in roadsides mowed once or twice per year with hay removal compared to unmowed roadsides, and also higher in roadsides mowed twice compared to once per year (with or without removal of hay), whereas mowing more than twice per year did not increase species richness.
Diversity of plants was also higher in roadsides mowed twice per year, but there were not sufficiently many studies for analysing the effects of hay removal on diversity. The variability was too large to allow conclusions on the effects of early vs. late mowing on plant species richness and diversity.
Similarly, mowing frequency influenced how mowing affected abundances of functional groups of vegetation. Mowing twice instead of once per year had a significantly negative impact on the abundance (cover) of graminoids (grasses, sedges etc.), with a parallel positive trend towards higher abundance of forbs. Woody plant abundance was usually lower in mowed than in unmowed roadsides, although also with the clearest effect in roadsides mowed twice per year.
Implications of the findings
Based on our review we conclude that vascular plant richness is likely to increase (i) if roadsides are mowed each year, (ii) if they are mowed twice rather than once a year (this can be expected to benefit forb diversity specifically), and (iii) if hay is removed after each cutting.
However, the review also shows that there are large knowledge gaps on how management of roadsides affects diversity of vascular plants and invertebrates. First, relevant studies on invertebrates were very few. Secondly, studies on vascular plant diversity are almost exclusively focused on mowing; few studies test effects of other management options such as burning or grazing, or effects of roadside management in combination with influx of chemicals, nutrients or salt. The studies also suffer from short experimental duration, limited geographic distribution, and lack of common research protocols.
How the review was conducted
This systematic review was conducted by a specially appointed team of researchers chaired by Regina Lindborg, professor of physical geography at Stockholm University, Sweden.
Initially, in 2017, the review team published a systematic map of the evidence on how different forms of roadside management affect the diversity and dispersal of species (see EviEM SR9). A systematic map provides an overview of existing studies of a certain topic, but it does not synthesise their findings. From the more than 300 studies that were included in the SR9 systematic map, we then selected those that described biodiversity effects of non-chemical vegetation removal. Usable data reported in these studies were extracted and synthesised.
The final report on the systematic review was published in July 2018. Results have also been published in a popular summary and a fact sheet (see links at right).
Regina Lindborg (chair), Dept. of Physical Geography, Stockholm University, Sweden
James M. Bullock, Centre for Ecology & Hydrology, Wallingford, UK
Kris Verheyen, Dept. of Forest and Water Management, Ghent University, Belgium
Simon Jakobsson (project assistant), Dept. of Physical Geography, Stockholm University, Sweden
Claes Bernes (project manager), Stockholm Environment Institute, Stockholm, Sweden